pulverised coal ash requirements for utilisation
TRANSCRIPT
lEA COAL RESEARCH
-Pulverised coal ash requirements for utilisation
Pulverised coal ash requirements for utilisation
Lesley L SlossIrene M SmithDeborah M B Adams
IEACR/88June 1996lEA Coal Research, London, UK
Copyright © lEA Coal Research 1996
ISBN 92-9029-270-9
This report, produced by lEA Coal Research, has been reviewed in draft form by nominated experts in member countries andtheir comments have been taken into consideration. It has been approved for distribution by the Executive Committee of lEACoal Research.
Whilst every effort has been made to ensure the accuracy of information contained in this report, neither lEA Coal Research norany of its employees nor any supporting country or organisation, nor any contractor of lEA Coal Research makes any warranty,expressed or implied, or assumes any liability or responsibility for the accuracy, completeness or usefulness of any information,apparatus, product or process disclosed, or represents that its use would not infringe privately-owned rights.
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Abstract
Coal ash is an inherently variable material due to differences in the mineral content of the source coal, combustion conditions,and ash collection and handling methods. This affects the marketability of pulverised coal ash. Markets in different countries arediscussed with respect to opportunities and obstacles to their development. Recent publications on applications for pulverisedcoal ash are reviewed briefly. A knowledge of the chemical, mineralogical and physical properties of fly ash and bottom ash isessential for the more specialised utilisation sectors such as cement and concrete. Methods for the classification of fly ash arereviewed. Standards and specifications used in various countries for use of coal ash in different applications are discussed alongwith the appropriate test methods. Quality assurance procedures which suppliers use to ensure fly ash meets these specificationsare also included. The marketability of fly ash may be improved by a number of beneficiation processes. Processes such asdewatering, blending, agglomeration, grinding, sieving, air classification, flotation, carbon burnout and electrostatic separationare discussed, including some examples of processes which are being actively developed.
4
Contents
List of figures 7
List of tables 8
Acronyms and abbreviations 9
1 Introduction 11
2 Ash markets 122.1 Market strategies 12
2.1.1 Markets in different countries 122.1.2 Overcoming the market obstacles 17
2.2 Applications 182.2.1 Engineered fill and fillers 182.2.2 Cement, concrete and mortar 202.2.3 Secondary products 232.2.4 Pollution control 252.2.5 Agriculture and fisheries 282.2.6 Materials recovery 30
2.3 Summary and comments 31
3 Physical and chemical characteristics 323.1 Origins of coal ash 323.2 Chemical composition 33
3.2.1 Major elements 333.2.2 Trace elements 353.2.3 Unburnt carbon 35
3.3 Mineralogy 363.4 Morphology 373.5 Physical properties 38
3.5.1 Particle size distribution 383.5.2 Bulk density 40
3.6 Chemical properties 403.6.1 Pozzolanicity 403. ).2 Reactivity 41
3.7 Summary and comments 42
5
4 Classification and specifications 434.1 Classification 43
4.1.1 Grading systems 434.1.2 Kocuvan'ssystem 45
4.1.3 Triangular system 454.1.4 ASTM system 464.1.5 Russian Academy of Sciences system 464.1.6 New systems 46
4.2 Specifications 574.2.1 Cement and concrete 48
4.2.2 Other applications 554.3 Test methods 55
4.3.1 Pozzolanicity 554.3.2 Fineness 564.3.3 Strength 564.3.4 Autoclave expansion 574.3.5 Permeability 57
4.3.6 LOI 584.3.7 Other methods of testing 584.3.8 Advanced techniques 59
4.4 Quality assurance and certification 594.5 Summary and comments 60
5 Quality control 615.1 Operating conditions of plant 61
5.1.1 Coal type 615.1.2 Combustion conditions 625.1.3 Pollution control systems 63
5.2 Storage and transport 635.2.1 Storage 635.2.2 Transport 64
5.3 Processing and beneficiation 64
5.3.1 Dewatering 655.3.2 Blending 655.3.3 Agglomeration 665.3.4 Grinding 665.3.5 Sieving 675.3.6 Air classification 685.3.7 Flotation 695.3.8 Fluidised beds 705.3.9 Carbon burnout 735.3.10 Electrostatic separation 745.3.11 Magnetics 755.3.12 Chemical treatment 765.3.13 Combined processes 76
5.4 Summary and comments 77
6 Conclusions 78
7 References 80
6
Figures
1 Proportions of fly ash used in different sectors worldwide 12
2 Distribution of coal ash use in the Electric Power Industry, Japan 15
3 Structure of asphalt paving and coal ash application 19
4 Relative compressive strength of fly ash concrete specimens(to that of concrete with no fly ash) as a function of curing time 22
5 The Aardelite process 23
6 Comparison of conventional system and Poz-O-Tec system 26
7 Average pH values for incoming and outflow water at grouted and ungrouted well areas 27
8 Types of coal ash artificial reef units used in Taiwan 30
9 Schematic diagram of the interactions of different inorganic components during combustion 33
10 Distribution of mineral matter following combustion in a power station 33
11 Particle size distribution of fly ash samples measured by different techniques 39
12 Classification of pulverised fuel ash based on self-hardening properties 44
13 Triangular graph for coal ash classification 45
14 Seven-day compressive strength trends of Ottumwa fly ash pastes 62
15 Moisture content versus dry density curve for typical lagoon and conditioned ash 64
16 Systematic diagram of pilot ash dewatering plant 65
17 Three-dimensional vibration sieving machine 67
18 Fineness of ash and its effect on compressive strength of mortars 68
19 A pilot-scale air classification process 69
20 KEMA air classifier 69
21 Particle size distributions of classifiedand ground fly ashes in comparison with the input fly ashes 70
22 Conceptual flow-sheet and solid balance for a 25 tph fly ash processing facility 71
23 Vibro fluidised bed 72
24 Reductions in unburnt carbon content in product fly ash from various raw fly ashes 72
25 Typical high efficiency centrifugal fly ash classification system 73
26 Process flow diagram for carbon burnout 74
27 Triboelectrostatic particle separation 74
28 Process diagram for electrostatic separation of fly ash 75
29 Separation schematic for a fly ash beneficiation process 76
7
Tables
I Ash utilised in various countries 13
2 Concentration of major and minor elements in coal ash and in fly ash 34
3 Chemical composition of North American fly ashes 34
4 Concentration of trace elements in coal ash and fly ash 35
5 Mineralogy of North American fly ashes 37
6 Classification schemes applicable to fossil fuel residues 44
7 Fly ash classification within ENV 197 47
8 Specifications for coal fly ash use in Portland cement in various countries 49
9 Specifications relevant to fly ash use in the USA 50
10 Chemical requirements for fly ash under ENV 450 and ASTM C618 51
II Physical requirements for fly ash under ENV 450 and ASTM C618 51
12 Blended fly ash cement as defined by ENV 197 52
13 Durability requirements related to environmental exposure according to ENV 206 52
14 Australian Standard AS3582.1 - specified properties of fly ash for use in concrete 53
15 Classes of concrete 54
16 Frequency of testing in Australian Standard 3582 59
17 Major deviations from ENV 450 60
18 Permissible variations from Australian Standard 3582 60
19 Comparison of compressive strength of ordinary Portland cement and fly ash of different fineness 67
20 Chemical composition and pozzolanic activity index of sieved classified fly ash 67
21 Analysis of ash fractions from triboelectrostatic separation 74
22 Chemical composition of magnetic and non-magnetic fractions of fly ash samples 75
23 Summary of properties of Class F t1y ash following beneficiation 77
8
Acronyms and abbreviations
AASHTOACAAAESAFBCAPIASTMATRAN
CAACAMCAPD
CARRCCBOCCSEMCCUJCEACEN
CNSDIN
ENELEPRIESPFBCFGDFTIRGDRIAEICPJISLOIMWPFBCSEM
US DOEUS EPAXRD
American Association of State Highway and Transportation OfficialsAmerican Coal Ash Association
atomic emission spectroscopyatmospheric fluidised bed combustion
American Petroleum InstituteAmerican Society for Testing and MaterialsAsh TRANsformations database
Coal Ash Advisor (database)carbon in ash measurement systemCoal Ash Properties DatabaseCoal Ash Resources Research Consortiumcarbon burnout processcomputer controlled scannir.g electron microscopyCentre for Coal Utilisation, JapanCanadian Electricity AssociationComite European de Normalisation
Chinese National StandardsDeutsche Industrie NormenEnte Nazionale per I'Energia Elettrica, ItalyElectric Power Research Institute, USAelectrostatic preci pitatorfluidised bed combustionflue gas desulphurisation
Fourier transfornl infrared (spectroscopy)(fomler) German Democratic RepublicInstitute of Applied Energy, Japaninductively coupled plasmaJapanese Industrial Standardsloss on ignitionmegawatt (electric)pressurised fluidised bed combustionscanning electron microscopyUnited States Department of EnergyUnited States Environmental Protection AgencyX-ray diffraction
9
10
1 Introduction
Coal ash has been used in the construction industry fordecades. The amount of ash being used is increasing.However, it is a buyer's market and, for success, there has tobe proof that ash is a marketable by-product. Specificationsexist for each product. Ash has to comply with thesespecifications and, in addition, must be economicallyattractive. New techniques are being developed to increasethe marketability of ash and new uses are being found.
This report deals with pulverised coal ash, that is, bottom ashand fly ash, although the majority of information reviewedrelates to fly ash. The report excludes consideration of ashescontaining sorbent for removal of sulphur dioxide (S02) influidised bed combustion and flue gas desuiphurisationsystems and also excludes gasification residues. These topicswere reviewed in previous reports by lEA Coal Research(Smith, 1990; Clarke, 1991, 1993a, 1994). This reportcomplements the earlier reports from lEA Coal Research onapplications for coal use residues (Clarke, 1992) andlegislation for their management (Clarke, 1994).
In July 1994 the Office of Fossil Energy in the USDepartment of Energy (US DOE, 1995) presented a report toCongress which listed II major 'institutional' constraints onfly ash use in the US. These were:
lack of familiarity with potential ash uses;lack of data on environmental and health effects;restrictive or prohibitive specifications;belief that fly ash quality and quantity is inconsistent;lack of fly ash specifications for non-cementitiousapplications resulting in substitution in these applicationsof the more restrictive specifications for use of fly ash incement and concrete;
belief that raw materials are more readily available andmore cost-effective;US EPA procurement guideline for fly ash in concrete isviewed by States as a rigid ceiling instead of a generalguideline for use;environmental agencies support beneficial ash uses inprincipal, but are frustrating the actual implementation;State regulation of fly ash as a solid waste;lack of State regulations on beneficial ash use;lack of clear Federal direction on regulation of beneficialash use.
Although these restrictions were listed in reference to theUSA, such barriers against the use of coal fly ash arecommon.
Coal ash is not normally a material produced to meet a givenspecification. Rather it is a by-product of power production.Its chemical and physical properties vary depending onpower plant operating conditions and uniformity of the coalsource. This is of great concern to those attempting to utilisethe ash. Thus successful utilisation of coal ash depends firstupon knowing the physical and chemical properties of flyashes, their composition, mineralogy and engineeringproperties. There are already various schemes used to classifyfly ash as well as standards and specifications for fly ash usearound the world. These are discussed in this report,including a short critical review of the test procedures used tocharacterise fly ash. Ash markets and market strategies indifferent countries indicate where coal ash is usedsuccessfully and how market obstacles are overcome. Finally,the review of quality control and beneficiation methods inuse shows how the marketability of fly ash may be enhanced.
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2 Ash markets
More details of the sectoral use and markets are given inSection 2.1.1 for specific countries and the opportunities andbarriers to use are summarised in Section 2.1.2.
Where available, the information on coal ash utilisation indifferent countries from Manz (I 995a) has been updated andis shown for countries giving data for 1992 onwards andcountries producing over 200 ktly in Table I.
The most recent data on market sectors, targeted by variouscountries for utilisation of coal ash is summarised inSection 2.1. It aims to see how opportunities for the useof coal ash are developed and how obstacles may beovercome.
other10.8
secondaryproducts13.4
8.52.2
10.7
ktly %
691184875
structural and landfill40.1
roadconstruction
7.4
None of the brown coal ash was used but has potential forthe production of marketable chemicals, magnesium as wellas in cement blends (Manz, 1995a).
2.1.1 Markets in different countries
Cement and groutStructural fill, specialised fillerTotal used
AustraliaAccording to a questionnaire study by Manz (1995a), the fivemajor coal burning states produced a total of 7.1 Mt ofbituminous coal ash and 0.3 Mt of brown coal ash in 1992.Bituminous coal ash was used mainly as cement raw materialand cement replacement, and smaller amounts as fill forexample, in embankments, roads and mines:
cement, concreteand grout
40.7
Figure 1 Proportions of fly ash used (Mt) in differentsectors worldwide (Manz, 1995a)
Market strategies2.1
Results of a recent questionnaire survey of coal ashutilisation in 42 countries are presented by Manz (I 995a).They indicated that about 459 Mt of coal ash was producedin 1992 and 153 Mt (33.3%) was used. The proportions usedin different sectors are shown in Figure I.
The detailed handbook already published by IEA CoalResearch on applications for coal-use residues (Clarke, 1992)demonstrates the wide variety of by-products developed. Thesuccessful marketing of pulverised coal ash depends on anunderstanding of the scope for deploying the specialcharacterisitics of the residues for specific uses. Lessdemanding use is as fill material for roadbase or for'dumping' in mines. In addition to more specialised bulk usesuch as in cement and concrete, there are also applicationsrequiring smaller or discontinuous use of coal ash which maybe of great importance at local sites. Hence recentdevelopments in the large- and small-scale utilisation of coalash are described in Section 2.2. This is by no meanscomplete and the interested reader is referred to the earlierstudy cited above.
12
Ash markets
Table 1 Ash utilised in various countries
Countries Fly ash, Coarse ash, Total ash, Ut'l, % Year Notes
kt kt kt kt
Australia 7,351 824 8,175 875 II 1992 Manz (1995a)
Austria 480 26 506 390* 77 199\ Neumann (1994)* includes 120 ktly used + 270 ktlyfor filling old mines and storage
Belgium 776 1\6 892 566 63 \992 Manz (1995a)
Canada 7,309 1,155 8,464 982 12 \992 Manz (l995a)China 80,641 10,498 91,139 34,100 37 1992 Manz (1995a)Colombia 924 126 1,050 590 56 1992 Manz (I 995a)
Czech Republic 7,063 2,371 9,434 1,455 1\ 1993 Manz (1995a)
Denmark 1,043 133 1,176 920 78 1992 Manz (I 995a)
Finland 490 90 580 230 40 1993 Manz (1995a)
France 1,436 287 1,723 1,636 95 \993 Manz (1995a)
Germany 14,300 5.740 20,040 19,840 99 1992 Manz (I 995a)
Greece 7,000 630* 7,630 6,800 89 1992 Manz (I 995a)* estimated from previous data
Hong Kong 841 71 912 553 61 1992 Manz (1995a)
Italy 648 90 738 587 80 1994 ENEL (1995)
India 35,000 3,889* 38,889 778 2 1992 Manz (1995a)* estimated from previous data
Ireland 200 22 222 80 36 1992 Manz (1995a)
Israel 462 61 523 479 92 1992 Manz (I 995a)
Japan 5,264 774 6.402* 3,955 62# 1993 CCUJ (1995),IAE (1995)* includes 402 kt FBC ash.# 100% if landfill is included
Korea (South) 1,868 3,701 5,569 5,217 94 1992 Manz (I 995a)
Malaysia 135 92 227 45 20 1992 Manz (1995a)
Mexico 1,700 300 2,000 30 2 1992 Manz (1995a)
Netherlands 841 74 915 941 103* 1994 Vliegasunie (1995)* includes 700 t imported fly ash
Poland 14,010 NA 11.995 86 1992 Manz (l995a)
Portugal 335 39 374 335 90 1992 Manz (I 995a)
Russia 56,513 5,487 62,000 21,078 34 1992 Manz (1995a)
Slovak Republic 2,250 250 2,500 250 10 1992 Manz (1995a)
Slovenia 958 126 1,084 131 12 1992 Manz (I 995a)
South Africa 24.000 2000* 26,000 4600# 18 1995 Kruger (1996)* mostly from coal gasification;# includes 3000 t backfill
Spain 7140 1319 8459 6112 72 \994 UNESA (1995)
Sweden 50 50 100 15 15 1992 Manz (1995a)
Taiwan 1,206 299 \,505 882 59 1994 Manz (I 995a)Thailand 2,000 400 2,400 0 0 1992 Manz (l995a)
Turkey 12,426 1,440 13,866 3,459 25 1992 Manz (I 995a)
UK 6,830* 1,613* 8,443 4,041 48 1994 National Power (1995),PowerGen (1995)* estimated
USA 49,780 16.897 66,677 19,183 29 1994 ACAA (l995a)
AustriaAbout two thirds of the electricity demand in Austria is metby hydroelectric power. Hence the quantity of fly ash andcoarse ash from coal-fired power stations is less to managethan in some other European countries. It amounted to about500 kt/y in 1991 which is regarded as more normal than inthe subsequent years when much less was produced. In 1992,increased use of hydropower resulted in scarcely more thanhalf the total pulverised coal ash being produced comparedwith 1991 (Neumann, 1994). Hard coal fly ash comprisesabout one third of the total in 1991 and is of a reliable
specification as it comes mostly from high quality Polishcoals. This is almost entirely used through long-termcontracts with the cement industry and a little for concrete.Neumann (1994) considers that an increase in fly ash fromgreater use of imported coal to meet increased electricitydemand in Austria would not present a problem with respectto the markets for ash utilisation.
Part of the brown coal fly ash is used as FLUAL (about70 kt/y). This is temporarily stored wet, to deactivate theunburnt carbon, then dried and finely ground for use in
13
Further information on marketing strategies WaS notavailable. This is due to the need for confidentiality in theintensified international competition for trade in fly ash(Poulsen, 1995).
after delivery of the fly ash. ELSAM sells the rest of the ashto other industries or dumps it. These users have theassurance that they are receiving only fly ash from firingbituminous coal. Hence the customer undertakes the mainanalysis of the fly ash purchased from ELSAM (Poulsen,1996).
GermanyThe amount of hard and brown coal ash produced in 1990 isestimated by VGB (1992) at 11.3 Mt for the statescomprising the Federal Republic of Germany prior to 1989.The brown coal ash produced in the former GermanDemocratic Republic (GDR) deceased from 14 Mt/y in 1989to II Mt/y in 1990 and to 8 Mt/y in 1991, with a small butincreasing addition of hard coal ash (50 kt/y in 1991). Noreliable data were available for the proportion of coal ashused in the GDR although large quantities of brown coal ashwere used in fill for mines, in recultivation of surface minedland and as an additive in cement. In the western states, theproportion of coal ash used in 1990 amounted to 91 % of hardcoal and 100% for brown coal ash. Nearly 6 Mt of hard coalash was used primarily in the following sectors in 1990:
Ash markets
concrete, for example in dams. Around 50 kt/y is used in thecement industry and about 5 kt/y in SAM, a flowable fill.The major proportion (about 270 kt/y) is used for filling oldmines or stored for future use (Neumann, 1994).
Hard coal fly ash for the concrete industry and FLUAL aresold for Sch 25~450/t and fly ash at Sch 4~70/t for thecement industry.
BelgiumMost of the coal ash used in 1992 went to the cementindustry (500 kt), the remainder mainly as filler for roadmaterials and in bricks or ceramics (Manz, 1995a).
The Recybel company was created in 1993 by Electrabel andthe cement industry with the aim of maximising use of flyash and minimising disposal. The cement industry increasedits use of fly ash from 738 kt/y in 1993 to 832 kt/y in 1994,or about 107% of the 1994 production (Electrabel, 1995).
CanadaThe 982 kt of coal ash used in 1992 were mainly deployed inthe cement industry (933 kt), with 10 kt for aerated blocks,21 kt as structural fill and 18 kt in other uses (Manz, 1995a).
Some of the major applications of fly ash are in theconstruction industry, as a cement in the petroleum industry,for stabilisation of wastewater and to some extent as afertiliser. Joshi and Achari (1992) list some constraints in theproductive use of fly ash:
Fly ashkt/y %
Coarse ashkt/y %
VGB (1992) considered that the above utilisation sectorswould be able to expand to maintain the 1991 utilisation ratesand include an estimated increase of 0.5 Mt/y of fly ash by1995. Most of the 5.3 Mt of brown coal ashes were used tofill old surface mines and in recultivation of the land. Otheruses would be explored for the future. Manz (1995a) gives99% coal ash utilisation for Germany in 1992 (see Table 1).This is composed of 60% as a fill in mines, 16% as otherstructural fill, 12% for cement and grout, 7% in non-aeratedblocks and around 5% in other applications.
proximity to the market (transport costs need to be lessthan disposal costs);ash quality (pozzolanic ashes find more outlets);limits as a cement replacement (up to 20% generally butmay increase to 50% with superplasticisers);environmental concerns;acceptance by architects and engineers;lack of collection and storage facilities at the site.
Joshi and Achari (1992) include an example of favourableeconomics for use of fly ash in Calgary. Fly ash is sold at$50/t. The price of cement is $130/t. At about 40%replacement of cement by fly ash for moderate strengthstructures, the saving for using fly ash is around $32/t ofcement.
Cement, concrete and mortarStructural fill (roads, mines)Building materialsTotal used
2140 61810 23110 3
3060 87
5101740640
2890
1757219S
DenmarkMost of the coal ash used in Denmark in 1992 went to thecement and concrete industry or to structural, land orembankment fill according to the survey by Manz (1995a).Poulsen (1995) confirms these data. giving a fly ashproduction of 0.8-1.5 Mt/y in recent years. This may varyconsiderably from year to year due to variations in theelectricity exchange with neighbouring countries andvariations in the ash content of the coal. Applications arefound on both the home and export markets.
The sale of fly ash to the cement and concrete industries aswell as some industrial customers is undertaken byDANASKE. This organisation carries out an extensiveanalysis of the chemical composition and other properties
14
Obstacles to the use of pulverised coal ash were viewed byVGB (1992) as being mainly in their acceptability,competition from traditional building materials and otherbyproducts as well as the seasonal demand being highest insummer while the supply is highest in winter. The latterpresents a storage problem which can only be solved bydeveloping planning procedures with the relevant localauthorities. Transport, preparation and storage costs have aconsiderable influence on the marketability of coal ash. Onaverage ash is transported 5~150 km. The acceptability ofhard coal ash has been considerably increased by technicalregulations and specifications. The use of brown coal ash hasprimarily been as a mixture with FGD gypsum in the miningsector but further uses are being sought, including thosealready developed in the former GDR (VGB, 1992).
Ash markets
application. Manz (1995a) gives a total use of coal ash in thecement industry of 1,025 kt in 1989 but only 242 kt in 1992.According to ENEL (1995), hard coal ash was used in thefollowing sectors in 1994:
The Residue Treatment and Valorisation Research Centrewas founded by ENEL to identify applications for coalresidues, which do not pose environmental risks, are coveredby legal norms and may be controlled by monitoring. Itoffers external services giving advice on such matters as howto optimise utilisation, undertakes characterisation of theresidues and the site for use, quality control and monitoringsurface and groundwater.
Bottom ashkt %
The amount of brown coal ash produced by VEAG powerstations in 1995 was around 4 MtJy and is estimated todecrease to around 3 MtJy at the end of the century (Reckerand Kahl, 1995). Applications for brown coal ash producedby the VEAG power stations are also described by Zabel(1994). The total brown coal ash used in 1988 amounted toonly 3.9 ktJy. The main sector was for concrete and structuralmaterials (2.5 ktJy). About 1 ktJy was used in roadconstruction and 0.5 ktJy in dams.
Regulations for the use of brown coal ash, similar to thosefor hard coal ash, would improve their marketability (Zabel,1994). According to Recker and Kahl (1995), an applicationto permit use of brown coal ash from the Janschwalde powerstation in concrete according to DIN 1045 has been made.This ash is consistent in its chemical characteristics andsimilar to hard coal ash. A certified brown coal ash would bemuch easier to market. The power station would install aquality control system giving the building industryguaranteed characteristics. Road base construction is animportant and expanding sector for brown coal ash in futurein Eastern Germany.
ConcreteCementStructural fill and building
materialsTotal used
Fly ashkt %
313 48220 34
20 3543 85
34.2
34.2
38
38
IndiaChandramouli and others (1995) estimate that a consumptionof about 160 MtJy of coal for power generation in Indiaresults in about 50 MtJy of fly ash. Coal consumption forpower stations is projected to increase to 260--360 MtJy inthe year 2010 (Daniel, 1995) with a corresponding increasein ash production. Complete data on the utilisation of coalash in India do not appear to be available. Trehan and Mittal(1995) report that the National Thermal Power Corporationhas promoted use of coal ash since 1991-92. Fly ashutilisation increased from 0.32 Mt then to 1.5 Mt in 1993-94.Applications include landfill, structural fill, dykes, brickmanufacture and road construction.
There is potential for massive use of fly ash as cementreplacement in buildings and as underground fill while roadsand embankments could use 5 MtJy and save otherconstruction materials. By using fly ash to construct ashponds, there is a potential saving of 50% of the landrequirements. Reclamation of ash ponds can make large areasof land available for human settlement (Kumar, 1994).
Constraints in the utilisation of fly ash in India are listed byChandramouli and others (1995):
users prefer traditional building materials like clay bricksand natural aggregates which are also very cheap;most power stations have wet ash handling systems butcommercial-scale fly ash utilisation requires a drycollection system;many power stations are located near coal mines at greatdistances from populated areas;initial investments for using fly ash are relatively high andare not considered as an integral part of the power station;lack of promotion of fly ash products.
ItalyIt appears that use of coal ash has grown in recent yearsfollowing a decline due to legislation to control its
JapanCoal ash production is estimated to increase from a total of6.4 Mt in 1993 (see Table I) to over 10 MtJy in the year2000. This increase is due mainly to growth in the electricpower industry where production of coal ash is predicted toincrease by a factor of 1.8 times the 1993 output (CCUJ,1995; Nagumo and Tsukuda, 1995). The Institute of AppliedEnergy (IAE) is exploring effective utilisation of coal ash ina study due for completion in March 1996. An analysis forFY 1993 indicates that 56% of the 4.4 Mt of coal ashproduced by utilities and 74% of the 2 Mt produced in theindustrial sector were put to effective use. These data includecoal ash from FBC boilers which are used increasingly in theindustrial sector. The rest of the coal ash is disposed of aslandfill for onshore and offshore reclamation which woulddoubtless be treated as a utilisation in some countries.
others 5%
agriculturaland fishery
2%
premixed concrete
1.9 Mt was used additionally in landfill
Figure 2 Distribution of coal ash in the Electric PowerIndustry, Japan (1993FY) (CCUJ, 1995)
15
Ash markets
The cement industry is by far the largest user of coal ashfrom both utilities and general industry, amounting to 2.8 Mtfor both sectors in 1992 (Nagumo and Tsukuda, 1995). Theypredict that the markets for effective use of coal ash mayexpand as follows:
Since the largest increase is anticipated in the electric powergenerating industry, the distribution of utility coal ash use inall sectors is highlighted in Figure 2. Effective utilisation ofcoal ash in the electric power industry increased from about1.5 Mt (41 %) in 1990 to 2.5 Mt (56%) in 1993 (CCUJ, 1995).This was mainly due to growth in the use of coal ash as araw material for cement (IAE, 1995). Investigations are beingundertaken to see how this trend could continue in future.
Even with this growth in utilisation of coal ash, about 46% ofthe 10.1 Mt coal ash anticipated for the year 2000 would beused effectively. This proportion would increase to 54% ofthe 10.9 Mt likely to be produced in the year 2010. Thusabout half the production of coal ash would need to be putinto landfill. Hence there is a continued need to explore newapplications suitable for bulk use of coal ash. Fielddemonstrations for large-scale utilisation of coal ash inharbour and airport construction are being surveyed by theCCUJ (1995).
Cement industryBuilding and constructionAgriculture and fisheriesOthersTotal
FY 2000ktJy
3300870190310
4670
FY 2010ktJy
45001010
190130
5830
Markets for coal ash in the Netherlands are developed by theDutch Fly Ash Corporation (Vliegasunie). This organisationwas set up in 1982 by the Dutch coal-fired electricityproduction companies. The Corporation sells 100% of the flyash produced in the Netherlands and has even reduced stockpiles of fly ash from previous years. Vliegasunie bvintegrated activities with the Dutch coal supply companyGemeenschappelijk Kolenbureau Elektriciteitsproduktiebedrijven (GKE) in 1994. It has reached aco-operative agreement with the large trading company inconstruction materials, Cementbouw, which will buy allconstruction materials not already sold to existing customers.
SpainIn 1994, the total production of fly ash and bottom ash frompulverised coal fired power stations amounted to 8.5 Mt,72% being used (see Table 1). Nearly half is used as landfill.The utilisation sectors in 1994 were distributed as follows:
Fly ash Bottom ash
kt % kt %
Cement and concrete 1898 26.6 38 2.9
Secondary products and others 4 0.1 40 3.0
Structural fill and pavementbase 93 1.3 68 5.2
Landfill 3336 46.7 638 48.4
Total 5331 74.7 784 59.4
Compared with 1993 data, there was an increase of 600 ktused for specific applications, excluding landfill. This wasmainly due to marketing efforts by electric utilities and to anincreased value accorded to coal ash by some cementmanufacturers.
Several problems in the utilisation of coal ash were outlinedas a result of the questionnaire sent to industries by the IAE(1995). These included: assurance of supply of coal ash tousers, its quality, costs of transport and of use. Suggestionsfor increasing the use of coal ash related to improvinginformation about its utilisation, deregulating legislation topromote effective utilisation, as well as several technologicaldevelopments.
The NetherlandsSince 1991, all fly ash produced by coal-fired power stationshas been sold and in recent years it has been imported too(Vliegasunie, 1995). About 906 kt of wet and dry fly ash(868 kt dry) was sold in 1994 to the following sectors:
kt %
Cement industry 542 60Artificial gravel 203 22Asphalt fillers 85 9Concrete fillers 51 6Other 25 3Total 906 100
Bottom ash was sold as a light foundation material for roadconstruction (73% of sales) and to the concrete blockmanufacturing industry (27%).
16
Reasons for the relatively low utilisation rates were:
long distances from power station to user;a 'prisoner' market by some cement manufacturers whodo not actually used all the ash they buy and prevent thedevelopment of other markets such as concretemanufacture;significantly lower prices for fly ash than in otherEuropean countries with higher utilisation rates;less intense trade in building materials in Southerncompared with Central Europe.
Quality does not seem to be an obstacle to marketingpulverised coal ash in Spain, for example a small-mediumcapacity lignite fired power station achieved a utilisation rateof nearly 100% over the past few years due to the proximityof the user plant to the power station. The development ofnational/regional environmental policies could encourage agreater use of coal ash in Spain even though there is alreadya regulatory framework on residue management (UNESA,1995).
TaiwanThe Taiwan Power Company used half of the 1.53 Mt of coalash produced in 1992, mostly (91.2%) in concreteproduction. The rest could be used in place of conventionalconcrete for constructing artificial reefs. However, public
Ash markets
opinion is resisting this because of possible environmentaleffects. Public education and dissemination of informationare needed to overcome this obstacle in future (Kuo andothers, 1995).
UKThe total coal ash produced in the UK amounted to 15.5 Mtin 1989 when 35.3% was used (Manz, I995a). The utilisationsectors were as follows:
techniques and several projects being adopted by powerproducers to maintain existing markets and to develop newbusiness opportunities. Technology development anddemonstration are important but there is also a need toestablish separate business enterprises to reuse fly ash and tomarket the technology. A key factor here is ashcharacterisation followed by targeting of different grades forappropriate markets.
The coal-fired power stations run by the utility PowerGensold 35 % of the fly ash produced in 1994. All the bottom ashwas sold and, including some produced in earlier years,amounted to 103% (PowerGen, 1995).
No breakdown of coal ash into fly ash and bottom ash ispublished by National Power (1995). In FY 1994-95,49% ofthe total coal ash produced was sold, mainly to the constructionindustry. The rest was disposed of in licensed sites.
kt/y %
Cement, mortars, concrete 1120 7.2Secondary products 2262 14.6Structural and landfill 890 5.8Other 1195 7.7Total 5467 35.3
By comparison the amount of coal ash produced in 1994 haddecreased considerably (see Table 1) but the quantity solddecreased to a lesser extent. It is difficult to obtain an overallpicture of ash markets in the UK because marketingstrategies are developed at each power station - incompetition with each other. Hence no data are available onthe applications in which coal ash is being deployed.
A major constraint on the profitable sale and use of coal ashis the distance to the site of use and the resulting transportcosts. A contract with an ash broker is one way to avoid suchdirect costs according to Rittenhouse (1995) who also refersto a project described by Hobson and Hammons (1995). Inthe example, an extension to a church in North Carolinarequired low cost landfill material. Coal combustionby-products from a nearby power station settling pond arebeing used for the landfill. This has the benefit of addingsix years to the life of the existing ash pond, at aconsiderable saving of more expensive disposal costs in anew facility.
There are barriers to coal ash markets because of regulations.Prior to 1993, most states regulated coal combustionby-products under hazardous or solid waste programmes andrequired a permit for disposal. Then the US EnvironmentalProtection Agency (EPA) decided to place them underSubtitle D as solid wastes under the jurisdiction of individualstates. Existing regulations are not expected to change much,owing to the lack of Federal guidance in the past. Hence coalash will continue to be mistrusted even where appropriateapplications are identified. The potential liability laws whichmay incur high costs to all parties are an added barrier toboth coal ash producers and users in the USA (Eylands,1995; Jagiella, 1994). Details on legislation for coal-useresidues are given by Jagiella for the USA and reviewed ingeneral by Clarke (1994). A summary of state solid wasteregulations governing the use of coal combustion by-productshas been published by the American Coal Ash Association(ACAA, 1995b).
Public acceptance may also be a barrier to wider use of coalash. For example, utilisation of fly ash in artificial reefs isviewed critically in the USA which has a relatively smallpopulation density in a large area where agriculture providesa surplus of produce and there is room for solid wastedisposal. The ocean is often considered untouchable. Afurther objection is due to the concept of zero discharge fromanything placed in a water body. In practice, however, thismay not permit a reasonable distinction between the amountof contamination from natural processes or from otherstructures.
II43
II29
Mt %
7.51.82.37.6
19.2
Cements and concreteRoadbase and asphalt fillerStructural, land and mine fillOtherTotal
USAThe production of pulverised coal ash in the USA decreasedfrom about 68 Mt in 1990 to 66 Mt in 1992 and thenincreased to 67 Mt in 1994 (see Table I). However theproportion used increased from 24% in 1990 to 29% in 1994(ACAA, 1995a; Manz, 1995a; Michalski and Glogowski,1993). The main utilisation sector was cement and concrete,the relative proportions of coal ash used in each sector in1994 being:
Despite the large amounts of coal ash used in the USA, thereis considerable scope for greater utilisation. Michalski andGlogowski (1993) concluded that restrictions on disposal andscarcity of land are likely to increase the cost of disposalgiving strong incentives towards by-product utilisation. Infuture US utilities will have to optimise the economics ofby-product managem..'~T1t through tailored disposal, storageand utilisation plans. Makansi (1994) describes the
2.1.2 Overcoming market obstacles
The previous section included some reference to thedifficulties encountered in specific countries whenestablishing markets for pulverised coal ash. Closeco-operation between companies promoting use of coal ash,COil] <lIppliers and the building industries further theestablishment of contracts for reliable markets. 'Prisoner'markets can result in areas where a large cement
17
Ash markets
manufacturer purchases most or all of the fly ash available.Sometimes in such markets, the total amount of fly ashpurchased by the cement manufacturer is not completely usedor even withdrawn from the plant, leading, in the absence ofpenalties, to permanent disposal of 'sold' ashes in landfill(UNESA, 1995).
Ash sales will increase if they are marketed properly and ifsupplies are managed professionally. Plant-by-plantbudgeting for ash marketing has been found to be verydifficult (Nerison, 1995). Rather 'system-wide' budgeting iseasier. Here many plants can contribute to a common ashmanagement group. The ash management group can thenmarket the ash from plants which have the greatest stockpileash or the ash most suitable for the needs of the externalcustomers. For example, Nerison (1995) describes ashmanagement by the Duke Power Company in Charlotte, NC,USA. Here the eight coal-fired power stations are charged afixed amount according to each tonne of ash generated andthis goes into a common reserve for the ash managementgroup. The larger plants, with dry collection systems, havecontinuous marketing for several applications. Other plants,with sluicing systems with ponds, take it in turns to supplyash for structural fill, the plants with the least remaining pondlife being marketed first. Nerison highlighted a potentialproblem that may arise with the formation of ashmanagement groups. Such groups, or companies, havedifferent viewpoints from the utilities on ash marketing. Theutilities wish to sell all the ash, whereas the marketingcompany wishes to be able to ensure a constant supply of ashto customers without risking running out.
In the Netherlands, the integration between Vliegasunie bvand the Dutch coal suppliers, GKE, (see Section 2.1.1) meansthat the supply of coal to power stations can be co-ordinatedwith the requirements of the power stations as well as thoseof the construction materials market. A constructioncompany, Vulstof Combinatie Nederland BV (VCN, asubsidiary of Cementbouw BV) has agreed to purchase allthe ash which has not already been purchased and will alsotake over Vliegasunie's long-term contracts. Thisarrangement has the advantage that the construction companyhas greater scope for reallocating ash supplies and developingnew markets (Vliegasunie, 1995).
Vliegasunie has established ISRA, an Information System forthe Marketing of Coal Residues. The system allows theorganisation of quantities and qualities of constructionmaterials. ISRA is linked as closely as possible with thepower stations and takes advantages of links with theweighbridge computers and the Laboratory InformationManagement System (LIMS). Using the information fromcoal analysis and taking into account the unit and burner inwhich the coal is to be fired, Vliegasunie can predict thecomposition and quality of the ash that will be produced andtherefore the possible applications and markets for the fly ashmay be determined before it is produced (van den Berg,1995).
The Energy and Environmental Research Center (EERC),ND, USA, has developed a model named ATRAN (ashtransformations) which uses advanced analytical
18
characterisation data of coal to predict particle size andcomposition distribution of the resulting ash. The Coal AshResources Research Consortium (CARRC), also at theEERC, has developed a Coal Ash Properties Database(CAPD). Over 800 samples have already been analysed andthe information incorporated into the CAPD. The CAPDcontains information on chemical, mineralogical and physicalcharacteristics as well as descriptive informatiun on theorigin of the ash. This allows correlations to be madebetween fuel properties and operating conditions. Acombination of databases and models has formed thecomputer program entitled Coal Ash Advisor (CAA). Aprogram such as this could be used for predictions such asthe compressive strength value of controlled low-strengthmaterial or the strength measurement value of a concretebrick, with given mix designs. The program will alsoeventually contain actual and recommended requirements forutilisation or disposal of ash, including economicinformation. The resulting output would be a list of potentialuses and or disposal options for the fly ash (O'Leary andothers, 1995).
2.2 ApplicationsThe utilisation of pulverised coal ash has been grouped inseveral different ways, none of which are completelysatisfactory. Manz (1995a) lists individual uses separately,whereas CCUJ (1995) and Clarke (1992) combine these usesin a few main groups. For example, the civil engineeringgroup includes such varied subdivisions as artificialaggregate, asphalt filler, backfill and embankment materials.Fly ash may be used as fill in a wide variety of applicationsand these comprise the largest sectors in Figure I. However,these are confusing because much of this use is regarded asdisposal in some countries listed in Table I. This report,giving a brief overview of recent information, describesapplications of engineered or structural fill and fillers,followed by more specialised uses such as cement andconcrete, secondary products (gravel, bricks and so on), inpollution control, in agriculture and fisheries and formaterials recovery. This section gives an introduction to theuse of fly ash in different applications and examples of wheresuch uses have been particularly successful.
2.2.1 Engineered fill and fillers
Engineered fills are described by Brendel (1995) as thoseapplications where coal ash is used as a substitute for soil toconstruct embankments, dykes or as general site fills. In suchapplications the fills are usually constructed in thin layers andcompacted to produce relatively incompressible support. Flyash has several advantages as a fill material, principally itslow unit weight and its ready availability in many areas.Structural fill and backfill applications require largequantities of raw materials such as fly ash. Fly ash also hasseveral physical and engineering properties which areadvantageous in construction, including:
a relatively high shear strength;relative ease of handling and compaction;for some sources, the ability to adjust the moisturecontent of the material easily at its source.
Ash markets
The disadvantages of fly ash as structural fill include itsfine-grained, non-cohesive nature which make it subject toerosion and dusting.
Compaction characteristics of fly ash from a single sourceshow little variance although stock-piled or lagooned fly ashmay have lower dry densitites because of their content ofcoarser material (PowerGen, 1995). Fly ash has a lowermaximum dry density (1.38 g/cm3) than clay(1.55-1.84 g/cm3) or sand (1.94 g/cm3). The optimummoisture content for fly ash is around 18% compared with28% for heavy clay, 14% for sandy clay and II % for sand(PowerGen, 1995). Fly ash from brown coal combustion canalso be used in construction. Brown coal fly ash hasself-hardening properties, compensates for the dry shrinkageencountered with some cements and can be used to createhighly dense structures. However, brown coal fly ash is proneto multiple hydration reactions and has longer setting timeswhich can create problems when it is used as a flowable fill(Zabel, 1994). Brendel (1995) notes that bottom ash, being acoarser material than fly ash, has many of the advantages andfewer of the disadvantages exhibited by fly ash, but isavailable in much smaller quantities.
Fly ash can be used in the construction of road embankmentsand as bulk fill material for land reclamation projects. The flyash is mixed with a predetermined amount of water and isthen compacted to a certain dry density to meet theengineering requirements. In the field, fly ash is easilycompacted by machinery such as vibratory rollers. Such useof fly ash is demonstrated in the fly ash embankment nearSamia, Ontario, and in the Canadian Brick Quarry landreclamation project (Chan and Carmichael, 1992). In India,3,700,000 m3 of ash from the stockpile of the NationalThermal Power Corporation Ltd was used to raise 13 hectaresof low lying land. Good compactibitility was achieved with a
vibratory roller, and a satisfactory bearing ratio was obtained.Rs 125 million was saved on the cost of purchasing soil forthis project. In addition, the saving in ash storage space wasRs 60 million (Trehan and Mittal, 1995). In the Netherlands,bottom ash is used almost exclusively as fill in lightembankment material or road base material (Lamers and vanden Berg, 1995).
Fly ash has been used for many years as a soil stabiliser toimprove the perfomlance of highway subgrade soils.Hydrated fly ash is fly ash which has been stored in pits andhas become wetted or partly hydrated long before its use. Thehydrated fly ash reduces the plasticity of clay soils andimproves the strength, upgrading the soil to a moreacceptable subgrade or fill material. The use of lime inconjunction with the fly ash dramatically increases thestrength possibilities (Pandey and others, 1994).
The lower permeability of fly ash, as compared to other fillmaterials, prevents the leaching of soluble material from thecompacted mass. For example, concrete structures adjacent tofly ash fills have not incured sulphate attack, despitecontinuous exposure over many years. However, a layer oftar, bitumen or polythene sheeting is normally used betweenconcrete and fly ash fill to prevent loss of moisture duringcuring of the concrete. Fly ash fill can be eroded by surfacewater. Intercepting drains and covering side slopes and otherprecautions are normally taken during construction and in thecompleted works. Shallow fills can become saturated andunstable due to water rising by capillary action even aftercompaction. A drainage layer of coarse material placedbefore the fly ash eliminates this problem. Some fly ashesmay be frost susceptible and should be kept at least 450 mmbelow the finished surface in susceptible locations(PowerGen, 1995).
H---------.. Effective use of bottom ash
---l.._-+- ~ Effective use of bottom ash
L- +- -+ Used in the Hokkaido area, Japan
H,Y ash as filler of asphaltadmixtures
Substructure
Surface layer
Road bed(about 1 m)
Subbasecourse
Tests are currently beingconducted on the applicability
~-l------------.I of fly ash stabilised with cementfor subbase courses
Uppersubbasecourse
This portion iscalled 'pavement'
Figure 3 Structure of asphalt paving and coal ash application (Kamada and Tezuka, 1995)
19
Ash markets
Fly ash and bottom ash are used for road construction inseveral ways, in the road bed and subbase courses as fill andthe fly ash is also substituted for the limestone powder fillerin the asphalt. For example, Figure 3 shows the structure ofan asphalt pavement and indicates the areas where coal flyash has been used (Kamada and Tezuka, 1995).
Fly ash may be consolidated with a relatively small amountof cement and water prior to use as an engineered fillmaterial. The material is pelletised and cured. This producesaggregates which may be used in place of sand and stone.The production of aggregates is discussed in greater detail inSection 2.2.3. The pellets may be crushed and then mixedwith water and a binder to provide base course material andbackfill material. Consolidated fly ash is lighter than soil andstone, has sufficient strength for applications such as bankingand for use as base course, and has a permeabilitycomparable to that of sand (Kondo and others, 1996).
The flowability and pumpability of engineered slurries can beenhanced by fly ash because of the large number of sphericalparticles in the ash. Fly ash can be readily pumpedunderwater as well as in open space. Various cementitiousmaterials such as cement, lime and lime kiln dust, have beenmixed with a Class F fly ash and water to produce flowablefill. The strengths obtained vary from 500 kPa to 1000 kPadepending on the mix design (Chan and Carmichael, 1992).
In the USA, controlled low strength materials (CLSM), orflowable backfills, are composed of fly ash, cement andwater. Coarse and/or fine aggregates and admixtures mayalso be included. The primary characteristic of a CLSM isthat it has a 28-day unconfined compressive strength of lessthan 1200 psi (8.28 MPa). This makes CLSM highly suitablefor use as backfills for bridge abutments. The CLSM are easyto place and eliminate settlement which normally occurs withgranular abutment backfills. They also have the advantagethat there is a reduction in lateral pressures on the abutmentwalls. Newman and others (1995) give several examples ofthe successful use of fly ash in CLSM in bridges inPennsylvania.
Low strength mortars are required for some applications, forexample Ambroise and others (1995) describe flowablemortars which need to be pumped over long distances andremain workable for up to 24 h. Such mortars are used asbackfill for excavation projects and fill for abandonedunderground facilities. The final set after 3 days needs to beabout 0.157 MPa (similar to the surrounding soil) and along-term unconfined compressive strength of 1 MPa isrequired after 28 days. A mortar was prepared with360 kg/m3 Class F fly ash, n kg/m3 blast furnace slagcement, 1120 kg/m3 sand and 430 11m3 water.Superplasticisers and a biopolymer were added to reducefluid loss, to give cohesiveness and improve the pumpabilityof the mortar. Ambroise and others tested the properties ofthe mortar with different cements and superplasticisers andconcluded that coal fly ash has a high potential as a majorcementitious component of backfill sanded grout.
A quick setting grout containing fly ash is being used inJapan including reinforcement of cracks in bed rock and dam
20
joints and backfill in tunnels. The mix proportion is varied inthe ranges from 1: 1 to 1:20 cementfly ash. The fly ashquicksetting (FQS) grout mainly developed by EPDC iscomposed of two liquids which are mixed near to theinjection hole. Liquid A contains about 100-210 kgim3
cement, 400---600 kg/m3 fly ash and 400---500 kg/m3 mudwater and the other liquid is 40---70 kg/m3 water glass wi thfresh water. The use of coal ash in the grout was calculatedto reduce material costs provided that the fly ash cost lessthan Y8000/t (the unit price was 7.48-8.62 ). FQS grout hasnow been used at several sites in Japan (CCUJ, 1994).
According to Yasuhara and Horiuchi (1996), foamed mortar,a lightweight fill could be a suitable application for fly ash.Foamed mortar made with concrete is often too strong and issubject to temperature increases caused by cement hydration.The high pozzolanic activity of some fly ashes and the lowspecific gravity could make them suitable as a partialsubstitute for cement in foamed mortar. The effect of fly ashreplacement on other properties of the material necessary forconstruction design, such as strength development, cohesionand creep have not been studied yet.
There are many examples of fly ash slurries used in civilengineering works. These include dams, tunnels, and bridgepier foundations. Fly ash slurries have been used in Japan forthe backfilling of tunnels and in the construction of artificialislands to support the tower substructure of bridges such asthe Hakucho bridge (45,000 m3) and the Meiko HigashiOhashi Bridge (9 kt). Fly ash from fluidised beds has alsobeen used for mud stabilisation for reclaimed ground offshoreof Haneda Airport (Kamada and Tezuka, 1995). Hanzawaand others (1996) report on the use of large quantities of flyash solids and slurries to reclaim land at two thermal powerplant sites. At one site fly ash was dumped from land bybulldozer to fill a depth of 10m. At another site the fly ashwas hydraulically pumped as a slurry to fill a depth of 20m.The fly ash was classified as non-plastic silt and had shearstrength characteristics similar to sandy soil in a loose state.
2.2.2 Cement, concrete and mortar
There are many successful applications of fly ash in cement,concrete and mortar and a few examples are presented here.Following this, the beneficial characteristics of fly ash incements and concretes are discussed. An excellent shortreview of the use of fly ash in concrete and the chemistryinvolved is given by Pratt (1990). The specifications and testmethods which apply are discussed in Chapter 4.
Pulverised coal ash may be used in the construction industryas a raw material for production of cement, replacing the claywhich is not always abundantly available. Experience inJapan suggests that a 10---20% substitution of the clay by coalash would be the limit (CCUJ, 1994).
Fly ash may be added to cement during its manufacture. Thiscan be done either by blending fly ash with cement whichhas already been processed or by intergrinding it with thecement clinker (Joshi and Achari, 1992). A recent example ofthis application is the fly ash product containing less than 3%carbon which was used to replace Portland cement for the
Boston Third Harbor Tunnel project in the USA (Makansi,1994).
Dhir and others (1994) set up six series of concrete mixes,each containing five standard strengths from 25 MPa to70 MPa, with fly ash contents ranging from 0-45% in stepsof 15%. They found that Portland cement/fly ash blendconcrete mixes can be used with up to 45% fly ash for designstrengths of 25-70 MPa at 28 days. This gives engineerssome flexibility in the choice of materials when designing fordifferent applications and exposure environments.
In Japan, a new material with a high content of fly ash hasbeen developed. This' Ashcrete' is a concrete in which all orpart of the aggregate in a plain concrete has been replacedwith fly ash. Ashcrete made with about 54% fly ash and 15%cement mixed with water has a high strength and is stable. Ifan activator (such as NaCI) is used the Ashcrete gains a highinitial strength of 100-400 kg/cm2 (9.8-39 MPa) at 28 days.Ashcrete has been used extensively in Japan for theconstruction of artificial reefs since 1980 (Suzuki, 1995).This is discussed in greater detail in Section 2.2.5.
Spoelstra and others (1992) report on a process developed byNOVEM (Netherlands agency for energy and theenvironment) for the production of ASC - activated slagcement - containing 70% coal fly ash. In theKOREL-process, developed in conjunction with the cementindustry, coal, fly ash and limestone are heated in a fluidisedbed then sent to a glass furnace where a vitrified slag isproduced. This slag is the raw material for activated slagcement which is highly resistant to acid attack. Comparedwith the normal process for the production of Portlandcement, the KOREL-process has a lower consumption of fueland marl (clay, silt and calcium carbonate), lower emissionsof C02 and NOx and can accept many different types of flyash, including those which otherwise could not be used.
Kilgour and others (1994) tested the feasibility of producingconcretes where coal combustion residues replaced most ofthe natural resources. The cement was replaced by fly ashfrom the Lansing power station and the Iowa State UniversityFBC power plant. Spent bed-offtake was used as areplacement for concrete sand. Fly ashes from both powerstations were agglomerated and used as a replacement forlimestone aggregate. The agglomerated fly ash from theLansing plant was found to be a suitable replacement forlimestone aggregate and the resulting concretes hadessentially the same strength as the control concrete. Whenthe Lansing fly ash agglomerate was used in conjunction withthe spent bed-offtake from the Iowa FBC unit (instead ofconcrete sand) the compressive strength was increased by25%. However, when the Lansing fly ash agglomerate wasused in conjunction with Lansing fly ash as the cementingagent the compressive strength decreased by 55%. Lansingfly ash is therefore a less effective cementing agent thanPortland cement. In concrete made entirely from coalcombustion residues (Lansing fly ash agglomerates, spentFBC bed-offtake and Lansing fly ash), deleterious reactionsoccurred causing extreme expansion and loss of strength ofthe concretes. Work is continuing to determine the optimumcombination of natural resources with coal combustion residues.
Ash markets
Technical benefits arising from the incorporation of fly ash incement and concrete, whether in addition to or as a partsubstitution for Portland cement (Mills, 1990), may include:
improved resistance to alkali-aggregate reaction;conversion of calcium hydroxide, the most solubleproduct of Portland cement hydration, to more stablecalcium silicate hydrate;improved water and gas tightness;lower creep and shrinkage;favourable pore size distribution.
Fly ash in cement and concrete can be used in a beneficialmanner to improve engineering properties such asworkability, strength, permeability, expansion and so on. Thefollowing subsections briefly discuss each of theseengineering benefits followed by examples of fly ashapplications.
WorkabilityThe addition of fly ash increases the workability of concretebecause (Pratt, 1990):
the volume of paste is increased since the density of flyash is less than the density of cement;fly ash particles have a non-absorptive glassy surface;the finer particles reduce bleeding by obstructing thebleed channels;fly ash acts as a water-reducing agent by dispersingflocculated cement particles.
The workability of cement and concrete is particularlyimportant where the product is to be poured or injected.Some ashes greatly enhance the flow, for example those fromhigh-temperature wet-bottom furnaces. Conversely, fly ashesfrom low-temperature dry-bottom furnaces can reduce theflow slightly (Pratt, 1990). Engineered fill was discussed inSection 2.2.1.
StrengthThe inclusion of fly ash in cement and concrete may alsoimprove the resulting compressive strength of the product. Anumber of factors influence the development of strength infly ash blends including particle size distribution, glasscontent of fly ash, the mix design and the relative humidityand temperature during curing. Finer particles in concreteimply greater strength. Fly ash must contain a significantproportion of fine particles (<10 /-lm at least) forimprovements in the transition zone microstructure to lead toimprovements in strength (Pratt, 1990). Beneficiation by airclassification and by grinding of the fly ash have been foundeffective in increasing the strength of fly ash blends.However, grinding can reduce the workability of the mortarsbecause of the unfavourable particle shape of the brokenspheres.
Several fly ash samples from some South African powerstations were examined by Cooper and Davies (1994). Theytested the strength gained by the samples when reacted withNaOH solution. The results indicated that factors such asglass content and fineness of fly ash play only a very minorrole in the strength development of the mortars. The only
21
Ash markets
40
Naik and Singh (1994) studied the compressive strength of
factor they could relate to the strength of the samples was theFe203 and Ah03 content of the glass. The Ah03 has apositive correlation with the strength and Fe203 has anegative correlation with the strength of the mortar.
various cement mortars containing fly ash. Within the testedrange of variables, the inclusion of Class F fly ashes caused areduction in compressive strength. Class C fly ashes, on theother hand, produced excellent results at mixes of 20%cement replacement. Higher replacement by Class C fly ashesgave lower compressive strengths. The optimum water tocement ratio varied in the range 0.35-0.6 for all mortar mixestested.
Cement mortars containing cement, sand, water and fly ashwere compared with a reference mortar mix having a cementto sand ratio of 1:2.75 by Naik and Singh (1994). A mortarwith 20% cement replacement by Class C fly ash cementdeveloped a higher compressive strength than the referencemortar with no fly ash (125% at 7 days and 112% at 28days). The mortars containing 20% Class F fly ash attained alower compressive strength than the reference (84% at 7 daysand 85% at 28 days). Those with 40% Class F fly ashreached much lower values of 56% at 7 days and 72% at 28days of the equivalent reference values. The maximumcompressive strengths at 28 days (read from a figure) appearto be about 5300 psi (37 MPa) for the mortar with 20% ClassC fly ash, 4000 psi (28 MPa) for the mortar with 20% ClassF fly ash and 3400 psi (24 MPa) for 40% Class F fly ashcement replacement.
An interesting comparison of fly ash concrete versus ordinaryPortland cement concrete is provided in two 10-year-oldconcrete bridges over the M56 motorway in Cheshire,England (Pratt, 1990). One of the bridges was constructedwith ordinary Portland cement, the other with 25% fly ashfrom the Fiddlers Ferry power station. Compressive strengthsof the concrete in both bridges at 28 days were well matched.However, the bridge built with fly ash has continued to gainin strength. The oxygen permeability and the waterpermeability are also lower, and thus better, in the fly ashbridge. Some signs of alkali aggregate reaction were found inthe ordinary concrete but none in the fly ash concrete.
absorptivity which is mainly due to capillarity - fluid istaken in by the material to fill cavities and voids;permeability, which is a measure of resistance topressure-driven fluid movement;diffusivity, the rate at which fluid or ionic movementoccurs due to a concentration gradient.
PermeabilityThe permeability or permeation of concrete is the ease withwhich gases, liquids or ions can move into or out of concrete(Dhir and others, 1994). Permeability is a factor indetermining the durability of concrete. There are threedistinct basic transport mechanisms, collectively termedpermeation, which can operate in a semi-permeable mediumsuch as concrete:
The use of fly ash in concrete will affect the microstructureand thus may alter the permeation properties. Fly ash tends toreduce permeation. The effect is more marked with age forwater-cured concrete (Dhir and others, 1994). Concrete formarine environments benefits from the inclusion of fly ashbecause fly ash reduces the permeability of concrete and ithas a high capacity to bind chloride ions (Amtsbiichler, 1994).
4 6 8 10 20 40 60 100 200
Curing time, days
Relative compressive strength of fly ashconcrete specimens (to that of concrete withno fly ash) as a function of curing time (Linand Lin, 1994)
2
30
<fl fly ash content
J:: 200,c~t5Q)
>"w(J) 10Q)
CiE0()
Q)
>~ 0CDIT:
-10
Because of the slow pozzolanic reaction of fly ash, fly ashconcrete in general has a lower compressive strength thanregular concrete within 28 days of the curing period.However, after this time the compressive strength of fly ashconcrete exceeds that of regular concrete by a large margin.Lin and Lin (1994) demonstrate this phenomena for thespecial case of a fly ash from a cogeneration plant for achemical fibre manufacturer. The plant was burning coalsfrom Australia or South Africa, and produced significantlyfiner fly ash than other coal fly ashes of similar composition.Figure 4 shows the increase in compressive strength withcuring time for concretes containing different proportions ofcogeneration plant fly ash. The trend is more pronounced forconcretes with higher fly ash contents. Concrete containing20% cogeneration plant fly ash developed a compressivestrength 29% greater than the concrete with no fly ash and3.3% greater than the other coal fly ash concrete after180 days of curing. This percentage difference would beexpected to increase further with increased curing time. Theincreased compressive strength compared with other fly ashconcrete was attributed to the finer particle size (no valuegiven). The fly ash concrete also had the advantage of alower diffusion coefficient, making it more waterproof.
-20
Figure 4
22
Ash markets
ExpansionCertain types of fly ash may cause expansion when used ascement extenders. Kruger (1994) investigated the cause ofdefects, the relationship between the degree of unsoundnessand the expansion of concrete made with such fly ash, aswell as the relevance of specification limits for soundness.Expansion is due to free lime. Free lime hydrates rapidly andis not a long-term problem when fly ash is used as a cementextender. The free lime forms at lower combustiontemperatures and gives rise to a fly ash which may causeexpansion. At higher temperatures the lime is assimilated intothe glass phase and does not cause expansion later. Kruger(1994) concluded that concrete with binder contents up to450 kg/m3 with the binder containing up to 70% fly ash byvolume, should not expand deleteriously, provided theLe Chatelier expansion of the fly ash does not exceed 10 mmwhen tested in a blend of 60% fly ash and 40% ordinaryPortland cement by volume. ASTM C618 specifies that theautoclave expansion of fly ash should not exceed 0.8% (seeChapter 4). Tricalcium aluminate in the cement participatesin deleterious sulphate expansion reactions (McCarthy andothers, 1990). Ettringite may form and contribute to thedevelopment of expansive properties. It is not considereddesirable in normal Portland cement concrete (Graham andRob\, 1994).
2.2.3 Secondary products
Artificial or synthetic aggregates, gravel, bricks, tiles andzeolites are examples of secondary products. They mostlyreplace conventional building materials.
Lightweight aggregate from coal ash may be used instead of
crushed stone in concrete. Lytag aggregate was developed inthe UK during the late 1950s and has used over 15 Mt of flyash over this time. The technology is licensed in theNetherlands and is being sought for fly ash utilisation inmany parts of the world including Australia, Poland and theUSA. It is used extensively in concrete constructionelements, for example meeting the Dutch standard NEN 3543for coarse aggregates in lightweight concrete. The resultingconcrete mix is 20% lighter than gravel concrete. Lytag ismanufactured by sintering pelletised fly ash at 11 00°c.Ideally the fly ash contains sufficient unburnt carbon (up to8%) for the sintering process or coal is added to the dry ash(Clarke, 1992; Dolby, 1995).
The Aardelite process was developed in the Netherlands (seeFigure 5). Aardelite gravel is incorporated as lightweightaggregate in concrete and concrete products or used on roads.The product has been produced commercially at plants inFlorida, USA since 1988, in the Netherlands since 1993 andsince 1994 in India. Fly ash, lime water and, if necessary,other calcic additives are mixed with or without steaminjection to a homogenous mass with specific properties(details are provided by Mahadew, 1995; van den Heuvel,1995; Voortman, 1995). The mixture is pelletised and thencured for several hours at 80-90oC at 100% relative humidityand atmospheric pressure. The product satisfies relevantASTM specifications for aggregates including ASTM C331for use in masonry units and ASTM C330 for structuralconcrete. The resulting concrete weighs about 30% less thanconcrete made of natural gravel and sand. It is stronger dueto the high absorption capacity of the Aardelite gravel whichproduces a good bond with the cement matrix. The productalso has other advantages such as better workability,
steam
recycleembedding
material
lime fly ash curingsilo
breaker
fractioningsieve
drum sieve
ai.~
roEuOJ
.DEOJ
Aardelite storage
Figure 5 The Aardelite process (van den Heuvel, 1995)
23
Ash markets
improved insulating properties and reduces the demand forexpensive cement.
More than 120 million masonry blocks produced withAardelite gravel have been marketed in Florida and otherstates. During the first year of operation of the plant in theNetherlands, 70 kt was produced and used mainly for blocksmarketed in Belgium and the UK. Wet fly ash from old ashdisposal sites will also be processed in the Netherlands(Mahadew, 1995). Jn India, uses for the aggregate from theTrombay power station include masonry blocks, cementbricks, concrete piles, prefabricated concrete and in thesubbase of roads (Chandramouli and others, 1995). Mahadew(1995) emphasises that the Aardelite process has theimportant advantage of using low quality fly ash which isdifficult to market. This category amounts to about 85% ofthe world production. In pricing the artificial aggregate, it isoften matched with natural gravel on a volume basis.However, the difference in weight of the two materials issuch that the price of the lighter Aardelite aggregate shouldbe 1.6 times the price of natural gravel.
In Japan artificial aggregate has been sold under the tradename FA-LITE since 1984 at about 70 ktJy. A lightweightartificial sand (fire beads) is also being developed by addinga small amount of cement and other constituents to coal ashand pelletising the mixture. Presently about 40% of thedemand for sand is met by using sea sand which has thedisadvantage of being salty and involving fishing rights. Asfuture demand for sand expands, an artificial sand from coalash will be able to displace the use of natural sand (CCUJ,1994). A sintered lightweight aggregate from fly ash andlimestone has also been developed in Japan. It is classified asclass H (specific gravity up to 2.0) and meets thespecification JIS A 5002. Tests to compare coal ashaggregate with crushed stone in concrete showed that the coalash aggregate had lower absorption and higher solid volumepercentage but greater abrasion loss. Only 50% of the watercontent was required to achieve the same slump. Dryingshrinkage was lower. The compressive strength was higherand the resistance to freezing and thawing of concrete usingcoal ash aggregate was equal to that using crushed stone(Sone and others, 1995). In their recent summary, CCUJ(1995) list sintered artificial lightweight aggregate as partiallycommercialised and an artificial sand reinforced with glassfibre which has been developed.
Some examples of lightweight aggregate using coal ash havebeen developed to use other by-products or wastes as well. InWisconsin, USA, a lightweight aggregate product was chosenfor using fly ash which was not suitable for cementreplacement on account of the high and varying carboncontent. The patented MINERGY lightweight aggregateproduction process combines fly ash with dewateredwastewater solids and a small amount of the natural claybinder, bentonite. The mixture is pelletised, heat treated, fedinto a rotary kiln for pyro-processing and then heat hardened.The product is cellular and of a strong construction gradeconforming to ASTM standards for lightweight structuralconcrete (ASTM 330) and concrete masonry units (ASTM331). There is typically a demand for this product throughoutthe year. The process binds any heavy metals present in the
24
constituents in the fused aggregate which is mostly furtherencapsulated in concrete (Nechvatal, 1995).
Research into the potential for utilising Class C fly ashand atmospheric fluidised bed combustion residues isdescribed by Bergeson and Waddingham (1995). Thelightweight aggregate was comparable in strength to a lowquality (class B) crushed limestone and met IowaDepartment of Transportation freeze-thaw specifications.The artificial aggregate could be stabilised with 15-25%Class C fly ash to produce a base or subbase material(ASTM C593). The efflorescence of sodium sulphate fromthis material may preclude its use under Portland cementconcrete but the potential for high-volume applicationappeared high.
Autoclaved aerated concrete blocks or autoclaved cellularconcrete was first introduced during the 1920s in Sweden andhas been used for over 25 years in the UK and China(Golden, 1995). In the UK, this application used over 1 Mt offly ash in 1990 (Clarke, 1992). In 1991, there were 67factories in Western Europe producing about 9,000,000 m3
and 120 factories in Eastern Europe producing10-12,000,000 m3 . In addition there are over 70 plants inAsia. Attempts to introduce the technology in the USA andCanada have been largely unsuccessful due to competitionfrom lower priced construction materials, which are mainlywood as opposed to masonry in Europe. Since then, a testhouse has been built of autoclaved, aerated concrete blocks inthe USA and is being compared (for energy efficiency) witha traditonal wood-framed home (Golden, 1995).
Autoclaved, aerated concrete is produced by mixing Portlandcement, lime, aluminium powder and water with a largeproportion of silica, usually sand. Fly ash may be substitutedfor the sand and may comprise as much as 75% of theconcrete by weight. The product is about one quarter to onefifth of the normal weight of concrete, it is a good thermalinsulator and sound barrier, insect repellant, rot and fireresistant and may be worked using standard carpentry tools.It is used for external and internal load-bearing walls and forcladding (Golden, 1995).
Bricks and tiles in which coal ash has replaced clay havebeen used commercially in Japan since 1982 (CCUJ, 1995).A slip casting process for making lightweight bricks isreported by Dinelli (1995) to have been studied in Japan.Anhydrite is produced by thermal treatment of FGD gypsum,fly ash and lime at 150°C, combined with water and castfollowed by steam curing. The resulting bricks have aspecific gravity of about 1000 kg/m3 and a flexural strengthof about I MPa. An alternative method has been studied byENEL in Italy in which all the large volume solid residuesfrom a coal-fired station, including bottom ash, may be usedin variable proportions according to output. The pressureforming process does not require thermal treatment toproduce anhydrite but uses FGD gypsum as received fromthe plant. This yields a higher strength than that of traditionalceramic bricks produced at about 1000ne. However, the slipcast method appeared more favourable when high strengthwas not required, when better insulating properties weredesirable and also for more complex geometries.
Sand-lime bricks are normally prepared from a mixture ofcalcined lime, sand and water. However, the sand can bereplaced in part by fly ash. In the Netherlands, NOVEM isdemonstrating this application at a sand-lime brick plant, DeHazelaar, Koningsbosch. Limestone is calcined in a fluidisedbed with carbon-containing coal combustion residues. In thisKaldin-process, the fly ash is introduced in a Lurgi-developedcirculating fluid bed calciner. The resulting fly ash-limemixture can be used as a binder for fly ash bricks and also inthe Aardelite process (see above) (Spoelstra and others, 1992).
One disadvantage of coal ash bricks might be their light greycolour for some applications. More decorative coloured ashbricks are being developed in a laboratory-scale study inIndia. In the study, pond ash was cited as containing 60-80%unburnt carbon, 5-10% hydrated lime, 2G-30% sand and0.2% dye were dry blended and then mixed wet. The brickswere pressure moulded and cured in autoclaves at 18G-1850 Cand pressure of IG-14 kg/cm2 (Trehan and Mittal, 1995).
Alkan and others (1995) in Turkey are studying thepossibility of producing a new building material made fromcoal fly ash and polyethylene bags. The tensile strength ofthe material reached a maximum at 20% fly ash. At fly ashconcentrations above this the tensile strength was reduced.
Zeolites are more specialised secondary products. The nameis derived from the Greek, meaning boiling stone. Zeolitesare crystals mainly of Si02 and Ah03 and occur naturally.They are used mainly as water softening agents, soilamendments and as a component in fertilisers. Other usesinclude: adsorbents (desiccants, deodorants, antibacterialagents), catalyst supports and geotextile filters. The estimatedmarket for natural zeolite is about 150 kt/y in Japan. Twotypes of coal ash zeolite are being produced for various usersat a demonstration plant built by Nippon Steel: a powderpriced at about Y40,000/t and pellets at Y70,000lt. If salescontinue to expand, the plant may become commercial(CCUJ, 1994). A marketing standard and applications forartificial zeolites are being surveyed in Japan (CCUJ, 1995).
The production and characteristics of coal ash zeolites aredescribed by Singer and Berkgaut (1995). The plentiful glassin fly ash is a readily available source of Si and Al for zeolitesynthesis. A hydrothermal treatment process enhances thezeolitic properties of fly ash to increase its suitability for useas a polishing treatment for contaminated waters. Treatmentof fly ash for 2-48 hours in 3.5 M NaOH at 100°C formsZeolite P and/or hydroxysodalite from the glassy part of thefly ash. The quartz fraction dissolves in this treatment and themullite fraction remains stable. Around 50% of the fly ashmay be converted to zeolites by this process. The zeolitisedfly ash has increased cation exchange properties and canextract cadmium, caesium, copper, lead, strontium and zincfrom solution at low concentrations. The fly ash wasparticularly effective at removing lead and zinc fromindustrial wastewaters. The treated fly ash could also be usedto improve sludge feasibility as a fertiliser/soil conditioner.Singer and Berkgaut (1995) emphasise that, once used, thezeolitised fly ash has been contaminated by the wastewaterand becomes a hazardous waste, requiring disposalaccordingly.
Ash markets
Lin and Hsi (1995) also reported on the use of fly ash toproduce zeolite-like materials. Four zeolites were identifiedunder different conditions:
zeolite P, 2-4 M NaOH at 7G-130°C;analcime, 2 M NaOH at 13G-170°C;hydroxysodalite, 4-10 M NaOH at 9G-200°C;cancrinite, at over 200°C.
The treated fly ash showed a strong affinity for copper andcadmium. However, the practical application of such zeolitesfor removing metals from wastewaters must be comparedagainst the use of naturally available aluminosilicates (Linand Hsi. 1995).
Alkaline activation of fly ash from the Teruel power stationin Spain is described by Querol and others (1995). Heresodium hydroxide, sodium hydroxide with potassiumhydroxide and potassium hydroxide were used to synthesisevarious zeolites (type NaPI being the most abundant).
2.2.4 Pollution control
Pulverised coal ash may be used to control environmentaldamage from waste materials by stabilising them in anadmixture or by constructing an impermeable liner or coverfor the landfill. Other applications for pollution controlinclude water treatment, use as a sorbent in flue gasdesulphurisation and in the construction of filters.
Stabilisation and/or solidification technology using coalash is discussed by Eylands (1995) and its use in landfillby Kusterer and others (1995). In the USA, fly ash andbottom ash was ruled in August, 1993 by the US EPA asnon-hazardous solid waste to be regulated under Subtitle Dof the Resource Conservation and Recovery Act (RCRA).State regulations for management of coal ashes varywidely but some permit the beneficial use of coal ash inlandfills.
The chemical and physical properties of fly ash (see
Chapter 3) allow its use as a binder for many wastes,including hazardous wastes such as radioactive salts, organicsludges and soils contaminated with heavy metals. The aim isto immobilise undesirable constituents in the wastes byforming insoluble compounds and/or by trapping them in aninert and stable solid. Binders may consist of Class C and Ffly ashes combined with lime, cement, kiln dust, clay,recycled rubber, asphaltene, blast furnace slag and anhydrite.In addition to the variable cementitious and pozzolanicproperties of fly ashes in waste solidification andstabilisation, the porosity is of prime importance. Finer ashesare more desirable because they tend to decrease the porosityof the solidified wastes and develop greater strength. Theleachability of the stabilised waste is a primary concern andcritical interpretation of leaching tests is required, especiallyregarding the pH both of the waste and the test solution andits applicability to the disposal environment (Eylands, 1995).An example of another use for fly ash in waste managementis a recent development in Germany. Oily, sandy wastesfrom garages are classified as special wastes under Disposalclass 5. Tests with fly ash binders aim to stabilise the waste,
25
Ash markets
allowing it to be classified under Disposal class 2 which ismuch less costly (Umwelt, 1995).
The Poz-O-Tee process was developed by ConversionSystems Inc, USA, in the 1970s in response to the need for aprocess to stabilise FGD sludges. The aqueous dischargefrom FGD scrubbers is dewatered and combined with fly ashproduced at the same facility. A small quantity of lime isthen added as the activating agent and the mixture is blended.The dry weight ratio of fly ash to FGD sludge is about 3: I. Ifthe material is to be used as a civil engineering material thenaround 1-3% lime is added according to the required qualityand the moisture content is adjusted to maximise the densityafter compaction. The mixture is self-hardening and can beused in land reclamation projects, on landfill caps and liners,in highway construction, in synthetic aggregate and for aquaculture ponds. A flow diagram of the process is shown inFigure 6 (CCUJ, 1995). By 1991,25 Poz-O-Tec facilitieswere in operation, mainly in the USA, producing 27,000 ktJyof cementitious land fi]]able material (Smith, 1991).
Conventional system
Kusterer and others (1995) investigated the suitability of flyash and bottom ash in the construction and operation of amunicipal solid waste landfill in Maryland, USA. Here flyash is to be deposited in layers between soil and the drainagelayer, made of soil and/or bottom ash, on the landfill baseand perimeter. It is also proposed that fly ash be used asdaily cover material either alone or mixed with soil for dustsuppression. About 645,000 m3 of fly ash could be used withanother 178,000 m3 of bottom ash for the drainage layer. Fillto cover the waste is estimated to require approximately306,000 m3 of fly ash. The nearby 830 MWe coal-firedpower station at Dickerson, MD, produces about 756 tJy ofClass F fly ash. Use of coal ashes would offset more thanhalf the estimated deficit of 1,990,000 m3 of infill materialrequired for the 45 year life of the landfill. Laboratory testsof physical and engineering properties indicated that fly ashcould be used as a structural fill and daily cover materialwhile bottom ash might also have potential use as an internaldrainage layer beneath the fly ash and as a subbase forlandfill access. Kusterer and others pointed out the need to
smoke stack
desuiphurisation
make-up water
wastewatertreatment system
POZ-O-Tee system
treated water
smoke stack
denitrification
make-up water
Figure 6 Comparision of conventional system and Poz-O-Tec system (CCUJ, 1995)
26
monitor ash properties throughout the construction andoperation of the landfill to assure that ash properties (whichvary considerably) remained consistent with the design values.
Other potential uses for coal ash in landfill were described byKusterer and others (1995). These included use of astabiliseI', for example class F fly ash with up to 15% hydratelime or cement to reduce the permeability and immobilisetrace metals. In Austria, brown coal fly ash is being activatedwith carbide lime from acetylene production and used to plugleaks in the ash disposal site of the Voitsberg power station.Impermeable layers are made to prevent leaching (Neumann,1994).
Water treatment using fly ash deploys certain uniqueproperties of fly ash, the most important according to Joshiand Achari (1992) are:
the adsorptive properties;the water extracts of lime and gypsum in some fly asheswhich precipitate inorganic phosphorus and neutraliseacid in polluted waters;as a conditioner of wastewater sludges for dewatering byvacuum filtration.
Inorganic materials may be effectively adsorbed frompolluted water because of the large surface area of fly ash(2000-6000 cm2/g). The adsorptive capacity of fly ash is afunction of the amount of carbon in the fly ash.
Ash markets
modified with various additives such as lime, gypsum,activated earth and charcoal, organic and inorganic salts.Laboratory- and pilot-scale tests showed that the treatmenteliminated the foul odour of the effluent and its colour. Thetotal solids and total dissolved solids could be reduced by anorder of magnitude and the treated water could be re-used forfurther dying operations. Vinod and others recommendedusing the approach on a large-scale plant and extending it toother effluents. The case study also demonstrates thepotential for use of appropriate mixes of fly ash with AFBCresidues containing lime, unspent sorbent and residual carbonor FGD residues.
Fly ash is also used to neutralise acid mine drainage (Clarke,1995b; Joshi and Achari, 1992; Kim and Ackman, 1995).This application deploys the alkaline components such ashydroxides in fly ash. Tests by the US Bureau of Minesinvolved subsurface injection of 192 m3 of grout made of105 m3 of fly ash with lime and for part of the time, 87 m3
of FBC residues. The area treated was a 1.2 ha portion of areclaimed surface mine site in Greene County, PA, USA, andcontinued over a period of eleven months. Application of thegrout increased the pH of the spoil significantly and that ofthe seep slightly (see Figure 7). Total acidity decreased in thegrouted area as did ferrous iron, tota.1 iron, aluminium andsulphate. Concentrations of heavy metals released from thefly ash were not hazardous to surface or groundwater.Continued monitoring will determine whether coalcombustion residues produce a long-term improvement inwater quality.
8.0
Zeolites made from coal ash may also be used in watertreatment but are described under Secondary products inSection 2.2.3.
Water soluble components of fly ash such as gypsum andlime release hydroxide ions and calcium during mixing of flyash with water. The hydroxide ions neutralise the hydrogenions released from acidic wastes. The calcium ions removephosphorus from wastewater, preventing 'blooming' oreutrophication. Fly ash is therefore used to clean up lakescontaining runoff from agricultural land. These favourablereactions are offset by oxidation of ferrous ions present in flyash which, with oxygen and hydroxyl ions, tend to lower thepH. There is also a limit on addition of calcium into a waterbody, through increasing the hardness of the water but anaddition of up to 10 giL does not seem to have any adverseeffect on aquatic life (Joshi and Achari, 1992).
Wastewater sludges are treated by dewatering by vacuumfiltration followed by incineration of the dewatered cake.Biological sludges have high compressibility and thereforerequire pretreatment to give sufficient rigidity and strength toform a filter cake. Fly ash is added to support the sludgeparticles. It has the added benefit of decreasing the amount ofwater requiring treatment because fly ash has a highabsorptive capacity for water. The technique has thedisadvantage of increasing the volume of filter cake to beincinerated. However, the fly ash filter cake has a higherporosity, proportionately lower residual water content andhigher fuel value (due to the carbon content of fly ash) (Joshiand Achari, 1992).
7.0
60
5.0(/)
<I.l:::Jtil 4.0>I0-
30
2.0
10
0.0Well A
Well AWellCSpoil 8Spoil ASeep 8Seep A
Well C Spoil 8 Spoil A Seep 8
incoming water outside mined areainside mined areabefore groutingafter groutingbefore groutingafter grouting
An example of the use of fly ash for treating effluentdischarged from dyeing acrylic and woollen gam (yarn orthread) in India is described by Vinod and others (1995). Inthis case fly ash from the Kota thermal power station was
Figure 7 Average pH values for incoming and outflowwater at grouted and ungrouted well areas(Kim and Ackman, 1995)
27
Ash markets
Flue gas desulphurisation using fly ash has been testedsince 1985 in Japan on a pilot- and demonstration-scale andstarted commercial operation in 1991 with a gas flow rate of644,000 m3/h at the No I unit of the Torrato-Asuma powerstation. Coal ash is mixed with lime and gypsum and formedinto pellets. Dry desulphurisation takes place by solid-gasreactions in a moving bed absorber, achieving an S02removal efficiency >90%. Reheating of the flue gas is notnecessary. The spent sorbent can be recycled as a source ofgypsum for construction. Simultaneous particulate removalhas achieved well over the design value of 85% and thedenitrification efficiency has been measured at valuesbetween 4.6% and 19%. A hot water curing process has beendeveloped to manufacture the sorbent in granular, slurry orpowder form for demonstration tests which are scheduled tofinish in 1996 (CCUJ, 1994; 1995).
Sorbents derived from pressure hydration of calciumhydroxide and coal fly ash were agglomerated with cokeprior to combustion. Although the desulphurisation of thecoke during combustion by the sorbent was proven, detailedcost analysis, reaction kinetics, and combustibility andcarbonisation studies need to be carried out to determinewhether the agglomerate can be used commercially as fuel(Otaigbe and Egiebor, 1992).
2.2.5 Agriculture and fisheries
Coal ash found a use many years ago as a fertiliser and morerecently as a soil amendment. Its use in fisheries is less direct- in stabilised ash concrete as an alternative to theconventional concrete used to construct artificial reefs.
AgricultureCoal ash may be simply added to the soil, for example as'green ash' the special fertiliser marketed in Japan in the1960s or made into a manufactured product such as thepotassium silicate fertiliser marketed since 1979. Currentlythere are two plants producing 28 kt/y and 18 kt/y of thisfertiliser in Japan (CCUJ, 1994; Kamada and Tezuka, 1995).Earlier studies from around the 1960s concluded that fly ashcontained most of the essential nutrients for plant growth sothat moderate additions to soil would correct deficiencies ofcertain elements in plants. High additions of fly ash couldresult in aluminium, boron or manganese toxicity. Also,nutrient availability is dependent on pH. Hence the fly ashcharacteristics (and those of the soil) should be studied beforeapplication (Joshi and Achari, 1992).
Ashes from certain coals such as Hazelwood, Huntly andYallourn coals from Australia and New Zealand are rich inlime and magnesium which improves the soil structure. Thealkalinity of these ashes (pH 9.7-10.9) increases the pH ofacidic soils. However, the possibility of contamination fromless desirable trace elements is investigated in accordancewith Environmental Protection Authority regulations (Jo andothers, 1994).
Soil amendments are being developed to meet more stringentenvironmental regulations and generally consist of a mixtureof coal ash with other materials such as organic substances(sawdust) and a binder (cement and gypsum) in an
28
appropriate blending ratio (Kamada and Tezuka, 1995). Otherresearch is investigating various mixtures of coal ash withmanure (Brodie and others, 1995; Beaver, 1995).
The manufacture of artificial soil from coal ash produced atthe Indian River Power Station, near Millsboro, Delaware,USA, was investigated by Brodie and others (1995). Theyidentified the following key requirements for a healthycompost:
C:N (ratio of organic carbon and nitrogen) in the rangeof 20: I to 35: I;optimum moisture range 40-60%;oxygen content ~5%;optimum pH control ranges between 6.5 and 7.2 (apH> 8.0 may lead to odours).
Balanced conditions for the first three requirements allowthermophilic aerobic bacteria to consume the organic matter,releasing C02, water vapour and heat.
Six compost mixtures were made with fly ash mixed withother by-products from the region (chicken litter, pine bark,sawdust) in various ratios, the proportion of fly ash up to40% by volume (0-64% by wet mass) and a seventh compostwas made with 25% bottom ash by volume (39% wet mass).During 8:;2 months composting under difficult winterconditions, the composts had undergone a heating cycle toabout 24°C and to nearly 60°C in some cases. A high ashcontent reduced the maximum temperature attained andresulted in greater concentrations of heavy metals than lowash composts. However all the composts had trace elementconcentrations below the ceiling limits set by the US EPA forbiosolids applied to land (Brodie and others, 1995).
Washington State University has a programme to compostover 900 t of coal ash produced each year from the universitypower plant with nearly 450 t of pulped food waste, wastepaper and landscape trimmings with over 9000 t of separatedcow manure. Test piles of separated cow manure alone andfour others with additions of coal ash at various proportions(5.4%, 8.7%, 18.8% and 31.8% dry weight) were compostedand tested. The pH of the coal ash alone was 11.6 but aftertwo weeks of composting, all the mixtures had a pH ofabout 8.7. The CN ratio was 23: I to 29: 1 by weight. Thetrace element concentrations in the compost mixes were allbelow the limits set by the proposed Washington StateDepartment of Ecology Compost Quality Guidelines and theEPA Sewage Sludge Regulations. Beaver (1995) concludedfrom a field test with barley that adding coal ash enhancedthe nutrient value of the compost and subsequent cropgrowth. The results reported of greenhouse tests with tomatoplants appear to be less encouraging.
FisheriesIn the early 1970s research started in the USA to preparestabilised ash concrete using fly ash for the construction ofartificial reefs at sea. Shieh and Duedall (1995) noted thatdespite this early start, no pilot-scale reefs had beenconstructed in the USA although some research activitiescontinued. The large-scale practice of constructing artificialreefs is only being considered seriously as an ash
management strategy in Japan and Taiwan - countries wherethere is greater emphasis on marine resources and much lessland for agriculture. Research is also in progress in the UKand Italy and has confirmed the lack of adverseenvironmental effects. However, regulatory and publicacceptance of the use of stabilised ash concrete blocks forreef construction is still a barrier in the USA.
In Japan research has been in progress since 1980 onartificial fish reefs made of fly ash, gypsum and cement. Theconcrete is made into hollow cubes, hollow hexagonalpyramids, mats to simulate soft ground and it is also injectedunderwater to form large-scale banks, designed to generateupwelling. Tests have confinned that marine creatures adhereto the reefs and that fish swarm there naturally (Kamada andTezuka, 1995).
Suzuki (1995) describes details of the fly ash concrete usedto make Ashcrete - a high-volume fly ash concrete forlarge-scale fishing ground development projects. The basicmixtures of fly ash, Portland cement, water, sand and gravelwere tested with different activators. Sodium chlorideactivator especially promoted the development of highcompressive strength between the 7th and the 28th day. Thelong-term compressive strength was, however, similar for alltypes of Ashcrete. The artificial reefs of Ashcrete have beeninvestigated since 1986 and have passed all the safetyrequirements. These include limits by the Fishery Agency ofthe Japanese government on the maximum content of ninetoxic substances. A further proposal is to construct 40 'superridges' (30 m high by 300 m long and 120 m wide at base)underwater at intervals of 150 m. This would generate15,000 m3/s of upwelling, estimated to bring sufficientnutrients to the photic zone to produce 8000 t/y of fish after30 years, having used 10 Mt of coal ash from a 1 GW powerstation.
Experiences of artificial reef construction using coal ash inTaiwan are described by Kuo and others (1995). Laboratorytests were carried out in the early 1980s to determine themost appropriate formula for the concrete. Different types ofreef models were then constructed in coastal waters tomonitor their short-term effectiveness. Finally a long-termevaluation has recently been carried out with respect to thefish communities there. Blocks were made of eight differentmixtures including waste carbide for one site and ten othersfor another site. The best formula from the point of view ofcompressive strength development was found to be 60% (byweight) of fly ash, 10% lime, 15% cement, 15% sand with awater to dry material rato of 0.42. After 28 days thecompressive strength was 227 kg/cm2 and met therequirements for artificial reefs in Taiwan. Several differenttypes of reef units as shown in Figure 8 were installed during1984-86, off the northern and southern coasts of Taiwan.Observations of the artificial reefs from 1986-90 showed agradual colonisation by fish and other marine life, 80% of thetotal biomass of fish being of commercial value andincluding juveniles. The results showed that the fly ash reefswere as good as those made of the usual concrete.
Experimental reefs of stabilised coal ash concrete wereconstructed in 1989 in the UK (Collins and Jensen, 1995)
Ash markets
and soon after in Italy (Relini and others, 1995). The reefblocks in the UK were made of pulverised fuel ash, gypsum,flue gas desulphurisation sludge and cement and comparedwith concrete controls. The compressive strength of the ashblocks increased with the time immersed and a replacementof calcium by magnesium was noted. Heavy metalconcentrations remained similar through a 4-year test periodwith no significant loss. Reef biota and reef predators did notshow excess bioaccumulation of heavy metals compared withthose from natural and concrete control reefs. Monitoring ofheavy metal concentrations in the reef blocks and associatedbiota is continuing in order to gain an understanding of thelong-ternl behaviour of the stabilised coal ash mixtures in themarine environment. The mixture chosen for theexperimental reef in Italy contained 49.5% fly ash, 24.7%bottom ash, 4.9% hydrated lime and 21 % water. Limitedleaching of some macroelements such as alumina, calciumand silica into sea water was observed but negligible leachingof trace elements such as arsenic, chromium, selenium andthallium which are of environmental interest. Some biotaappeared to favour the ash-based blocks to those made onlyof concrete but there was no explanation at this stage.
In Canada use of stabilised fly ash blocks for artificial reefsin freshwater is being investigated by Ontario Hydro andNorth American studies from the 1970s and 1980s arereviewed by Chan (1992). Various mixed of fly ash concretewere tested as an artificial reef in Lake Michigan. Fly ash,cement and aggregate were used and in some cases P4(Class C) fly ash was used as the binder without cement. Thefly ash blocks needed to be sufficiently strong for handling inplacement, cured prior to immersion in water and to conformto ASTM C618 specifications (see Chapter 4). The fly ashblocks had a low initial strength (350-500 psi (2.4-3.45 MPa»but gained strength and retained their physical integrity in thelake water. This was attributed to the calcium-silicate-hydratebinding the concrete together; also destructive forces in thelake were minor and due to 12 m of water depth, a minimalfine sediment flow < 0.09 mls and lack of dissolved salts inthe fresh water. Chemical tests showed that trace elementconcentrations remained well within water quality criteriaguidelines although the concentration of arsenic increased inthe water. The content of the radioactive elements 226Ra and40K in the fly ash blocks were only a few times more thanthose in naturally occurring sediment. Natural sedimentationdeposited about 54 Mt/y. This is much greater than the massof about 270 t of fly ash blocks. Hence the addition wasunlikely to increase the radiation dose to the environmentsignificantly.
A study, cited in Chan's review, by the State University ofNew York used stabilised fly ash with FGD sludges forartificial reef construction in fresh and sea water. The mixdesign selected to give a minimum strength of 2000 kPa wascomposed of 34% fly ash, 8% bottom ash, 31 % FGD sludge(dry solids), 5% hydrated lime, 2% Portland cement and 20%water. After 5 years of laboratory and filed investigationswith 3 years of monitoring the artificial reefs, it wasconcluded that the coal residue blocks were stable orincreasing in compressive strength. Elements ofenvironmental concern remained in the blocks or had veryslow leaching rates. Calcium from the blocks did dissolve in
29
Ash markets
o~-30em~
To 0 30em
00 1~
~oem
a) small four-hole simple block
/ ......OIilil--- 48 em--~ /
~-----""T
38 em
1r8
)
b) small one-and two-hole simpleblocks
c) cylindrical block
1.5 m
V
00
e) cubic block
IT1.65 m
1\. 4 m-------.~Id) giant turtle block
0 0 0 0
Figure 8 Types of coal ash artificial reef units used in Taiwan (Kuo and others, 1995)
sea water but the rate of leaching decreased exponentiallyupon submersion and remained slow. No bioaccumulation ofpotentially toxic elements from the blocks was found in reefanimals studied.
The study by the University of Delaware, reviewed by Chan(1992), investigated coal residue blocks for oyster and fishreefs in sea water. The minimum strength required was400 psi (2.8 MPa) and the best mix tested was composed of58% fly ash, 38% FGD sludge and 4% lime. This materialwas suitable for reef construction in a marine environment,attracted colonisation by reef organisms and was not toxic tothe environment.
size for construction was I" xl" x 2" (30 x 30 x 60 em) also the standard size used in the concrete block industry.Current concrete block manufacturing technology could beused to construct stabilised fly ash blocks. Chanrecommended Ontario Hydro to conduct similarinvestigations to improve understanding of the costs andbenefits of using stabilised ash blocks in fresh water. Thisshould include design of a mix to achieve the minimumstrength requirement and also maximise utilisation of the ash.A demonstration site in a lake needed to be selected forplacement of a reef and then monitored continuously.
2.2.6 Materials recovery
Chan (1992) also reviewed studies from the 1980s fromelsewhere in the USA, Japan and Taiwan. Overall it wasconcluded additionally to the above that the optimum block
Aluminium and iron may be recovered from fly ash bylime-ash sintering, high temperature chlorination or acidleaching processes. Estimates from around 1980 indicated
30
that a high Ca ash showed an aluminium recovery of75 tJIOOO t of raw ash using direct acid leaching butproduced residues amounting to 457 tJI000 t of feed(Hemmings and others, 1989). Also a 1980 study cited by Joand others (1994) estimated that fly ashes with aluminacontents ;:::::23% can be used for the economic recovery ofaluminium. For example fly ashes from several bituminousand subbituminous Australian and New Zealand coals wereapplicable for aluminium extraction. Iron extraction wasconsidered highly viable for Yallourn and Hazelwood browncoal ashes which contained only small proportions of silica.The extraction of alumina, magnetite and ferrosilicon fromfly ash is being studied in Japan (CCUJ, 1995). Thetechnology for the production of iron and aluminium oxidesfrom Kazakhstan coal ash is being developed at the SiberianMetallurgical Institute and is described by Pavlenko andothers (1995) and a process for aluminium sulphatemanufacture from fly ash in India by Trehan and Mittal(1995).
Iron and iron oxide were concentrated in two stages from ashwhich was removed hydraulically from the Omskaya thermalpower station. As a result of this process, the magneticconcentrate contained on average 56-76% iron oxides,7-IO% aluminium oxide and 13-27% silica. The iron oxidecontent needed to be increased to 85-90% for use as aferriferous concentrate. This required separation of the silicaand alumina, producing other by-products such as liquidglass and zeolite in addition. A method developed by theInstitute of Metallurgy and Concentration of the Academy ofScience of Kazakhstan was tested. This involved dissolvingthe alumina and silica in an alkaline solution, leaving the ironconcentrated in a sludge (80-93%). The alkaline solutioncould be leached in an autoclave to remove over 95% Si02and over 75% aluminium oxide. Pavlenko and others (1995)concluded that the raw material extraction wastechnologically feasible but used only 10% of the powerstation ash. Hence further research was required to achievecomplete ash utilisation.
A pilot plant for manufacture of a low iron contentaluminium sulphate from fly ash is to be set up for theNational Thermal Power Corporation Limited in India(Trehan and Mittal, 1995). The process involves magneticand gravity separation followed by digestion withhydrochloric acid of the ferric iron in the finer portion of flyash. The aluminium sulphate, ferrous sulphate and excesssulphuric acid is neutralised with hydrated alumina toproduce a solution of aluminium sulphate and ferroussulphate.
Materials recovery from pulverised coal ash is beinginvestigated but would be appropriate for specific marketsrather than for bulk use of ash as for example in theconstruction industry. The relative deployment of theseapplications in different countries is assessed in the nextsection.
Ash markets
2.3 Summary and commentsSuccessful applications for the use of pulverised coal ash arefound in several sectors and in many countries. Over half isused as a fill for mines and quarries, for structural andlandfill, although some of these sectors may be regarded asdisposal in some countries. Over a quarter of the tonnage offly ash used is deployed to manufacture cement, mortars andconcrete. Secondary products and road construction comprisethe major part of the remainder of coal ash utilised (seeFigure 1). The tonnage of pulverised coal ash utilised(including landfill) varies considerably from country tocountry, being highest (>18 MtJy) in China, Germany, Russiaand the USA. However, the ash utilised as a percentage ofthat produced is highest (>99%) in Germany, Japan and theNetherlands where disposal is strongly discouraged (seeTable 1). Clearly, there is considerable scope for usinggreater quantities of coal ash. Continued and increasing useof pulverised coal ash is hindered by various obstacles whichare being addressed.
Legislation may be a constraint or a benefit to the utilisationof pulverised coal ash. For example in the USA potentialliability laws may threaten to incur high costs to all parties.On the other hand, coal use residues are not regulated ashazardous wastes; and more technical regulations andspecifications to control the quality of coal ashes and give aguarantee of their suitability for specific applications areopening up opportunities for more bulk use of coal ash,especially in the building and construction industries.Transport costs are often major obstacles but strategies todevelop more site-specific markets can optimise such costfactors. Strong competition for ash markets, even betweenpower stations in the same country, has led to commercialconfidentiality in several European countries. However,dissemination of information is also essential in order toexpand markets by marketing the technologies beingdeveloped for specialised applications.
Most progress towards overcoming obstacles is probablymade by co-operative ventures promoting use of coal ashwith coal suppliers, ash suppliers and the building industries.These not only overcome preferences among potential usersfor conventional materials but provide educational andinformation services to the public in order to improveunderstanding of the environmental issues and improveacceptance of the use of coal ashes.
By far the largest specialised application for fly ash is incement and concrete. In this sector, it is essential tocharacterise the coal ash. Then there are opportunities formarketing as a result of regulated specifications, qualityassurance and certification. These aspects are discussed ingreater depth in the following chapters.
31
3 Physical and chemical characteristics
Coal combustion by-products exhibit a broad range ofchemical, mineralogical and physical characteristics due tothe variable nature of the parent coal, the conversionprocesses, and the emissions control systems. It is possible topredict, within a range of error, the characteristics of the ashfor a given coal and installation. Characterisation of ash isalso necessary to provide information on its behaviour duringhandling and treatment and to assess its utilisation potential(Kuhnel and others, 1992).
This chapter starts by outlining the origins of coal ashfollowed by the chemical composition, mineralogy andmorphology. The physical properties are discussed inSection 3.5. Section 3.6 explains how the chemical propertiesof coal ash relate to its performance when utilised.
3.1 Origins of coal ashThe combustion of coal in electric utilities produces largeamounts of ash. Three types of ash precursors have beendefined (Kamer and others, 1994):
Excluded mineral grains which break free of coalfragments during crushing and are relatively large. Thesegrains may be fragmented during combustion and partlyor wholly fused depending on temperature, compositionand volatile content.Included mineral grains which are retained in coalfragments and tend to be smaller. These grains areaffected by coal swelling, char fragmentation andcombustion, and take part in several reactionsincluding fragmentation, volatile migration, coalescenceand fusion.Organically associated inorganic elements, which aredistributed within the coal macerals. These elements arefreed during combustion to form vapour and fineparticles that typically adhere to and coat larger particles.Figure 9 shows the interactions of different inorganiccomponents during combustion.
32
The solid by-products from coal combustion are mainly flyash, bottom ash and boiler slag. Figure 10 shows where thedifferent ashes are formed. The proportion of residuescollected as fly ash varies with the furnace system. Forexample a 'dry-bottom' pulverised coal fired boiler, so-calledbecause the bottom ash is removed from the base of thefurnace as discrete, non-molten particles or clinkers, willproduce 80-90% of the residues in the form of fly ash. In a'wet-bottom' furnace most of the residues are collected as
Fly ash refers to the finer fraction of coal ash which formsparticles suspended in the flue gases from combustion ofpulverised coal in electricity generating plants (Joshi andLohtia, 1995). Particulate emission standards in mostcountries require the separation of fly ash from the flue gases(see Soud, 1991; 1995). This may be achieved by:
electrostatic precipitators (ESP);fabric filters (baghouses);cyclones and other mechanical collectors;wet scrubbers.
Fly ash can be transferred to silos or bunkers and supplied asa dry product. Dry handling and transport facilitates fly ashutilisation. However, if required the residues can bedampened and stockpiled (Clarke, 1992). This is discussed inmore detail in Section 5.2.
Bottom ash, in dry bottom systems, falls to the bottom of thefurnace and typically passes through grates into a water filledpit where it is cooled. The residues are removed periodicallyand coarse material may be crushed to break up the largeragglomerates. In some systems the bottom ash is thende-watered before temporary storage and distribution forutilisation. In other plants the residues are removed fromhoppers by high pressure water jetting along sluices, andcollected in ponds. Material accumulated in the furnace isremoved periodically by de-slaggers. In wet-bottom systems,all the ash collected in particulate control devices is fed back
Physical and chemical characteristics
a) Discrete mineral particles
condensedparticle
devitrification
possible expansion
exsolved gases ....
glassy particles
---~.
•fusion/partial melting
---~ •pyrite 1100°Cclays 1300°Cquartz 1550°C
mineral particle
b) Minerals inclusionscondensation
'<0,1 ~
••• <2~•e 10-20~
•solid
surface enrichment
porous
coalescence
vapourphase
elements
-_fly ash
particles
agg lomerationof droplets
charcoal particle
vaporisation
, I' liquid
•
mlnera, grains , dr.o~~ts11'
, " ," , burning ~ ~~••••••••••• ,•••,"., ": . . : ,~;.
hollow cenospheres"----------. <t:) 10-250~
Figure 9 Schematic diagram of the interactions of different inorganic components during combustion (Clarke, 1993b)
Combustor Particulate control
coal electrostaticprecipitator
additionalgas
cleaningstack
1economiser
ashfly ash
-90% in dry bottom furnace-15% in wet bottom furnace
solid residues maycontain some fly ash
bottomash
-10% in dry bottom furnace-85% in wet bottom furnace
Figure 10 Distribution of mineral matter following combustion in a power station (Clarke, 1992)
into the boiler. The bulk of the mineral matter is removed asslag which is quenched in water at the furnace bottom. TIlesolidified ash is then either sluiced to settling ponds orde-watered similar to the dry-bottom furnaces (Clarke, 1992).
on the inorganic constituents of the parent coal and thecombustion conditions. The composition of coal ash isdiscussed in this section as major elements, trace elementsand unburnt carbon.
3.2 Chemical composition 3.2.1 Major elements
The chemical characteristics of coal ash are dependent largely Over 80% of the ash from coal consists of silica (Si02),
33
Physical and chemical characteristics
alumina (Ah03) and iron oxide (Fe203). A typical coal ashalso contains oxides of calcium, magnesium, titanium,sulphur, potassium, sodium and phosphorus (Itkonen andJantunen, 1989). The concentrations of the major and minorelements in coal ash, including the general ranges, and in flyash are shown in Table 2.
As coal is burned these compounds form spherical, glassyparticles with other compounds such as silicates in smalleramounts (Gutierrez and others, 1993). For some time theAmerican Society for Testing and Materials (ASTM) hasdivided fly ash into two groups:
Class F, pozzolanic, Si02 + Ah03 + Fe203 = 70 wt%minimum;
Class C, cementitious, Si02 + Ah03 + Fe203 = 50 wt%minimum.
Table 2 Concentration (mg/g) of major and minorelements in coal ash and in fly ash (Itkonen andJantunen, 1989)
Element Coal ash Fly ash,average range average
0 484 467-501Si 263 213-324 184Al 133 50-190 103Fe 63 14.7-112 75C 17K 8.9 1.1-13.5 13Ca 15 2.8-46 42Na 5.4 2-8.1 5.3Ti 22.4 2-69 6.4Mg 15 2.6-47 12Mn 39 0.3-180 10S 12.1 3N 0.8 0.6-1.3P 2.1 1.4--3.3 0.3
This classification does not consider the compound form inwhich these oxides are present in the ash. Class F ashes canhave a wide range of lime (CaO) contents, and some with ahigh CaO content may show cementitious behaviour. Class Cashes almost always have more than 20% CaO (McCarthyand others, 1990). A full discussion of ash classificationschemes is given in Chapter 4.
Based on their database of 178 North American fly ashes,McCarthy and others (1990) concluded that as CaO increases,Ah03, Fe203 and Si02 decrease. Fe203 shows littlecorrelation across the classes of fly ash. The results of theiranalysis are shown in Table 3. Analytical Fe203 in fly ash islargely dependent on the pyrite content of the source coal,which can be quite variable. McCarthy and others (1990)made some further observations:
In fly ash from low rank coal it is thought that theCaO originates mainly in the organic portion of thecoal, and that calcite (CaC03), dolomite (CaMg(C03)2)or gypsum (CaS04.2H20) make only a minorcontribution.MgO behaviour parallels that of CaO. A typicalhigh calcium fly ash has about 5% MgO and25% CaO.Sulphite (S03) increases with CaO content as a result ofsome capture of flue gas S02 by the basic fly ash.Analytical S03 also increases with available alkaliscontent because of increased formation of alkalisulphates.In low rank coals, Na is present in smectites and onorganic ion exchange sites.The ratio of Na20 to K20 generally increases withincreasing CaO content. The general increase in availablealkalis in the higher calcium ashes results from anincreased portion of the Na20 being present in solublesulphates, instead of in less soluble glass.
Table 3 Chemical composition of North American fly ashes (McCarthy and others, 1990)
SiOz Ah03 Fez03 Sum 1-3 S03 CaO MgO NazO K20
Low calcium (<10% CaO) fly ash (45 samples from 32 plants)Number of samples 45 45 45 45 45 45 27 13 13Mean, wt% 52.5 22.8 7.5 82.8 0.6 4.9 1.3 1.0 1.3Sd*, wt% 9.6 5.4 4.3 13.1 0.5 2.9 0.7 1.0 0.8Rei sd*, % 18 23 57 16 72 59 54 100 59
Intermediate calcium (10-19.9% CaO) fly ash (36 samples from 19 plants)Number of samples 36 36 36 36 36 36 23 16 16Mean, wt% 48.5 19.6 6.2 74.3 1.3 15.2 3.2 1.5 0.8Sd*, wt% 4.8 3.6 2.1 4.4 0.8 2.5 1.2 1.7 0.4ReI sd*, % 10 19 34 6 61 16 36 114 55
High calcium (>20% Ca) fly ash (97 samples from 32 plants)Number of samples 97 97 97 97 95 97 64 36 36Mean, wt% 36.9 17.6 6.2 60.7 2.9 25.2 5.1 1.7 0.6Sd*, wt% 4.7 2.7 1.1 5.2 1.8 2.8 1.0 1.2 0.6Relative sd*, % 13 15 18 9 61 II 20 69 106
* standard deviation
34
Physical and chemical characteristics
Table 4 Concentration (JLg/g) of trace elements in coalash and fly ash (Itkonen and Jantunen, 1989)
The maximum LOI for Class F pozzolans for use as generaladmixtures in Portland cement is 6% (ASTM C618-89a).
Certain characteristics of the coal, the boiler design, or themaintenance and operating conditions of the boiler mayprevent complete combustion. Incomplete combustion mayresult in fly ash containing 0.02-15.34% unburnt carbon(Joshi and Lohtia, 1995) (see Section 5.1.2). It may bepossible but uneconomic to change the combustionconditions. Conditions in the boiler that are most conduciveto complete combustion may also be those most likely toproduce high levels of nitrogen oxides (NOx = NO and N02)(Cochran and others, 1995). Loss on ignition (LOI) is theterm used to specify the amount of unbumt carbon, and it isgenerally related to the amount of carbon or unburnt coalconstituents in the fly ash. However, for some fly ashes thereis a significant difference in the carbon content and LOI,because other components in the fly ash may be volatile ormay decompose on heating (Joshi and Lohtia, 1995). LOI canbe determined in accordance with ASTM C114 (Kilgour,1992). This is discussed in more detail in Section 4.3.
3.2.2 Trace elements
Fly ash may contain traces of arsenic, barium, beryllium,boron, cadmium, chromium, cobalt, copper, selenium, silver,strontium, tin, vanadium, zinc and other trace elements.These inorganic trace elements, which include radionuclides,can condense or be adsorbed on the particulates in the fluegas as it cools, largely depending on the surface areaavailable. Thus particle size is important, and theconcentration of volatile elements and compounds generallyincreases as the particle size decreases (Poole and Bayat,1993). It is considered that about 75% of the trace elementsare captured, with the exception of the more volatile such asmercury, selenium and metal halides and selenium of whichonly up to 30% may be retained in particulate controlsystems (Joshi and Achari, 1992).
Trace elements may be divided according to their associationwith the organic or mineral fraction of the coal. Over 90% ofthe trace elements found in coal fly ash originate from theinorganic fraction (Itkonen and Jantunen, 1989). Theseinclude arsenic, cadmium, lead, mercury and zinc. Traceelements from the organic fraction include beryllium, boron,gallium, germanium and titanium. Table 4 indicates theconcentrations of trace elements in coal ash. Trace elementsin coal are covered in more detail in another IEA CoalResearch report (Davidson and Clarke, 1996).
The methods to determine the distribution of an element(Polyak and others, 1994) within or among small particles arebasically:
analysis of single particles by scanning electronmicroscopy (SEM);direct species determination for mineralogical phases byX-ray diffraction (XRD) or Fourier transfonn infraredspectroscopy (FTIR);solvent leaching for the information on the behaviour ofmetal pollutants under real environmental conditions.
Trace elements in fly ash include the radioactive elements238U, 232Th and 4OK. Using the fission track registrationtechnique, Jojo and others (1994) found the average uraniumconcentration to be 29.1 ppm in fly ash, 25.7 ppm in slag and17.1 ppm in coal. Uranium exists in coal as the silicatemineral (coffinite) and uraninite (U02). During combustionthe refractory coffinite is distributed in equal concentrationsin fly ash and bottom ash, while uraninite is vaporised andlater condensed on fly ash when the flue gases cool. Uraniumhas only a slightly higher concentration in fly ash than inbottom ash.
Font and others (1993) quantified heavy metals in ash byX-ray fluorescence spectrometry and inductively coupledplasma atomic emission spectroscopy (ICPAES). The amountof radionuclides that are captured depends on theirconcentration in the original coal used, that of the ash itproduces, the efficiency of the filtering system employed andthe combustion efficiency of the power plant itself. Theconcentrations of some primary radionuclides in fly ash areestimated to be 265 Bq/kg for 4OK, 200 Bq/kg for 238U and240 Bq/kg for 226Ra.
Element Coal ashaverage range
Ag 2.1 0.5-4.6As 18 3.4--88B 338 98-465Ba 1367 300-3500
Be 9 0.7-21Br 25.4 19.5-31.9Ce 84 84--84
Cd 0.8 0.2-2.4
CI 2451 900-8081
Co 23 3.2-51Cr 241 57-533Cu 117 15-201Cs 48 0.9-199F 313 103-636Ga 69 7-97Ge 25.0Hf 1.3 0.7-2Hg 0.264 0.014--1.11La II 8.3-14Li 65 65-65Mo 14.6 4.2-53.5Ni 55 15-108Pb 32 9.9-87Rb 177 102-310Sb 5.6 0.7-12.1Sc 9 4.5-22Se 14.3 3.0-39.4Sn 38 10--79Sr 264 127-533Th 13.0 2.3-42.4Tl 3.8 0.8-6.9
U 3.8 1.0-11.1V 224 80--570Zn 110 50--152
3.2.3 Unburnt carbon
Fly ash,average
180220
8
230
250250
250530
20
252324350600
35
Physical and chemical characteristics
Hower and others (1995) examined fly ash samples frommost of the power stations in Kentucky. They found that overhalf of the units burning coal with <5% sulphur had over10% carbon in the fly ash. Many of the units were relativelysmall «200 MWe) and several were quite old (built before1960). PetrographicalIy the carbon is categorised as isotropicand anisotropic coke and inertinite. Inertinite includes theinertinite macerals, presumed to be mainly fusinite, whichpass through the combustion process relatively structurallyintact (Hower and others, 1995).
Another reason for the high LOI values of low calcium ClassF North American fly ashes is the higher content of unburntcarbon in the bituminous coal ashes (McCarthy and others,1990). Apparently the less dense low-rank coals burn morecompletely. An important point is that LOI data for fly ashinclude a component of weight gain on ignition due to theoxidation of Fe304 to Fe203 (see Section 4.3.6). Thus inhigh-iron ashes there can be even greater amounts of unburntcarbon than suggested by the LOI data. There is a wellestablished, positive correlation between the waterrequirement (water necessary to achieve a flow) and highLOI and/or coarse particles.
Carbon in fly ash is detrimental to the quality of concretebecause it adversely affects air entraining admixtures whichare commonly used to impart desirable properties such asincreased durability to the concrete. The presence of carbonmeans that the air-entraining admixture dosage must beincreased, and the admixture is less effective. As a result, theconcrete manufacturers lose some quality control, especiallywith extended mixing times that are common to ready-mixoperations (Cochran and others, 1995). In the US concretemarket some specifying agencies have set maximumpermissible LOI levels for fly ash to be used in concrete,which range up to 6% LOI. However, for many US marketsthe LOI is set at 4%, which is considered liberal by someproducers (see Section 4.2).
3.3 MineralogyThe chemical composition of a fly ash obviously depends onthe coal and impurities from which it is formed. Mineralsdominate the inorganic components of high-rank coals, whileorganicaJIy bound inorganic elements are most abundant inlow-rank coal and often exert a strong influence on ashdeposition behaviour (Karner and others, 1994).
GeneraJIy, fly ash is predominantly amorphous glass orcarbon but can contain about 11-48% crystalIine matter. Themost common crystalline phases are mullite, quartz, hematite,magnetite, anhydrite, tricalcium aluminate (3CaO.Ah03),melilite, merwinite, periclase and lime (McCarthy and others,1990). Of these, muJIite is usually the largest fraction, at6-15% of the total ash content (Joshi and Achari, 1992). Thesilicates are generated from alite (tricalcium silicate;3CaO.Si02) and belite (dicalcium silicate; 2CaO.Si02).Aluminates such as tricalcium aluminate which are present infly ash particles react to produce ettringite(Ca6Ah(S04)3(OH)1226H20) and monosulphates. Afterhydration, the calcium:silicon ratio in the particles is found tobe in the range of 1: 1.5 (Joshi and Achari, 1992).
36
The crystalline and glassy mineral phases of ash may bedetermined by X-ray diffraction (XRD) (McCarthy andothers, 1990). Each mineral has a unique pattern of X-rayreflexes which can be used for its identification. Thedisadvantage of this method is that the mineral must bepresent in amounts greater than its detection limit (Kuhneland others, 1992). Trace components, as identified by XRD,are defined as those present at less than 10%. Therefore, onlythe formation of predominant mineral species exhibitingcrystallisation are identified (Booher and others, 1994). Alsothe crystallinity of the material may cause diffuse reflexes.However, XRD is stiJI a very valuable tool (Kuhnel andothers, 1992).
Mullite (AI6Si2013) forms by direct crystallisation from themelt as ash particles start to cool (McCarthy and others,1990). It forms during the decomposition of kaolinite, illiteand other clay minerals present in the base coal (Booher andothers, 1994). Mullite is rich in Ah03 and does notparticipate in cementitious reactions. It is the principalcrystalline phase containing Ah03 in low calcium ash. Lowcalcium, high alumina ashes generally contain 2-20%muJIite, whereas high calcium ashes generally contain <6%mullite. There is less mullite in high calcium ashes because:
Ah03 is also crystallised in tricalcium aluminate and thegehlemite content of melilite;there is less Ah03 available in the low-rank coal derivedhigh calcium ashes.
Quartz (Si02) in fly ash originates as silt and clay sizedquartz grains in the coal that lack the time during combustionto be incorporated into the melt by other inorganics. Quartzwas detectable by XRD in every fly ash sample fromMcCarthy and others' (1990) collection. There was nosubstantial difference in quartz content among the classes andcompositions of fly ash. For some ashes, almost half of theanalytical Si02 was present as non-reactive quartz. Thismeans that caution should be exercised when assessing thepozzolanic activity of a fly ash in placing too much emphasison the Si02 content (McCarthy and others, 1990; see alsoChapter 4).
Magnetite (ferrite spinel) ((Mg, Fe)(Fe,Alh04) andhematite (Fe203) are magnetic iron oxides which are presentin fly ash. In the oxide ferrite spinel, AI, Mg and Ti are oftensubstituted for Fe. Magnetite is pure Fe304. Most fly ash alsocontains some hematite. Ferrite spinel and/or hematite wasobserved in all the fly ash. Magnetite was present at roughlythe same level in aJI the fly ashes, and hematite was commonin the low calcium ash, but rare in the higher calcium ashes(McCarthy and others, 1990). Iron minerals present probablyoriginate from iron pyrite. Pyrite usually occurs in a widevariety of sizes and associations in the host coals.Combustion behaviour of this material is expected to result incrystalline particles, as found by Booher and others (1994).Booher and others also found that the potential for crystalformation appears to be greater in lignitic fly ash than in flyash from other coals. The reactivity of a fly ash is influencedby the portion of total analytical Fe203 present asnon-reactive crystalline oxides versus the portion present inthe glassy phases, which is presumably reactive. Thus it is
important not to put too much emphasis on the Si02+ Ah03+ Fe203 sum as the principal measure of the availablepozzolanic material in a fly ash (McCarthy and others, 1990).
Anhydrite (CaS04) is a characteristic phase in high calciumfly ash, but it is also found in others. It fOffilS from thereaction of CaO, S02 and 02 in the furnace or flue. Theamount of anhydrite increases with increasing S03 content,but for most ashes only about half of the S02 is present asanhydrite. The other principal speciation of S03 is in alkalisulphates (Na,K)2S04. Anhydrite is important in hydrationbehaviour as it participates with soluble aluminates such astricalcium aluminate in the formation of ettringite. Thisprocess. ettringite formation, contributes much of theself-hardening characteristics of fly ash (McCarthy andothers, 1990).
Tricalcium aluminate (3CaO.Ah03) is an importantcrystalline phase to identify and quantify in fly ash as itparticipates in beneficial self-hardening reactions throughettringite formation, and harmful sulphate expansionreactions. It was found in all the high calcium ashes andabout half the intermediate ones. However, its presence ishard to quantify with certainty because the XRD peaks areoverlapped by those of merwinite, mullite and hematite(McCarthy and others, 1990).
Melilite (Ca2(Mg,AI)(AI,Si)207), merwinite (Ca3Mg(Si04)2)and peric1ase (MgO) all appear to be related to the MgOcontent of the fly ash. Melilite and merwinite have beenmissed in prior XRD studies, probably because of peakoverlaps between melilite/anhydrite and tricalciumaluminate/merwinite. Periclase is essential in high calciumash and common in intermediate calcium ash. However, thesephases are not confined to higher calcium ashes. Typicallyabout half of the MgO in a fly ash is there as periclase,
Physical and chemical characteristics
which originates largely in the organic portion of the coal.Melilite and merwinite are common in metallurgical slagswhere they fornl by crystallisation on a cooling melt, whichmay also be how they form in fly ash (McCarthy and others,1990). Latrobe Valley (Victoria, Australia) brown coal flyash is unusual, largely due to its very high MgO content(18%). This fact, together with its high sulphate levels,makes its direct use as a replacement for Portland cementunsatisfactory. However it may be co-blended to produce apaste of adequate short-term strength (Macphee and others,1993).
Lime (CaO) was detected in all the high calcium ashes, mostof the intermediate calcium ones and some of the lowcalcium ashes. It can be detected at concentrations as low as0.2 wt%. Only a small fraction of the analytical CaO isspeciated as lime and may be referred to as 'free lime'. Mostof the analytical CaO in high calcium fly ash originates in theorganic portion of the coal (McCarthy and others, 1990).Lime-rich fly ashes have hydraulic properties (see
Section 4.1.4).
The mineralogy of the North American fly ashes studied byMcCarthy and others (1990) are summarised in Table 5.
3.4 MorphologyScanning electron microscopy and electron microprobeanalysis provide the micrometer scale size, shape andchemical data that are necessary to characterise fine-grainedcoal constituents, intermediate combustion products, and theresultant by-products (Karner and others, 1994). Computercontrolled scanning electron microscopy (CCSEM) is a wellestablished analytical procedure that is frequently used inevaluating ash-fouling problems. Statistically significantnumbers of particles are automatically located, measured, and
Table 5 Mineralogy of North American fly ashes (McCarthy and others, 1990)
Ah Mu Qz MI Hm Ca3A Mw Sp Lm Pc Sum
Low calcium (<10% CaD) fly ashNumber of samples 20 45 45 3 24 44 15 9 45Mean, wt % 0.8 11.8 8.0 1.9 2.0 23.9Sd*, wt% 0.4 5.0 5.1 1.4 1.7 8.3ReI sd*, % 50 42 64 73 82 35
Intermediate calcium (10-19.9% CaD) fly ashNumber of samples 31 36 36 13 4 20 19 30 28 32 36Mean, wt % 1.0 7.6 8.6 0.8 3.7 2.7 0.7 1.3 25.5Sd*, wt % 0.5 3.1 4.7 0.5 1.8 1.4 0.6 0.7 8.9ReI sd*, % 51 40 55 60 49 51 86 55 35
High calcium (>20% CaD) fly ashNumber of samples 97 97 97 97 2 97 97 97 97 97 97Mean, wt% 1.5 5.6 6.5 1.7 3.2 6.9 1.9 1.2 2.7 32.4Sd*, wt% 0.5 2.5 2.8 0.8 1.4 2.8 1.0 0.7 1.1 7.2Relative sd*, % 34 45 43 47 42 41 52 57 39 22
* standard deviation
Ah - anhydrite Mu - mullite Qz - quartz MI - melitite Hm - hematiteCa3A - tricalcium aluminate Mw - mcrwinite Sp - ferrite spinel Lm - lime Pc - pcriclasc
37
Physical and chemical characteristics
chemically analysed to determine their size, shape andcompositional category (Kamer and others, 1994). For moreinformation on CCSEM see the IEA Coal Research report byCarpenter and Skorupska (1993). Inductively-coupled plasma(ICP) can be used after SEM for analysis of fly ash(Georgakopoulos and others, 1994). If only bulk ASTM ashcompositions are available and chemical and physicalreactions are difficult to interpret, point counting provides amore detailed chemical analysis of crystalline and glassy ashphases. However CCSEM analysis provides a detailedinterpretation of the origin of ash during coal combustion,whereas ASTM ash analysis provides only a fraction of thenecessary information (Kamer and others, 1994).
Three categories of ash particle size have been identifiedaccording to their origin and formation:
an increasing modifier ion concentration will favour theformation of amorphous phases (Dewey and others, 1994).
From a database of 178 North American fly ashes, McCarthyand others (1990) concluded that most of the ashes with alarge fraction exceeding 45 j..Lm diameter were rich in quartz,possibly because the coal grinders were not working wellwhen the ashes were produced, or because excess mineralmatter was mixed with the coal. It appears that high calciumfly ashes are more consistently fine. Lime (CaO) influencesparticle size by reducing aluminosilicate polymerisationwhich decreases viscosity. Less viscous liquids can formsmaller droplets that freeze to form fly ash particles. Theyalso freeze faster and result in a higher glass content in thefiner ash fractions (McCarthy and others, 1990). Most fly ash(60-90%) is in the glass phase (Gutierrez and others, 1993).
3.5 Physical propertiesThe physical properties of fly ash determine the suitability offly ash for its use. This section concentrates on the basicphysical properties which are inherent to the fly ash itself,namely particle size distribution and density.
The morphology of the various kinds of fly ash substancesseems to be primarily detemlined by the mode ofcombustion, that differs for lignite (1000-1 150°C) and forbrown or black coals (I 400-1 600°C) (Rausch and others,1993)
Bottom ash from dry-bottom furnaces is a mixture of glassymaterial, partly fused residues and agglomerated ash.Agglomerated material may be relatively soft and friable,while fused residues are hard and glassy. Bottom ash or slagfrom wet bottom boilers is generally black, shiny andvitreous (Clarke, 1992).
Particle size distribution
Viscosity also affects the shape of ash particles. Low calciumfly ashes have a higher viscosity which enhances the trappingof gases in cenospheres (hollow spheres) and plerospheres(hollow spheres inside larger hollow spheres). Spherical flyash particles show that shape is a result of thecooling-solidifying process undergone by the coal mineralmatter (Gutierrez and others, 1993). The proportion ofcenospheres in a fly ash is partially dependent on the ironoxide content and the combustion temperature. Plerosphereformation is generally thought to be the result ofencapsulation during particle formation. (Poole and Bayat,1993). Non-spherical particles cOiTespond to particles ofunburnt coal.
3.5.1
The particle size of fly ash is a crucial diagnostic featurewhich is already incorporated in the quality requirements andspecifications for various applications. Particle sizedetermines the amount of surt'ace area which affects the ratesof some chemical reactions and the extent of surt'acephenomena. Particle size and geometry control the packing,porosity, pore-size distribution, and pore geometry whichdetermine the permeability and flow of gases and liquidsthrough the porous media (Kuhnel and others, 1992).
Most fly ash particles examined by SEM range in size frombelow I j..Lm up to 400 j..Lm. The small particles are generallysmooth-surfaced spheres in a range of sizes. The largerparticles (>250 j..Lm) are irregularly shaped and contain someincompletely combusted material. The large particles mayalso have finer particles on their crystalline surfaces (Mollahand others, 1994; Poole and Bayat, 1993).
Fly ash particle sizing is usually based on the aerodynamicproperties of the particles, and the supermicron and micronmodes are difficult to separate in practice (Itkonen andJantunen, 1989). The mass median aerodynamic diameter ofparticles >1 j..Lm is generally about 10 j..Lm in a range of3-30 j..Lm. The peak of the submicron mode is near 0.1 j..Lm.
supermicron particles are >5 j..Lm and consist of mineralagglomeration;
micron particles are 1-2 j..Lm and result from the sizedistribution of clay mineral inclusions in coal;submicron «0.5 j..Lm) particles form by the gaseousbursting of aluminosilicate spheres and homogeneousgas-phase nucleation with subsequent agglomeration andcondensation.
Booher and others (1994) used inteIierence contrast polarisedlight microscopy to image the internal components of isolatedglassy, spherical fly ash particles. This technique is able toidentify, instantly, hollow as opposed to solid fly ash spheres.Fly ash samples were taken from five commercial utilities.The overall crystal size ranged from below I j..Lm up to45 j..Lm. Crystals were found on or in spheres of 8-50 j..Lm.
As a result of the dynamic temperature environmentassociated with combustion, ash particles can form intoprimary or multiple phase systems when the liquid andvapour phases are cooled. Amorphous phases(alumino-silicate glasses) will occur in ash particles when therate of cooling is too fast for the structural arrangement ofthe atoms into an orderly crystalline matrix. Also, thepresence of a large proportion of modifier ions (K, Ca, Mg,Na) will inhibit the formation of crystalline phases and resultin the creation of alumino-silicate glasses. Thus theprobability of amorphous phases OCCUlTing in smallerparticles is much greater because the rate of cooling increaseswith decreasing particle size. Also, for a given particle size
38
Physical and chemical characteristics
11mc) Particle size distribution of a fly ash sample using the laser
diffraction technique
11ma) Particle size distribution of a fly ash fraction «80 11m) using the
Lumosed technique
80
120
20
100
60
15
80
40
10
6040
5
20
20
o
o
o
50
100
100
80
#- 60cDco:;E:oJ() 40()
<l:
20
40
#- 302"co:;E:oJ() 20()
<l:
10
80
#- 60cDl§:oJE:oJ() 40()
<l:
20
11mb) Particle size distribution of a fly ash fraction «37 ~) using the
cascade impactor technique
Figure 11 Particle size distribution of fly ash samplesmeasured by different techniques (Gutierrezand others, 1995)
Class F fly ash from the Shin-Ta power plant in Taiwan wassieved using a three-dimensional vibrating sieving machine(Sheu and others, 1990). The sieving machine and theimprovements to the quality of the tly ash are discussed inmore detail in Section 5.3.5. In general, the finer the ash thelower the LOI. In addition, the concentration of Ah03 ishighest in the finest ash. The sieved ash gave highercompressive strengths in concrete with lower water demandthan the original coarse ash. However, samples made with
Sedimentation in a liquid medium, using a Lumosedinstrument, on the 37-80 f.Lm diameter sample;Sedimentation in an air flow, using a Cascade impactoron the fraction below 37 f.Lm;Fraunhofer diffraction of a laser beam through asediment was used on the unsieved original sample,where the upper limit for particle size was 120 f.Lm.
The results are shown in Figure II. Although the resultsobtained from the three instruments were for different sizefractions, there were overlaps in the ranges covered whichallowed some comparisons to be made. The results obtainedby the different instruments were in agreement, despite thedifferent physical principles used in each. Combined resultsfrom the graphs imply that most fly ash particles are in therange 1-150 f.Lm (Gutierrez and others, 1993). Other studieshave reached similar conclusions. In the Latrobe Valley,Victoria, Australia, Macphee and others (1993) found that inone plant 75% of the fly ash particles were less than 75 f.Lm,and in another 90% of the fly ash particles were smaller thanthis. In a third plant, 24% of the fly ash was in the range212-150 f.Lm, 43% was 150-75 f.Lm and only 25% was lessthan 75 f.Lm.
Mollah and others (1994) collected a tly ash sample fromTexas lignite and leached portions of the fly ash using bothacidic (pHd.O) and buffer (pH<5.0) solutions. The particlesize was determined using the principle of laser diffraction todetermine the particle size distribution by surface area. Theparticle size ranged from 0.4 f.Lm to >400 f.Lm, although thebulk was in the size range 100-400 f.Lm.
There are several methods available to determine particle sizedistribution. Gutierrez and others (1993) used threetechniques on a fly ash sample from the Abono power plant,Asturias, Spain. First the sample was sieved through 37 f.Lmand 80 f.Lm sieves. 17 wt% of the sample was retained by the80 mesh sieve, and 60 wt% was below 37 f.Lm. Thetechniques used were:
Particle size distribution of fly ash is of particular interestwhen the material is used as a substitute for a portion of thecement in concrete and in soil-stabilisation mixtures. Particlesize distribution may have an effect on properties such as:rheology, floc structure of the cement paste, permeability ofthe hard paste and rate of strength gain (Gutierrez and others,1993). For example, it is generally accepted that fly ash withfine particles will participate more rapidly and completely inthe cementitious reactions of blended cement concrete thanfly ash with coarse particles. ASTM C618 specifies that amaximum of 34wt% of the fly ash can exceed 45 f.Lmdiameter (McCarthy and others, 1990; see Section 4.2.1).
39
Physical and chemical characteristics
finer fly ash were found to have greater shrinkage than thosemade with the original un-sieved ash.
The specific surface area of fly ash particles may beconsidered more important than particle size distribution. Thespecific surface area of particles relates to the amount ofsurface available for reactions or interactions with otherparticles. Sneddon (1995) argues that the specific surface areashould be a parameter used in any fly ash classificationsystem (see Section 4.1.6). However, particle size distributionor fineness is easier to measure and the test is morereproducible than any available test method for detelminingthe specific surface area (see Section 4.3.2)
3.5.2 Bulk density
Bulk density is a measure of the density of a granularsubstance. It is calculated for a unit volume of the substanceincluding the pores or spaces between the grains. Thus it isalways less than the true density of the material. The truedensity of a material is determined after grinding. Therelative density of a material is its density relative to water at4°C.
Poole and Bayat (1993) used a Tap-Pak Volumeter and apycnometer to determine the bulk and true densities of flyash samples. The bulk density ranged from 1.04 to1.43 g/cm3 and the true density from 2.03 to 2.61 g/cm3.
Both bulk and true density increased as particle sizedecreased. The major density differences were probably dueto the influence of distributions in mean wall thickness, voidvolume and particle diameter. It appears that the true densityof the fly ash samples and sized products depends more onvoid structure than on the composition of each particle. Veryfine particles «22 fLm) had true densities of 2.5-2.6 g/cm3
and were either thick-walled and void free, or composed ofmore dense glasses and crystalline components.
In their assessment of fly ash from Iowa, USA, Bergeson andSchlorholtz (1992) found that reclaimed ash aggregate haslow bulk relative densities and high absorption values in therange 24-28%. These are the values of a lightweightaggregate. Their freeze-thaw data combined to suggest thatthe reclaimed material may have a very open pore system asonly 28% was lost in the freeze-thaw test.
Analysis of the North American fly ash database (McCarthyand others, 1990) shows a clear trend in the variation ofrelative density with CaO content. Low calcium ashes havelower relative densities and a greater range of values. Thehigh calcium ashes have a mean relative density 19% greaterthan the mean of the low calcium ashes. There are threereasons for the lower relative density of low calcium fly ash:
The glass phases have a more open structure. As networkmodifiers such as Ca, Mg, Na and K increase inconcentration, the bridging oxygens in the network arebroken and open spaces are filled, increasing the densityof the glasses.The low calcium melts have a higher viscosity whichenhances the trapping of gases. Thus more cenospheresand plerospheres are fonned.
40
Low calcium ashes contain a greater amount ofcarbonaceous material which has a lower relative density.
These factors may also be partially responsible for the greatervariation in the relative density of low calcium fly ash.
The relative density of fly ash depends on the shape andchemical composition of the fly ash particles. Themorphology of fly ash may cause problems. For example,during particle size analysis mechanical dispersion canfracture the hollow spheres. Coal particles (containing lessminerals) have a relative density of 1.3-1.6. Spherical lightbrown-black particles in fly ash (containing 50-95% iron asmagnetic iron oxide) have a relative density of 3.6-4.8. Asthe amount of mullite and quartz increases, the densitydecreases. On the whole, the bulk density of fly ash is1.9-2.9 g/cm3 (Joshi and Achari, 1992).
3.6 Chemical propertiesThe physical and chemical characteristics of coal ash controlits behaviour. Many of the engineering properties of fly ashare specific to the application. For example, ease of handling,compressive strength, permeability and expansion areengineering properties of final products made with fly ash.These are discussed in more detail in Chapter 4. Moregeneral properties of fly ash are discussed here pozzolanicity and reactivity.
3.6.1 Pozzolanicity
A pozzolan is a siliceous or siliceous and aluminous materialwhich in itself possesses little or no cementitious property butwhich will, in divided form, combine with CaO in thepresence of water to form cementitious compounds. Apozzolanic reaction is a cementitious hydration reaction ofsilicon oxide; aluminium oxide; andlor iron oxides with analkali which is typically calcium hydroxide (Ca(OHh)·Normally the presence of all three pozzolan oxides isrequired to optimise both the chemical binding and thephysical properties such as compressive strength (Baldwin,1993). Tests for pozzolanicity are discussed in Section 4.3.1.
Portland cement is a hydraulic cement - it requires theaddition of water to react. The main constituents, such astricalcium silicate, dicalcium silicate and tricalciumaluminate, react with water to produce calcium silicatehydrates and calcium hydroxide. The calcium silicatehydrates formed are the main components of the bindingmechanism that makes Portland cement so widely used as acementitious binder (Morris and Bergesen, 1994).
Pozzolans closely resemble Portland cement in their chemicalcomposition except they lack lime and so cannot formcalcium silicate hydrates. However, the hydration process ofPortland cement releases an excess of calcium hydroxide, soa mixture of ordinary Portland cement and a pozzolan suchas fly ash will react to form additional calcium silicatehydrates and therefore increase the cementitious properties ofthe final product. Fly ash finer than 5 fLm reacts with the freelime as soon as it is formed after the addition of water to aPortland cement/fly ash mixture. Coarser fly ash (0-45 fLm)
reacts similarly at first, and then shows little reactivity within56 days (Morris and Bergesen, 1994).
Fly ash with low calcium content is pozzolanic, whereas flyash with high calcium content is hydraulic. Fly ash reactswith lime to form water insoluble calcium silicate andcalcium aluminate, which are highly cementitious. Thepresence of Ca-aluminosilicates in fly ash is thus one of thesources of self-binding properties. The ratio of CaO/(SiOz +Ab03) is important in predicting the self-binding propertiesof fly ash (Barta and others, 1990). This characteristic is usedin the construction industry (Joshi and Achari, 1992).
Fineness/surface area correlates well with pozzolanic activity.For properly cured fly ash blended pastes the most importantmicrostructural feature is the reduction in the coarse porosity>37 nm and the increase in fine porosity as the pozzolanicproducts fill the space. During the pozzolanic reaction, thealuminate and silicate ions dissolved from the glassy phasereact with calcium and hydroxide ions to calcium aluminateand calcium silicate hydrates (Pratt, 1990).
When pozzolanic fly ash is mixed with water, hydrates areformed which deposit on fly ash spheres as a film. The filmis composed of amorphous calcium hydroxide and calciumsilicate hydrate gel. The glassy amorphous surface is the mostreactive of fly ash particles. The densification of these gelsresults in the formation of hydrated shells (Joshi and Achari,1992). McCarthy and others (1990), in their survey of 178North American fly ashes, found that less crystalline, moreglassy ashes had a smaller range of pozzolanic activity whichmay be due to the identity of the glassy phases. For example,a 40% crystalline ash that includes anhydrite, calciumaluminate and lime in its crystalline phases, and has morereactive glassy phases, would be expected to attain greaterstrengths in the lime pozzolanic activity test than an ash with40% of inert quartz, mullite and iron oxides, and a lessreacti ve glass.
The pozzolanic activity of fly ash is also influenced by theratio of illite:quartz glass, as quartz is coarser than illite. Thisis a factor in predicting the pozzolanic activity of fly ash as itis thought that the illite-type glass material is more active inthe binding process than the quartz. The activity is alsoenhanced by the finer size distribution of illite glass (Bartaand others, 1990).
Larbi and Bijen (1990) examined the effects of threelow-calcium fly ashes with different particle fineness,mineralogical composition and glass contents on theevolution and distribution of lime in Portland cement pastesand mortars. Also they studied the influence of these flyashes on the extent of the interfacial zone and the orientationof calcium hydroxide crystals within this zone. Theyconcluded that:
During the first month of hydration of the Portlandcement studied, there was little evidence of pozzolanicreaction from calcium hydroxide measurements. ButSEM studies showed evidence of reaction products in theimmediate vicinity of some of the ash particles withinseven days. Also the ash particles served as nuclei for
Physical and chemical characteristics
precipitation of calcium hydroxide and calcium silicatehydrates. Some calcium hydroxide crystals remained inintimate contact with the ash particles even after aboutsix months of hydration.The fly ash had a great effect on the interfacial zone ofthe cement-paste aggregate. The thickness of the zonewas reduced and the degree of orientation of the calciumhydroxide crystals at the interfacial zone was decreased.These effects are thought to be due to improved particlepacking at the interface and reduced micro-bleeding, andto the pozzolanic reaction between the ashes and calciumhydroxide at the interface. The reduction in the thicknessof the interfacial zone is considered to be a majorcontributor to the effects of fly ash on concreteproperties such as strength and permeability.
The pozzolanic reaction of the interface appears to beconspicuous at an earlier age than in the bulk of the paste.1l1is may be due to accumulation of finer fly ash particles at
the intelface.
3.6.2 Reactivity
The reactivity of most mineral admixtures depends on thereactivity of the glass phase (Bijen, 1994). Much attentionhas been devoted to the difference between various fly ashes.In general, particle size has been identified as a major factorwith respect to reactivity. Fine solids react faster than coarseones. Bijen (1994) concluded that differences in the glasschemistry of fly ashes do not have a pronounced effect on therate of reaction of fly ash. Therefore differences in thereactivity of various fly ashes are likely to be due mainly tothe differences in the specific sUlfaces.
The reaction of fly ash with calcium hydroxide as a result ofits glassy character also reduces the risk of alkali-aggregatereaction. This characteristic may necessitate a new approachto mix design, involving the provision of sufficient fly ash toabsorb reactive species released by the Portland cement
(Mills, 1990).
In a hydrating Portland cement environment, fly ash particlesreact with hydroxide ions that enter vacancies in the glassstructure. These spaces are produced when the modifier ionsmigrate from the aluminosilicate glass structure to the poresolution. Thus modification of the glass structure, with alkalior alkali earth elements, makes it more susceptible to reactionwith hydroxide ions. Substitution of the glass matrixelements by Fe, Cr or Ti decreases the reactivity becausethese elements replace AI and Si and are more resilient tohydroxide ion reactions. Also, in aluminosilicate glasses ahigher Si concentration enhances reactivity becausealuminium atoms are less susceptible to reaction withhydroxide ions than are silicon atoms (Dewey and others,
1994).
Temperature and the pH development of the blended cementpaste have an etlect on fly ash reactivity. It is unclearwhether ash chemistry and/or ash glass structure also have aninfluence, as is the case with some blast furnace slags.Temperature and pH seem to have a more pronounced effecton fly ash glass dissolution than differences in ash chemistry,
41
Physical and chemical characteristics
although the influence of chemistry may not be entirely ruledout. For example, fly ash glass with a low alumina contentand (possibly) a high calcium content may react faster thanthat with a high alumina content (Pietersen and others, 1990).In the study by Larbi and Bijen (1990) on the effects ofdifferent low-calcium fly ashes on the lime in Portlandcement pastes and concretes it was concluded that - the finerand and the more glassy the fly ash, the greater appears to bethe consumption of lime.
Chemical phases that are unstable like lime, brucite(Mg[OHh), some Ca-silicates and soluble salts, will causeincreased reactivity when in contact with water or watervapour and possibly swelling, heat liberation and increasedalkalinity. These reactions will have an immediate effect onthe mechanical stability of the product (Kuhnel and others,
42
1992). The dissolution of fly ash is sensitive to thecomposition of the surrounding solution so the alkalinity ofthe pore water is important (Bijen, 1994).
3.7 Summary and commentsFly ash from coal combustion is complex and variable innature but has many properties which are useful to certainapplications. In order to ensure that fly ash is utilised tomaximum advantage it must be adequately characterised.Microscopic and spectroscopic techniques can providevaluable information on the nature of individual fly ashsamples. However, for bulk uses on a larger scale, simple andbasic classification schemes are required. These are thesubject of Chapter 4.
4 Classification and specifications
Pulverised coal fly ash is an important commodity toindustries such as those producing cement and concrete.However, fly ash is variable in characteristics and the buyerneeds to be able to determine that the fly ash is suitable foruse in a particular application. Classification andspecification schemes are necessary in order to ensure properand predictable pelformances of fly ash for each required use.There appears to be some confusion in the literature over themeaning of the terms classification and specification. For thepurpose of this report, classification schemes are those whichcategorise the fly ash itself on intrinsic properties, regardlessof end use. The term classification is also often used to meanthe physical separation of fly ash into batches based on acertain characteristic. For example, some fly ashclassification plants separate fly ash into batches based onparticle size. Actual physical treatment of fly ash in this wayis discussed in greater detail in Chapter 5. In this chapter, thetype of classification discussed is the definition of a fly ashsample as belonging to a particular class or category.Specification schemes are those which define properties offly ash which are necessary for its use in differentapplications. There is an inevitable overlap betweenclassification and specification. Classification schemes for flyash most commonly rely on properties which determine theapplicability of the fly ash as an admixture in cement.Specifications may simply require that fly ash of a particularclass be used or may require far more detailed properties tobe determined and limited. Test procedures for classificationand specification are the same and are thus discussed in aseparate section, Section 4.3.
4.1 ClassificationMattigod and others (1990) cite a definition for the purposeof classification:
... to organise knowledge. to provide a mnemonic forproperties and to discover new principles andrelationships between properties.
There are two levels at which fly ash may be classified.Grading systems classify the fly ash in a simple manneraccording to basic physical characteristics. These systems aresummarised briefly in Section 4.1.1.
There are two types of classification system which are moredetailed and which may be applied to fly ashes: taxonomic,based on their characteristics, and utilitarian, based on theiruse. According to Mattigod and others (1990), one taxonomicand three utilitarian schemes have been proposed forclassifying ashes. These schemes are summarised in Table 6.The US EPA (US Environmental Protection Agency)classification scheme may be used to evaluate all disposablesolid wastes (including fly ash) according to leaching risks.The wastes are classified as either toxic or non-toxicaccording to measured concentrations of trace elements. Thisclassification scheme is not relevant to this report and is notdiscussed further. Since classification schemes generallydevelop from or improve upon older schemes it is necessaryto include the earlier schemes in the following sections.
4.1.1 Grading systems
The simplest types of 'classification' system for fly ash aregrading systems such as those based on fineness and/or losson ignition (LOI, see Section 3.2.3). For example, within theAustralian Standard for fly ash use with Portland cement(AS 3582.1), fly ash is initially categorised into 3 grades:
fine - where 75% by mass passes through a 45 j.Lm sieve andthe maximum LOI is 4%;medium - where 60% by mass passes through a 45 j.Lm sieveand the maximum LOI is 6%;coarse - where 40% by mass passes through a 45 j.Lm sieveand the maximum LOI is 12%.
Another grading system which could be loosely defined as a'classification system' for fly ash is the state in which it isdelivered for usc. Pulverised coal fly ash can be supplied for
43
Classification and specifications
Table 6 Classification schemes applicable to fossil fuel residues (Mattigod and others, 1990)
Basis
Utilitarian
Utilitarian
Utilitarian
Taxonomic
Differentiating criteria
Particle sizeModulus of acidityLoss on ignitionSilicate modulusGypsum contentAnthracitic/bituminous(Si02 + Ah03 + Fe203) 2'"70%
Lignitic/subbituminous(Si02 + Al203 + Fe2 03) 2'"50%
Concentrations of specifiedelements and organics in extracts
Sialic, calcic, and ferric contentspHTexture
Classification
4 Classes6 Classes
Class F
Class C
Toxic, non-toxic
7 Groups3 Classes12 Classes
Reference
Kocuvan (1979)t
ASTM (1988):1:
USEPA (1982, 1986)§
Roy and Griffin (1982)~
t Particle size classes (A, B, C and D) based on defined limits on particle size distribution.Modulus of acidity: (Si02 + Ah03 + Fe2 03)/(CaO + MgO - 0.7S03); <l = strongly basic; 1-2 = basic; 2-3 = neutral: 3-10 = weakly acid;10-20 = medium acid; >20 = strongly acid. Loss on ignition undefined in classification.Silicate modulus: Si02l(Ah03 + Fe203); aluminate modulus: Ah03/ Fe03. Gypsum content: 1.7 S03
~: Additional properties are same for both classes of fly ashes: for example SOJ $5%; alkali as Na20 $1.5%; moisture content $3%;loss on ignition $6%; amount retained on 45 fLm sieve $34%; and other specified engineering properties
§ EP test: Extraction made with 0.5 M acetic acid (pH 5.0 ± 0.2) and analysed for Ag, As, Ba, Cd, Cr, Hg, Mn, Se and six organiccompounds. TCLP test: Extraction made with acetic acid solution at pH 2.9 or 4.9 and analysed for same eight elements as in EP test and44 organic compounds. Classification into toxic and non-toxic classes is based on whether concentrations exceed prescribed limits
~ Sialic = percentage of (Si02 + Ah03 + Ti02), calcic = percentage of (CaO + MgO + Na20 + K20), ferric = percentage of (Fe203 +S03);groups: sialic, fersic, calsialic, modic, ferric, calcic, and fercalcic; pH classes: acidic, neutral, and basic
engineered fill in several forms. In the UK, conditioned flyash is defined as freshly produced ash which has had acontrolled amount of water added to it prior to immediatedelivery to the site. TIle strength of conditioned ash increaseswith time after compaction and the extent of strengthdevelopment varies - not all ashes develop strength.Conditioned fly ash is considered a selected cohesive fill andtherefore is suitable as a structural fill behind retaining wallsand in reinforced earth. For this purpose, the conditioned flyash must meet strength criteria. Stockpiled fly ash andreclaimed lagoon ash have been stored for some time beforeuse and can only be used for general fill which have nostrength criteria. The water content of stockpiled ash can becontrolled less easily than conditioned ash. Stockpiled ash isconsidered poorer because the water content is fixed at theequilibrium water content achieved during storage. There willbe no further gain in strength for stockpiled ash and it maycontain agglomerated lumps (Clarke and Coombs, 1995). Thecurrent Department of Transport specification in the UK forpulverised fuel ash classifies the ash as either fresh ash,suitable for selective cohesive fill, or stockpiled ash, forgeneral fill. TIlis leads to the inference that fresh ash is betterthan stockpiled ash. Yang and others (1995) disagree andhave proposed a classification system for fly ash basedsimply on self-hardening properties. The proposed system isoutlined in Figure 12. According to Yang and others (1995),not all fresh ashes are self-hardening and some stockpiledashes are. In order to establish the classification of an ash,triaxial tests should be conducted after 1 day then after 28
44
days on saturated samples to give the worst case scenarioresults.
4.1.2 Kocuvan's system
The major use of tly ash is in the cement and concreteindustry and therefore classification schemes tend toconcentrate on the characteristics of fly ash which make itmost suitable for this purpose. The scheme suggested byKocuvan (1979) was designed to determine the suitability offly ashes as pozzolanic and cementitious materials and wasbased on differentiating criteria such as particle sizedistribution, acidity, ratios of concentrations of majorelements, gypsum content and LOr. Four classes (A, B, Cand D) can be differentiated by particle size and a further sixsub-classes may be defined according to acidity. Mattigodand others (1990) point out that Kocuvan did not specifydifferentiating criteria for all the properties included andtherefore the classification system was incomplete. Roy andGriffin (1982) suggested that the system by Kocuvan wasdesigned to suit the needs of research into concrete madefrom fly ash and was not a classification system for fly ash ingeneral.
4.1.3 Triangular system
The taxonomic system proposed by Roy and Griffin (1982)differentiates according to chemical composition, thehydration pH, and particle size distribution. Fly ashes can be
Classification and specifications
fresh ash
fresh conditioned stockpiledash ash
self-hardening non self-hardening self-hardening non self-hardeningash ash ash ash
selected cohesive general selected cohesive generalfill fill fill fill
Figure 12 Classification of pulverised fuel ash based on self-hardening properties (Yang and others, 1995)
(Rare or not known to exist)
oo
Calcic
\\
\\
\
Calsialic
Fercalsic
Fersic
//
//
/
12 ~ -\
Ferric
12 29 52
Calcic group, %
Figure 13 Triangular graph for coal ash classification (Roy and Griffin, 1982)
45
Classification and specifications
classed into seven groups based on their sialic, calcic andferric contents as defined by the percentages of the sum ofoxide concentrations:
sialic - (Si02 + Ah03 + Ti02);calcic - (CaO + MgO + Na20 + K20);ferric - (Fe203 + S03).
The triangular graph used in this coal classification system isshown in Figure 13. Within these taxa the ashes can besub-classified into three pH classes (acidic, neutral and basic)and 12 textural classes. Mattigod and others (1990) warn thatthe major limitation of this system is that a large number offly ashes fall into the same group whereas some groupscontain few fly ashes. The triangular system cannot be usedto predict properties and thus is not a useful classificationscheme.
4.1.4 ASTM system
In a similar manner to the scheme suggested by Kocuvan(1979), the ASTM standard is designed to determine thesuitability of fly ashes as pozzolanic and cementitiousmaterials. The scheme proposed by the ASTM (1988)defined two classes of fly ash (Class F and Class C) based onsource coal and specified major element oxide contents (seealso Chapter 3). Mattigod and others (1990) summarise theseas:
Class F - derived from either anthracite or bituminouscoal and contain a minimum of 70% (Si02 + Ah03 +Fe203);Class C - derived from lignite or subbituminous coal andcontain a minimum of 50% (Si02 + Ah03 + Fe203).
There are also additional criteria for each class based onchemical, physical and engineering properties.
Although Class F and Class C fly ashes are commonlyassumed to arise from the different coal types, Class F fly ashcan be produced from coals which are not bituminous, forexample some subbituminous Alberta coals. Similarly,bituminous coals can produce ash which is not Class F. TheASTM classification itself does not require that the fly ash isproduced from any particular coal type (Joshi and Lohtia,1995). According to Manz (1995b) little or no referenceshould be made to the type of coal from which a fly ashsample arises. Rather, Manz suggests that lime content ismore important. He cites Idom (1982) who defined twodistinct groups of fly ash for use in cement: 'the lime-rich,which are cementitious, ie they possess enough free lime tolaunch their own hydration; and the lime-poor, which aretruly pozzolanic ie they need an activator to becomehydraul ic.' Manz (1995b) therefore recommends that Class Cand Class F be redefined as follows:
Class F - exhibiting no hydraulic properties unless in thepresence of a solution saturated with Ca(OH)2;Class C - (lime-rich) having hydraulic properties.
The remainder of the classification parameters - chemistryand so on - should remain the same. Manz (1995b) also
46
suggests that a simple performance test be included onstrength of cubes containing fly ash but no cement. If thestrength at 3 days is greater than 500 psi (3.45 MPa) then thefly ash is Class C, if it is lower then the fly ash is Class F.This type of classification scheme is tending more towardsbeing almost a specification and assumes that the fly ash isgoing to be used in cement, concrete or similar product. TheASTM classification scheme is included in the often updatedASTM C618 - Standard specification for fly ash and raw orcalcined natural pozzolan for use as a mineral admixture inPortland cement concrete. Since the remainder of ASTMC618 refers to specification of ash for use in concrete, it isdiscussed in more detail in Section 4.2.
4.1.5 Russian Academy of Sciences system
Savinkina and others (1990) at the Institute of the Chemistryof Solid Fuels and Mineral Processing at the SiberianDepartment of the USSR Academy of Sciences suggest aclassification scheme based on calcium oxide content andcalcination losses, the two characteristics that they define asthe most important for industrial quality. Calcination losscharacterises the amount of unburnt material, water andCaC03 in the ash. Ashes of all classes can be used in theproduction of clay bricks and cement and, at a calcinationloss of 10%, used as an additive to clinkers. There are fiveclasses defined:
Class I - low calcium ash with calcination loss <10%. Thisash can be used instead of sand and fine filler in reinforcedconcrete and mortar. A plasticising effect is provided.
Class II - medium calcium ash with calcination loss <10%and high calcium ash with calcination loss between 5% and10%. This ash has high pozzolanic activity and can be usedin mixed binders as hydraulically active additives to cement,lime or gypsum. A plasticising effect is provided.
Class III - high calcium ash with calcination loss below 5%.This is the most variable type of ash and can be used to makemortar and ash concretes for road and low-rise residentialbuildings. This class of ash may be added to cement up to30% without reducing the quality grade of the cement butimproves the plasticity and frost resistance of products.
Class IV - ultra-high calcium ash with a calcination lossgreater than 5%. This ash is recommended as the lime-silicatecomponent for production of cement from a two-componentblend. If the calcination loss is below 5%, mechanochemically activated ash can be used to make autoclaveproducts and mixed binders with special additives can also beproduced.
Class V - High content of unburnt material, a light porousfiller of claydite or agloporite type can be produced.
4.1.6 New systems
New European standards are being prepared and within theproposed standard for fly ash use in cement, ENV 197, twotypes of fly ash are defined within the standard with respectto their CaO content. Fly ash with a reactive CaO content
Classification and specifications
Fly ash classification within ENV 197 (vom Berg,1993)
less than 5% is 'siliceous' whereas that with reactive CaOgreater than 5% is 'calcareous' (vom Berg, 1993). Moredetails on the two classes of fly ash are given in Table 7 andthe specifications included in the table are discussed in moredetail in Section 4.2.
Table 7
Parameter Siliceousfly ash
Calcareousfly ash
4.2 Specifications
Microscopy techniques are being used more and morecommonly in the classification of fly ash. Dewey and others(1994) recommend scanning electron microscopy, electronmicroprobe analysis and energy dispersive X-rayspectroscopy to identify phase composition within individualfly ash particles. The information gained from thesetechniques could be used to correlate ash composition withconcrete performance.
concrete products. Instead, a two digit classification schemeis proposed. The fly ash will be awarded a value betweenoand 10 for its cementitious properties and another value forits pozzolanic properties. In this scheme a I% fly ash wouldbe highly cementitious, a 0/1 0 would be highly pozzolanicand a 010 would be an inert filler. Although such a schemehas not been tried and tested, Hassett and Eylands suggestthat heat of hydration could be an important parameter. Onadding to water, fly ash gives off heat at a specific raterelated to the type of minerals present (for example, brownmillerite, mayenite and other calcium aluminate phases).X-ray diffraction of the sample following hydration provideseven more information on the minerals present. A study isunder way to determine any con-elations between temperaturechange and other empirical test results. A protocol is beingdeveloped for an improved classification scheme for coal ash.
* for fly ash containing 5-15% reactive Caai' for fly ash containing>15% of reactive Caa
5>525*10'1'10
5<525
Loss on ignition, max, %Reactive Caa, %Reactive Sia2, min, %
Strength at 28 days, min, Nlmm2
Le Chatelier expansion, max, mm
Joshi and Lohtia (1995) consider that cun-ent classificationsystems for fly ash do not adequately define the type ofbehaviour to be expected when the materials are used inconcrete. Instead, they suggest that LO} is the principalparameter for determining fly ash performance in cement andconcrete. They argue that the variability of the LOI isessentially due to the coal mineralogy and that aclassification system is possible based on LOI for the use offly ash as a pozzolanic admixture in cement and concrete. Anextensive review of appropriate literature lead Joshi andLohtia to suggest that fly ash generated from bituminouscoals with 1-6% LOI is invariably pozzolanic but notself-cementitious. Fly ash produced from bituminous,subbituminous and lignite coals containing less than 1% LOIare self-cementitious to some degree as well as beingpozzolanic. The degree of self-hardening is very variable.Joshi and Lohtia therefore suggest that the most importantparameter is the LOI and that fly ashes should be classifiedas low and high ignition loss fly ashes rather than Class ForClass C. These two proposed classes are summarised asfollows:
Several authors (Dewey and others, 1994; Sneddon, 1995)recommend that a new classification system should bedeveloped based on those characteristics which have thegreatest impact on the performance of the fly ash. Forexample, important parameters would include particle size,amorphous (alumino-silicate glass) content and compositionand reactive water soluble phase content. Sneddon (1995)suggests that the most important parameters with respect tostabilisation applications are 24-hour compressive strength ofash mortar, free lime content and specific surface. Sneddon isthus arguing that fly ash should be selected by a classificationsystem including more tests which are likely to indicate thecompliance of fly ash with the specifications for its use incement or concrete. Specifications are discussed inSection 4.2.
Class I: pozzolanic but not self cementitiousfor example, bituminous ash having generally Lor >1%Class II: pozzolanic and self cementitiousfor example, subbituminous and lignite ash, LOI <1 %
There is already equipment available on some modern powerplants for the determination of the carbon content of fly ashwhich could automatically classify the fly ash produced(Joshi and Lohtia, 1995).
Hassett and Eylands (1995) warn that the ASTMclassification system described in Section 4.1.4 above doesnot have any indication of pozzolanic or cementitiousproperties associated with it, although this is often implied.No current classification method provides a continuous-scalerating for pozzolanic/cementitious behaviour of fly ash andnone provides adequate infonnation to assess the reactivity ofthese materials outside of their limited use in cement and
According to Fitzgerald and others (1994) there are threetypes of specification system which may be applied to coalash:
Ingredient based specification. For example, ASTMC618. Such specifications are stringent on chemical andphysical characteristics of the fly ash.Product performance based specification. Here the coalash is used as an ingredient in the manufacture of aparticular product. The characteristics of the finalproduct are specified but those of the individualingredients are not or are easily met. Such applicationsinclude flowable fill, stabilised road base, fertilisers,potting soil and sludge stabilisation.Non-specification requirements. Applications of this typemay have a specification to meet but one that iscustomer or market prescribed rather than being anindustry standard. An example of this is structural fill.Coal ash may be blended with a binder such as cement
47
Classification and specifications
to produce a higher peIiormance and more valuableproduct whilst saving on crushed stone and pavementcosts.
Ingredient based specifications define the type of fly ashwhich may be used in a specific application. In thesespecifications, sometimes referred to as prescription basedspecifications, fly ash which does not meet the specificationscannot be used. In product performance specifications any flyash may be used as long as the product is of the requiredstandard. PeIiormance based specifications are also known as'end product control' and are criteria which the final productmust meet - commonly strength, dry density and so on. Sincethese are product-specific, any raw material, including flyash, which provides the final characteristic product may beused. In some cases this may be fly ash which has failed tocomply with ingredient based specifications. For example,according to Poulsen (1996) the only specification inDenmark for fly ash use in filling materials for civilengineering and landscaping and so on is that it must besuitable for direct compressing. For use in aerated concreteblocks it must only be of a steady quality (Poulsen, 1995).Non-specification requirements are product-specific and evenuser-specific and therefore are not discussed further in thisreport.
As mentioned in Section 4.1.6 above, many of the proposedclassification schemes for fly ash separate the ashesaccording to criteria which make them suitable for use incement or concrete. That is, some classification schemesconcentrate on criteria which will determine whether a flyash sample complies with specifications set for cement orconcrete manufacture. However, such classification schemescan only apply directly to ingredient based (prescription)specifications. They can never guarantee peIiormance. In theUSA there is much debate over peIiormance versusprescription based specifications (Manz, 1995b). Dewey andothers (1994) and Khiinel and others (1992) are of theopinion that the ASTM standard of Class C and Class F ismore of a quality control tool than a predictor of strength ordurability for concrete since it is based on limited physicaland chemical ash characteristics. The current ASTMprocedure for specifying fly ash for use in concrete thereforedoes not provide a reliable indicator of expectedperformance. Sneddon (1995) also emphasises that theuncritical use of ASTM 618-91 may not assure a satisfactoryperformance in applications in the construction industry notinvolving concrete. Further, failure of an ash to meet ASTM618-91 should not preclude its use. Other standard testmethods more relevant to the applications and finalperformance should be selected and, if necessary, modified tomeasure the relevant properties. Nunes and others (1995)argue that the ultimate aim should be the introduction ofspecifications which are blind to the source of the materials.End performance specifications should be promoted based onsimple and contractually sound test methods. For example, asupplier of granular material for pavement constructionshould have to prove that his material is suitable for the joband not have to pretend that it is a soil. Clarke and Coombs(1995) agree that acceptance criteria for uses such asengineered fill are sometimes based on incoITect testspecifications, such as soil tests being used for testing other
48
materials such as fly ash. Some materials may then beexcluded from use despite the fact that they meet designcriteria. Rockliffe (1995) agrees and points out that, withrespect to the use of fly ash in earthworks, compliance with'compactibility' criteria rather than 'fit for purpose' criteriacan lead to the use of higher quality aggregates than isstrictly necessary. The new European (CEN) standards aredesigned to ensure that baniers to trade are removed for'products based on the market' and the new standards aim tobe blind to the source of the material and to reflect the widerange of national practices across Europe (Rockliffe, 1995).
Specifications for fly ash usc in different applications varyfrom country to country but generally the same types ofcriteria apply to the fly ash. Over 40 countries around theworld have set specifications for the use of fly ash inconcrete (Manz, 1995b). European standards are continuingto be set for the use of fly ash in building materials.Countries within the European Union are required to eitheradopt such standards or follow their own similar, if not morestringent standards. In the USA most of the test methods andspecifications for the use of coal ash are found in theAmerican Society for Testing and Materials (ASTM) booksof standards. Identical or similar standards are also publishedby the American Association of State Highway andTransportation Officials (AASHTO) as well as various stateDepartments of Transportation. Additional specifications arepublished by the American Petroleum Institute (API), theSulphur Institute, the mineral wool industry and WestVirginia University. According to Manz (1994) it is difficultto keep up with the most recent specifications, especiallythose from the ASTM since the committees meet twiceyearly and have time consuming ballots to edit and amendspecifications. ASTM issues specifications for blendedcement containing fly ash, for sulphate resistance, and foralkali aggregate reaction as well as for fly ash for use inconcrete, in oil well cement and in grout. Coal ash isspecified for use in ash-lime stabilsation, as lightweightaggregate, for mineral filler and for structural and flowablefill.
Since many of the specifications and standards in eachcountry or group of countries are similar, a few examples arechosen for discussion here. The following sections deal firstlywith specifications for cement and concrete, followed byspecifications for other applications.
4.2.1 Cement and concrete
Cement and concrete provide the largest market for fly ashand therefore the majority of specifications are for theseapplications. A short introduction to cement and concrete isincluded here. However, Chapter 2 deals with fly ash use inall applications, including cement and concrete, in greaterdetail.
Cement is defined as any bonding agent used to uniteparticles in a single mass or to cause one surface to adhere toanother. Portland cement is obtained by burning lime andclay. When mixed with water and sand or gravel, Portlandcement turns into mortar or concrete. For example, in theNetherlands pulverised fuel ash is used almost exclusively as
Table 8 Specification for coal fly ash for use in Portland cement in various countries (based on Manz, 1995b)
South USA USA
Australia Austria C<U1ada China Denmark EU Finland France Germany India Israel Japan Korea Malaysia Africa Spain Sweden Taiwan Turkey Russia UK ASTM AASHTO
Loss on ignition, max wl% 12.0 5.0 12.0 5.0 5.0 5.0 5.0 7.0 5.0 12.0 8.0 5.0 12.0 6.0 5.0 6.0 5.0 12.0 10.0 10 6.0 12.0 5.0
(6.0>" Class A (6.0)a (6.0)a
Moisture, max wt% 1.0 1.0 3.0 1.5 3.0 1.0 1.0 1.0 1.5 3.0 3.0 3.0 3.0
Autoclave expllilsion or soundness 0.8b 0.8b 10mm'" 0.8 b
max % or max mm 10 mm" 10 mm" 0.8b 10 mm" 0.8b 0.8b
Si02. min wt% 25 40 35.0 45 40
Si02 + A1203. min wt% 70
Si02 + AI203 +Fe203, min wt% 70.0 70.0 50.0 70 70.0 70 70
(50)" (50)a (50)a
MgO.J max wt% 4.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
CaO (total), max wl% 10 8.0 10.0 30
(4O)a
CaO (free),c max, wt% 3.0 1.0 1.5 5.0 2.0
S02. max wt% 3.0 5.0 5.0 3.0 4.0 3.0 3.0 2.5 3.0 2.75 5.0 5.0 2.5 2.5 4.5 4.0 5.0 3.0 2.0 5.0 5.0
CI, max wt% 0.1 0.1 0.1 0.05 0.1 0.1 0.1 0.1Na20 available alkalies,t", max wt% 1.5 6.0 4.0 1.5 1.5 1.5 1.5 1.5
Particle size
Specific surface (Blaine), min rn2/kg 200 320 240 160 300
<20 ~m. min wt% 30 20
<40 fJ-m. min wt% 50
<50 tJ-m. min wt% 40
<80 tJ-m. min wt% 70
<200 tJ,m, min wt% 0.3
<315 fJ-m, min wt% 98
>45 J.Lm, max wt% 25g 34 12 40 40 25 12.5 12.5 40 34 12.0 34 34
>90 J.Lm, max wt% 55 15>150 J.Lm, max wt%
Pozzolanic activity with cement
7 days. min % 68 75 7528 days. min % 75 75 70 80 75 60 75 70 75 70 85 80 75 75 ()
90 days, min % 85 85 70 70h 90 85h Pi"(f)
Water requirement, max % of control 95 105 105 105 100(f)
102 105 95 100 105 95 :;;With lime, MN/m2 @ 7 days 4.0 5.5 5.5 5.5
()
~0'
replaced by ENV 450 ::J
IIIbracketed values are for ash with high lime content (Type C) ::JASTM C 15 autoclave expansions; 20% fly a,11/80% cement a.
(f)ENV 450 Le Chatelier Soundness Test; 50% fly ash/50% cement '0Australia - 4% OK if aUlocalve not >0.8 CDEurope<m Community - may be higher than I% but less thill1 2.5% if soundness limit met ()
::;;Denmark - water ,oluble; France - Na20 + K20; Germany, India, Sweden, Taiwan and USA - Na20 equivalent n'fine ash ~91 days 0'
~ ::JCO (f)
Classification and specifications
a pozzolan or filler in concrete (Lamers and van den Berg,1995). Fly ash may be usd as a pozzolan within Portlandcement, or simply to replace the sand or gravel bulk, or as acombination of both. Fly ash as an inert filler cannot replacesand and gravel but can be used as a supplement to improvethe granulometric composition of sand and gravel. Forexample, ASTM C593 for blended cement allows up to 40%fly ash to be added to cement during its manufacture. This flyash may be blended with already processed cement or may beground in with the cement clinker (Joshi and Achari, 1992).
Since cement is used in the manufacture of concrete there is asignificant amount of overlap between specifications for bothapplications. Some of the specifications are prescriptionbased whereas others are performance based. Many containboth prescription and performance based criteria. Therefore,for clarity, the following sections pull together available dataon specifications for the various applications in differentcountries. Since the standards set in the USA and by theEuropean Union often form the basis of standards in othercountries, it is necessary to introduce ASTM and ENVstandards before those set elsewhere. Over 20 countriesaround the world have specifications for fly ash use inconcrete. According to Manz (1995b) these are all verysimilar to ASTM C618. Specifications used around the worldare summarised and compared in Table 8. It is important tonote that some of the data in this table date back as far as1981 and no further data have been made available. All the
specifications include a LOI limit (from 4% to 12%), mosthave S03 limits, and almost all have a limit on fineness andcompressive strength. Many of the specifications includeMgO, (SiOz + Ah03 + FeZ03), autoclave expansion,available alkali, free lime, chloride and moisture contentlimits.
The USA has perhaps the most established specifications forfly ash use. ASTM and other specifications used in the USAfor fly ash use in various concretes and cements aresummarised in Table 9. If the fly ash is to be used as blendedcement then ASTM C595-93 applies (Manz, 1994). Here lessthan 25% should be retained on a No 325 sieve and thereshould be a minimum of 75% pozzolanic activity withcement. There is also a maximum set for alkali reactivity(0.05%). When the fly ash is to be used as a pozzolan thenASTM C618-93, ASTM C311-87 and AASHTO M295-93 allapply. ASTM Cl50 applies when fly ash or bottom ash is tobe used as a raw material in concrete. TIJis standard sets thespecifications based on chemical limits (Manz, 1994).ASTM C618 is the 'specification for fly ash and raw orcalcined natural pozzolan for use as a mineral admixture inPortland cement concrete'. A summary of ASTM C618requirements was given in Table 8. More details are alsoincluded in Tables 10 and 11. Fly ashes which meet thespecifications set out in ASTM C6l8 result in concreteswhich have lower water requirements, increased flowability,lower early strengths, higher long-term strengths and lower
Table 9 Specifications relevant to fly ash" use in the USA (Manz. 1994; 1995b)
Application
Readymix concrete
Mass concreteConcrete pipe
Concrete block
Grout
Fly ash slurry
Well cement
Sulphur concrete
High flexural strengthceramics
Mineral woolAsh brick
Standard
ASTM C618-93ASTM C33ASTM C618-93ASTM C595
ASTM CI4ASTM C618-87ASTM C331-87
ASTM C618-87ASTM C938
ASTM C618-93 orASHTO MZ95-93ASTM C618-93API 10Sulphur Institute
Details
cement and concrete. see Section 4.2specification for concrete aggregates40-50% replacement for cement with fly ash, since the heat of hydration is loweredblended cement; <20% retained on No 325 sieve; >75% pozzolanic activity;(Si02+ Ab03 + Fe203) >70% for moderate sulphate resistance; alkali resistivity <0.05%minimum 470 lb cement/yard3of concretecement and concrete, see Section 4.2bottom ash as lightweight aggregates for concrete masonry units. LOI <12%. Organicimpurities set in ASTM C40cement and concrete. see Section 4.2practice for proportioning grout mixtures for preplaced aggregate concrete. Trial batchto contain 2: I Portland cement:fly ashcement and concrete, see Section 4.2
cement and concrete. see Section 4.2materials and testing for well cementsClass F fly ash - 100% passing 200 mesh; bottom ash - ASTM C33, ASTM C 127 andC128 apply, moisture absorption <2%, <2% loss by weight by leaching in 24 hours in a140°F solution of70 wt% HCIpassing 100 mesh. Na20 and K20 content %
bottom ash - acid/base ratio 0.8-1.2 (wt% Si02+ Ah03)/(CaO + MgO)fly ash and slag - at a ratio of 3 or 4: I; slag - 10-20 mesh; fly ash - 3-6 fLm (modal).4--9 fLn1 (median). Water soluble mineral content <6.5% «2.5% recommended).Water soluble sulphur content < 1.24% «0.35% recommended). Carbon <3%(6-7% acceptable). Iron <20% (otherwise causes shrinkage and high bulk density).Calcium and magnesium compounds overcome the colouration caused by the ironand cause a narrow vitrification range
* all details apply to fly ash unless otherwise stated
50
Classification and specifications
Table 10 Chemical requirements for fly ash under ENV 450and ASTM C618 (vom Berg, 1993)
Si02 + AI20, + Fe20" min, % 70,0Loss on ignition, max, % 5,Ot 6,Oj:Chloride, max, % 0.10Free calcium oxide (CaO), max, % 2,5§Moisture content, max, % 3,0Available alkalies ~ as Na20, max, % - 1,5
Within Europe, almost every country currently has a separatespecification for the use of t1y ash in concrete (Manz, I995b).The European Commission is working on a commonspecification. As discussed in Section 4.1 above, fly ash isclassified into two groups, siliceous and calcareous, under thenew European cement standard ENV 197. Table 12 showsthe draft of the standard for the four different blended
* all requirements specified as characteristic values (lO%-fractile)t tly ash with LOI up to 7% by mass also be accepted on
national basis~: up to 12% may be approved by the user if either acceptable
performance records or laboratory test results are made available.§ more than I% and less than 2.5% is acceptable if complying
with soundness requirements11 optional requirement
There is a draft standard in Europe, ENV 206 - 'Concrete performance, production and compliance'. This standard setscriteria for the amount of cement in concrete and thewater/cement ratio in order to ensure durability. The limitvalues depend upon the type of concrete and onenvironmental conditions (exposure classes) as shown inTable 13 (vom Berg, 1993). ENV 206 allows a reduction inthe minimum cement content and a water to cement ratiogreater than the maximum according to national regulations.In a number of countries, including Italy, fly ash is notallowed to be taken into account as an active addition withinENV 206 (ENEL, 1995). A new draft of ENV 206 iscurrently being prepared by a CEN task group.
The European ENV 450 standard has been directly compared
ENV 450 is the new European standard for 'Fly ash inconcrete - definitions, requirements and quality control'. Itdeals mainly with hard coal fly ash since brown coal fly ashis only of regional significance in Europe. Fly ash fromlignite may be accepted if the CaO content is below 10% bymass and if it complies with other requirements of thestandard. ENV 450 recommends that fly ash to be used as anadditive in cement should meet the requirements listed inTables 10 and II (vom Berg, 1993). Within ENV 450, allrequirements are specified as characteristic values.Continuous statistical quality control is required from thesupplier at every generating facility to ensure that the fly ashconforms with the standard (see Section 4.4). ENV 450 hasthe status of a DIN standard in Germany and is used incountries such as Italy and Denmark (Poulsen, 1995). It willsoon be adopted in the UK to replace BS 3892.
cements which include fly ash. The four cement types aredivided according to fly ash content and by the type of flyash (vom Berg, 1993). ENV 197 also includes quality controlcriteria to be adopted for production and delivery (ENEL,1995). The fly ash used must comply with ENV 450 (vom
Berg, 1993).
ENV 450* ASTM C618Class F
RequirementParameter
heat of hydration (see Chapter 2). However, the amounts ofimprovement in each parameter are fly ash specific and arenot defined by ASTM C618 class designations (EPRI, 1987).AASHTO M295-93 is similar to ASTM C618 but includesan additional criteria of a maximum of 30% CaO for Class Ffly ashes and 40% for Class C. The limit of 1.5% foravailable alkalis, the same as in ASTM C618, is optional asis a dry shrinkage maximum of 0.03%.
Table 11 Physical requirements for fly ash under ENV 450 and ASTM C618 (vom Berg, 1993)
Parameter
Amount retained when wet sieved on 45 fLm sieve, max, %Activity index at 7 days. min, percentage of controlActivity index at 28 days, min, percentage of controlActivity index at 90 days, min, percentage of controlActivity index with lime at 7 days, min, kPaWater requirement, max, percentage of controlAutoclave expansion or contraction, max, %Le Chatelier expansion, max, mm
Uniformity requirements:Percent retained on 45 fLm, max, percentage points from averageSpecific gravity. max, variation from average. %Specific gravity, max. variation from average, kg/m'
* all requirements specified as characteristic valuest only for tly ash with free CaO <2.5% and> I%
51
Classification and specifications
Table 12 Blended fly ash cement as defined by ENV 197 (draft) (vern Berg, 1993)
Proportion by mass*
Cement Designation Notation Clinker Slagt Siliceous Calcareous Minortype fly ash fly ash constituentst
Portland fly ash cement IIIA-V 80-94 6-20 0-5IIIB-V 65-79 21-35 0-5IIIA-W 80-94 6-20 0-5IIIB-W 65-79 21--35 0-5
II Portland composite IIIA-M 80-94 < 11-35§ >cement 65-79 < 35-55§ >
IV Pozzolanic cement IlIA 65-89 11-35~ 0-5IllS 45-64 35-55~ 0-5
V Composite cement VIA 40-64 18-30 18-30# 0-5V/S 20-39 31-50 31-50# 0-5
* refers to the cement nucleus, excluding calcium sulphate and any additivest granulated blast furnace slag:1: for example, filler, or may be one of the major constituents if not already included in the cement§ fly ash, slag, silica fume, pozzolans, burnt shale, limestone, filler or mixes~ tly ash, silica fume, pozzolans or mixes# fly ash, pozzolans or mixes
Table 13 Durability requirement related to environmental exposure according to ENV 206 (vern Berg, 1993)
Requirements Exposure class
2a 2b 3 4a 4b Sa 5b 5c
Max w/c ratio* forplain concrete 0.70reinforced concrete 0.65 0.60 0.55 0.50 0.55 0.50 0.55 0.50 0.45prestressed concrete 0.60 0.60
Min cement content* in kg/m3 forplain concrete 150 200 200 200reinforced concrete 260 280 280 300 300 300 280 300 300prestressed concrete 300 300 300 300
* for minimum cement content and maximum water/cement ratio laid down in this standard only cement listed in clause 4.1(cement ENV 197 with exceptions) shall be taken into account. When pozzolanic or latent hydraulic additions are added to the mix, nationalstandards or regulations, valid in the place of use of the concrete, may state if and how the minium or maximum values respectively areallowed to be modified
with the USA's ASTM C618 and the results are shown inTables 10 and 11 (vom Berg, 1993). Within the chemicalrequirements (Table 10) there are a few differences betweenthe two standards. ENV 450 has no requirement for (Si02 +Ah03 + Fe203), moisture or alkalis, whereas ASTM C618does not limit the content of chloride or free lime. ENV 450includes a limit for chloride because most concretespecifications limit the chloride content of all constituents.However, according to vom Berg, the chloride content ofhard coal is generaJJy less than lO% of the limit. The contentof free lime is limited in ENV 450 since above 2.5% it cancause unsoundness in a mix under unfavourable conditions.An additional part of the standard specifies that ash with freelimit between 1% and 2.5% may only be accepted when a
52
50:50 mix of fly ash and cement passes the Le Chateliersoundness test. ENV 450 does not include a limit formoisture content since the fly ash since it is generaJJyremoved from the ESP, stored and delivered in a dry
condition.
A comparison of the physical requirements in ENV 450 andASTM C618 (Table II) show similarities with respect tofineness. ENV 450 contains no requirement for the activityindex at 7 days. According to vom Berg, this is because thestrength contribution from pozzolanic reactivity at that age islimited. The same requirement of 75% activity at 28 days isspecified in both standards. ENV 450 also includes anactivity index at 90 days. Differences in test methods must be
taken into account. In ASTM C618, 20% by mass of thecement is replaced by fly ash whereas in ENV 45025% isreplaced. Further, in ASTM C618 the water content of themix is adjusted to obtain a constant flow, in ENV 450 themixes are kept at a water binder ratio of 0.5. The effect ofthese differences on the test results cannot be predicted.
Classification and specifications
Austrian standard ONORM B3320-82 for fly ash use inconcrete sets an LOI of 5% wt, lower than the 6% set in theASTM standard. The moisture limit is also lower at amaximum of I% (Clarke, 1992). It is not known whether theEuropean standard ENV 450 has been or will be adopted in
Austria.
Although Canada has adopted many of the ASTM standards,the country has its own standard CSA A362 for blendedhydraulic cements. Within this standard, three differentcement types are defined:
Instead of the autoclave expansion test specified in ASTMC618, ENV 450 includes aLe Chatelier soundness test whichrequires that a 50:50 mix of cement and fly ash at a standardconsistency meets the same requirements as ENV 197 cement(see above). This test need only be applied when the freelime content of the fly ash is between I % and 2.5%.
Overall, vom Berg (1993) concludes that ENV 450 andASTM C618 give similar quality profiles for fly ash but thatENV 450 is more stringent with respect to pozzolanic activitywhereas ASTM C618 is more stringent with respect to theuniformity for fineness.
Type lOS
Type IOSM
Type lOP
Portland blast-furnace slag cement, havinga slag constituent between 25% and 70%
by mass;Slag modified Portland cement, having a slagconstituent less than 25% by mass;Portland pozzolan cement, having a fly ashconstituent up to 40% by mass.
Australian standards for fly ash use in concrete are includedin AS 3582 (1991) - Supplementary cementitious materialfor use with Portland cement. This standard sets out therequirements for fly ash as a component material for use inhydraulic cementJconcrete. Part I of this standard appliesspecifically to fly ash. The foreword of this standardemphasises that, although different fly ashes can behavedifferently with various cements, the short history of tests onthis in Australia means that there are insufficient data tospecify individual limits. The requirements for AS 3582 areshown in Table 14. The main way in which the Australianstandard differs from other standards in that the fly ash isgraded into three types - fine, medium and coarse, to whichdifferent criteria apply.
Table 14 Australian Standard AS3582 - Specifiedproperties of fly ash for use in concrete(Standards Australia, 1991)
Property Specification limits Referencemethod
Moisture content «1.0% AS 3583.2Loss on ignition 'fine'- 4%
'medium' - 6%'coarse' - 12% AS 3583.3
Fineness 'fine' - 75% <45 fLm'medium' - 60% <45 fLm'coarse' - 40% <45 fLm AS 3583.1
Sulphur anhydride «3.0% AS 3583.8Magnesia (MgO) «4.0% * AS 3583.9Autoclave expansion * AS 3583.4Available alkali optionaH AS 3583.12Relative density arbitrary AS 3583.5Relative water
requirement t AS 3583.6Relative strength t AS 3583.6Chloride ions 1" AS 3583.13
* A magnesia contcnt of greater than 4.0% is acceptable providcdthat the autoclave expansion does not exceed +0.8%. An autoclavecxpansion lcst is only necessary if thc MgO excceds 4%t These values are determined and the information made available tothe supplier with the test certificatc. No limits are set
In the Type lOP cement, although a maximum amount of flyash is specified, there are no constraints on the ash to be usedin its manufacture. In this respect, this standard is of theperformance type with the quality of the final product beingthe criterion of acceptance. However, the chemical andphysical requirements for final cements of Type lOP includevalues such as LOI maximum (6%) and criteria for S03,MgO. Fineness and pozzolanic activity are also included.Therefore, in order to assure the compliance of the finalproduct with these criteria, the ash supplier must determineand control the properties of the ash (CEA, 1983). TheCanadian standard for fly ash use in concrete, CSA A23.5, isvery similar to ASTM C618 (Manz, 1995b; see Table 8).
In China there are several Chinese National Standards, CNS,relating to the use of fly ash. These include CNS 11270which defines different classes of cement, CNS 11271 whichoutlines the specification for fly ash use in Portland fly ashcement, and CNS 3036 which is the specification of fly ashand raw or calcined natural pozzolan for use as a mineraladmixture in Portland cement concrete (Sheu and others,1990). In general, Class C fly ash must not be used at over20% replacement (the classification of fly ash as Class C orClass F is the same as in the USA). However, CNS I] 270defines three classes of Portland fly ash cement:
A 0--10% fly ashB ]0--20% fly ashC 20--30% fly ash.
This fly ash must meet the requirements set within CNSI] 271 where the Si02 content must be greater than 45%, themoisture must not exceed ] % and the LOI must not exceed5% (Kuo, ]995).
In CNS 3036, for Portland cement concrete, the maximummoisture and the maximum LOI are the same as in ASTMC6]8. There are optional chemical limits for MgO (5%) andavailable alkali (1.5%). Physical requirements includefineness, pozzolanic activity index, soundness anduniformity. There are also optional supplementary physicalrequirements for shrinkage and alkali reactivity. There is a
53
Classification and specifications
further standard, CNS (3090), for the use of admixtures suchas fly ash for use in ready-mixed concrete (Kuo, 1995).
The Danish standard for fly ash use in concrete, OS 411-84,has been replaced by the new European standard ENV 450,as described above. This is also true of the old Germanstandard, DIN 1045, originally issued by the Institute forCivil Engineers (IfBT) in Berlin (Clarke, 1992) and, if notadopted already, may be adopted soon in the Netherlands toreplace CUR Aanbeveling 12.
A decree issued by the Decreto del Ministero Dell'Ambientein Italy in September 1994 stated that ashes from coalcombustion 'constituted by aluminium, calcium and ironsilicates, with an unburnt carbon content between 2% and10% and heavy metal traces' can be reutilised for cementproduction, in addition to Portland clinker, for precast and insitu concrete production, for clay brick production and as apartial replacement (up to 20% dry weight) for embankmentsand road construction. Bottom ash can also be used in theconcrete industry provided that they have not beenhumidified with sea water, due to the chloride contamination.Italy has also adopted the European standards ENV 197 andENV 450 (ENEL, 1995).
Japanese Industrial Standard, JIS, R 5213 specifies criteriafor Portland fly-ash cement. Three classes of cement aredefined, the same as those defined in China (see above),although in Japan cement class A includes fly ashreplacement at 5-10% rather than 0-10%. For each of theseclasses different criteria are set for compressive strength.However, the criteria for specific surface area, setting time,stability, MgO (5% max), SO) (3%) and Lor (3%) were thesame for all classes of cement until 1 April 1995. After this
Table 15 Classes of concrete (Davies and Kitchener, 1995)
date, the LOI was limited to 3% for class A concrete only, nolimit being set for class B or C. Fly ash to be used inPortland cement concrete must comply with JIS A6201specification for fly ash to be used as an admixture in mortaror concrete (JIS, 1991). The Si02 content must be more than45%, the moisture is limited to I% and the LOI is limited to5%. Physical property requirements are specific gravity,fineness, unit water content ratio and compressive strengthratio at 28 and 91 days (JIS, 1992).
In the UK, the current British Standard BS 3892 Part I(1983) is the specification for pulverised fuel ash for use withPortland cement. The standard outlines the requirements forfly ash as a cement replacement in blended cements andconcretes and the limits are included in Table 8. The mainproperties for fly ash specified for use in concrete under BS3892: 1982 Parts I and II are fineness (by wet-sieving) andloss on ignition. Part I is a rigorous specification for the useof fly ash as a cementitious component of structural concreteand Part II covers other concrete applications. Although BS3892 regulates the use of fly ash in concrete by stipulatingthe characteristics of the fly ash itself, fly ashes which do notcomply with this standard may still be used in concrete underBS 8110 and BS 5382 (see Section 4.2.2) (Cabrera andWoolley, 1995). BS 3892 is similar to the proposed ENV 450discussed above (National Power, 1995) and will soon bereplaced by this standard.
Several classes of concrete have been specified in the UKand the allowable range percentage of fly ash which may beused is defined. These classes are summarised in Table 15(Davies and Kitchener, 1995). Another British standard,BS8110, limits the content of sulphate in ashes to be used instructural concrete. Different classes of concrete are defined
Cementitious contentMax, Min,kg/m3 kg/m3
Class ofconcrete
CIC2C2WC3C4C5C7C9CIILW
Class ofconcrete
ClC2C2WC3C4C5C7C9CIILW
54
Strength, Max Max water:N/mm2 aggregate cementitious
size, mm ratio
45 20 0.4830 20 0.5030 20 0.5020 20 0.5530 20 0.5510 20 0.7045 10 0.4840 20 0.4045 20
Area of use
structural concrete including primary containmentstructural concrete for the remainder of other structuresreservoirsthick floor topping concreteair-entrained road concretemass fi II concreteconstruction openings and areas of congested reinforcementseawater culvertssecondary containment dome
480400400380380300480450600
350310360250260200370400450
Available rangeof pu Iverisedfly ash, %
30-4030-4030-4030-40o5030-4030-400-40
by the sulphate concentrations. Fly ash is commonlyclassified as a class 3 material but, depending on the source,may also be classified in class 2 or class 4.
4.2.2 Other applications
Fly ash may be agglomerated into fly ash pellets or artificialaggregates which may then be used in concrete as areplacement for other aggregates. The commercial productionof artificial aggregates for this purpose is discussed in moredetail in Chapter 2. Any light-weight aggregate for use inmasonry units in the USA must comply with ASTM C331.Those for use in structural concrete must comply with ASTMC330. These specifications require limits on such parametersas unit weight and voids, soundness, organic impurities, firetests, specific gravity and absorption, and thermalconductivity (Mahadew, 1995).
If fly ash is to be used as a mineral filler in asphalt, ASTMD242-85 and AASHTO M17-88 both specify that 100% mustpass through a No 30 sieve (600 f.Lm), 95-100% through aNo 50 sieve (300 f.Lm) and 70-100% through a No 200 sieve(75 f.Lm). The LOI must be below 12% (Manz, 1994).According to Poulsen (1995) fly ash for use in asphalt fillerand roof sealer in Denmark must also meet a certain LOI andsize distribution limit (Poulsen, 1995).
BS 3892 Part 2 (1984) is the specification for pulverised fuelash for use in grouts and for miscellaneous uses in concrete.According to National Power (1995), this standard does notgenerally ensure a quality assured material and thereforetesting is only carried out routinely to give indicative valuesfor various properties. In many countries, including Denmark,there is no specification for fly ash use in grout (Poulsen,1995).
Fly ash used in applications other than cement and concreteor related media generally has to perform totally toperformance related criteria which are application specific. Iffly ash is mixed with a cementitious material such asPortland cement, and a suitable amount of water, it can beturned into a versatile flowable and pumpable fill material. Inmany countries, such as Canada (CEA, 1983), engineered fill,or controlled density fin, is not specificany covered either asa grout or as a concrete.
In Europe, Eurocode No 7 is the code of practice forfoundation design and anows end product control based oncompaction criteria. However, the code does not give anyrecommendations for tests to determine strength nor does itlist permissible values. Within Eurocode No 7 eitherconditioned or stockpiled ash may be used (Clarke andCoombs, 1995). Pulverised coal fly ash can be used inearthworks for capping, the application of a layer of coarsegranular material to improve the strength of the subbaselayers. The European Standards Committee is in the processof setting a new European standard on unbound aggregateswhich will include specifications for capping materials.Currently the ash, like any other bulk waste, must satisfy aset of simple criteria, the most essential of which iscompaction (Clarke and Coombs. 1995). A new ASTMstandard for the use of coal combustion fly ash in
Classification and specifications
structural fill is currently being composed in the USA(Brendel, 1995).
4.3 Test methodsSpecifications and appropriate test methods are published bya number of institutions within each country. For example, inthe USA alone test methods are published by the ASTM, theAASHTO, the US Army Corps and Engineers, the AmericanConcrete Institute and by regional departments of highways.It is beyond the scope of this report to review all thepublished test methods in detail. However, an indication isgiven of the main tests used and how they are performed.Any problems specific to the study of coal fly ash arediscussed. ASTM methods are commonly adopted or copiedby other countries, and therefore fornl the basis of thediscussion. Where possible, comparisons with other publishedtest methods are drawn.
ASTM C311 lists the test methods for testing for ASTMC618. The tests include S03 analysis, analysis for availablealkalis, LOI, specific gravity, moisture, 325 sieve finenessand pozzolanic activity. There is no ASTM C618requirement for the relative density (specific gravity) of anash, but the information is necessary to formulate blendedcements by mass rather than volume. The relative density ofresidues can be determined in accordance with ASTM C188'Standard test method for the density of hydraulic cement'(Kilgour, 1992). According to Manz (1995b) ASTM testmethods for the alkali aggregate reaction, sulphate resistanceand pozzolanic activity determination are in question.
ENV 450 refers to a number of relevant test procedureswithin ENV 196. These include tests for strength, chemicalanalysis of cement, determination of setting time andsoundness, fineness, sample preparation, chloride, carbondioxide and alkali content; free calcium oxide and so on.
Standard methods for testing fly ash have also been set inAustralia, AS 3583 - Methods of test for supplementarycementitious materials for use with Portland cement. AS3583 comprises 13 individual test methods for parameterssuch as fineness, moisture content, LOI, autoclave expansionand so on. Table 14 showed the appropriate test methods inAustralia for use with AS3582.
Chinese National Standards for the use of fly ash in cementand concrete arc supported by a standard test procedure, CNS10896 - Method of test for fly ash or natural pozzo1ans foruse as a mineral admixture on Portland cement concrete. Thetest method and the specifications are reported to be 'similarto ASTM C311-77' (Sheu and others, 1990).
The following sections summarise the test methods used todetennine the most important criteria for fly ash use.
4.3.1 Pozzolanicity
The principle of pozzolanicity was discussed in Section 3.6.1.According to Joshi and Lohtia (1995), a simple test forevaluating pozzolanicity has so far evaded the scientificcommunity. Although fineness and chemical composition are
55
Classification and specifications
important, the pozzolanic activity in coal fly ash comeslargely from the strain in glass. However, measurement ofstrain in glass is not practicable. Because pozzolanicity is sodifficult to measure, many organisations do not even identifyfly ash as a pozzolanic admixture, but rather as a mineraladmixture in cement concrete. At the moment it is necessaryto make trial tests with fly ash concrete to obtain validestimates of potential strength development (Joshi andLohtia, 1995).
The pozzolanic activity test with Portland cement specifiedwithin ASTM C618 involves replacing a portion of Portlandcement in a concrete mortar with fly ash. Cubes are mouldedand then cured for 24 hours at 100% relative humidity at23°C, followed by curing at 38°C for 27 days. Thecompressive strength (see Section 4.3.3) is then comparedwith a standard specimen. ASTM C618 specifies that the testspecimen must have a minimum of 75% of the strength ofthe control. The high calcium fly ashes which have their ownactivators and are already cementitious perform better in thistest than low calcium ashes (McCarthy and others, 1990).
Manz (1995b) discusses the controversy over pozzolanicactivity tests with cement and lime. There is often poorcorrelation between results of either test and actualperformance in the field when the fly ash is used in concrete.In ASTM C618 there is a footnote to the effect that 'neitherthe pozzolanic index with Portland cement nor the pozzolanicactivity with lime is to be considered a measure ofcompressive strength of concrete containing the mineraladmixture.' The footnote also says 'pozzolanic activity indexwith Portland cement is a measure of reactivity with a givencement and may vary as to the source of both the fly ash andcement.'
Tashiro and others (1994) have evaluated an electricresistance measurement method for the determination ofpozzolanic activity. When an activator is added to thepozzolan this results in a suppression of heat liberation andimprovement in chemical resistance, amongst other things.Higher resistivity indicates a superior pozzolan.
Researchers at the Energy and Environmental ResearchCenter (EERC), North Dakota, USA, have developed a newtest procedure for pozzolanic/cementitious materials such asfly ash (Hassett and others, 1995). Preliminary work hasconcentrated on the rate of heat generation from the heat ofhydration reaction that occurs when cementitious materialreacts with water. Results so far indicate that there is nocorrelation between the rates of heat of hydration andstandard ASTM results. It is suggested that the correlation ofthe heat of hydration results with those of advancedanalytical techniques such as electron microscopy mayprovide predictive capabilities for coal ash. More work isneeded to evaluate this.
4.3.2 Fineness
The workability of concrete containing fly ash is verydependent on the fineness of the fly ash. Fineness affectspozzolanicity, which is enhanced by finer grained fly ash.The current Japanese Industrial Standard for fly ash, JIS
56
A6201, determines fineness by the specific surface area asmeasured by the Blaine method (JIS, 1991). However, it hasbeen argued that the specific surface should not be regardedas the best parameter for the classification of fly ash becausereproducibility is low. Wet sieving is preferred. ASTM C618used to have a specific surface requirement but this has beenremoved (Manz, 1995b). Similarly, in Australia, StandardsAustralia also omit use of the Blaine surface areadetermination from their AS3582.1 standard for fly ash use inhydraulic cement/concrete as it has a limited application tofly ash. The 45fLm sieve test is considered more appropriateand more convenient (Standards Australia, 1991). However,the standard wet sieve test for fly ash fineness on the 45 fLmsieve, ASTM C430-83, has limitations. This test methodspecifies the use of the 'current lot of NIST standard sampleNo 114' (a reference cement) for calibration of the sieve. Inthe British and Australian versions of this standard it isrecognised that a reference fly ash rather than a referencecement should be used for the sieve calibration. However,differences of up to 50% have been reported between testresults obtained using different national tests. It is reportedthat the ASTM C430 fineness test procedure does notproduce reliable test results for coarse fly ash samples.According to Butler and Kanare (1989), despite the use ofreference materials for sieve calibration, the size of the sieveopenings still influence the test results and that care shouldbe taken when selecting sieve cloth material.
4.3.3 Strength
According to Clarke and Coombs (1995) there are severaldefinitions of strength and a number of techniques to measureit. Strength can be considered as being formed of fourcomponents:
cohesion due to suction pressures;cohesion due to inter-particle bonding;inter-particle friction;interlocking of particles.
Following compaction, ash has immediate strength due tosuction pressures and internal friction. These dissipate tosome extent with time and pozzolanic activity occurs in someashes to give permanent cohesion. Strength of the ash alsodepends on the density and shape of the particles.
Compressive strength is measured by preparing cylinders ofmortar which are allowed to set for a prescribed period oftime. The material is tested in unconfined compression andthe compression strength is reported as PIA where P is thefailure load and A is the cross-sectional area of the cylinder(Sneddon, 1995). Shear box tests are used to determine theeffective strength of pulverised fuel ash. Results of these testsdepend considerably upon sample preparation and achievingsaturation of the fly ash in a shear box may be difficult (TriUtomo and Clarke, 1995; Clarke, 1995a). It is important totake account of the relative compaction actually achieved inthe field when applying the results of shear tests. At 90%compaction the apparent cohesion is one-third less and theangle of shearing resistance one-fifth less than valuesobtained with 100% compaction. However, these results areonly valid immediately after placing and increase with time
as the concrete ages (PowerGen, 1995). Triaxial tests may bemore appropriate for fly ash. For triaxial tests, samples arestatically compacted to the maximum dry density.Compaction is then tested with a calibrated weight. Storageof samples is critical since changes in water content canchange the development of strength. Compaction is tested atdifferent levels of saturation (Tri Utomo and Clarke, 1995).
Strength tests are specific to individual applications. Forexample, determining the strength of fly ash used inengineered fill is performed with strength parameterscorresponding to the amount of pressure that the ash will berequired to stand in the depths to which it will be applied(Clarke and Coombs, 1995). Since strength tests apply to theperformance of the final product rather than the contributionfrom individual components such as fly ash, there may oftenbe a choice of applicable test methods. For example, there areseveral methods which are suitable for determining strengthof flowable fill materials. These include ASTM D4429 - theCalifornia Bearing Ratio (CBR); ASTM D 1194 - platebearing tests; and ASTM D2922 for density testing.Kuloszewski (1995) compared these three strength tests forcharacterising fly ash/boiler slag cement mixtures forflowable fill replacing compacted soils. Kuloszewskiemphasised that lateral containment is essential for loadbearing and therefore, since sites vary, the applications needto be studied on a case by case basis. The standards were notof use in evaluating the bearing capacity of the flowable fillbacause they were designed for soil. It was recommendedthat the flowable fill be tested in accordance with concretestandards.
There can also be confusion over which sampling protocolsare most applicable to some applications of fly ash. One ofthree different ASTM sample preparation procedures may beapplied to determine the compressive strength of flowablefills containing fly ash (Kuloszewski, 1995):
ASTM C39compressive strength of cylindrical concrete specimens(for 6 x 12 inch cylinders prepared in masonry block moulds)ASTM CI09compressive strength of hydraulic cement mortars(for 2 x 2 x 2 inch cubes prepared in)ASTM CI019sampling and testing grout (4 x 4 x 8 inch prisms)
Kuloszewski (1995) detailed a study by the Niagra MohawkPower Corporation, NY, USA, of the use of fly ash as aflowable fill. It was not clear which of the above standardswas applicable for measuring the compressive strength of thenew flowable fill so all three standards were used and theresults compared. When four different mixes of fly ash,boiler slag and cement were tested with all three protocolsthe compressive strengths were found to differ with theprotocol used. With ASTM C I09 the compressive strengthwas increased by 21 % more at 28 days than with ASTMCI09 and 60% more than with ASTM C39. This was due todifferences in the testing procedures themselves. In ASTMC39 the specimens are produced in plastic moulds while inASTM CI09 brass moulds are used. In ASTM CI0l9specimens are produced in masonry blocks which allow
Classification and specifications
excess water to leach out. This reduces the water to cementratio and thus increases the compressive strength. ASTM C39should give the most accurate representation of fieldconditions because this is closest to the actual fieldconditions for flowable fill replacement for compacted soils.However, Kuloszewksi recommends that all three tests beperformed to serve as a base for strength testing of the filland that the accepted ASTM C39 be taken as the preferredprotocol. This is because ASTM C39 has many advantages:
plastic moulds give protection during curing andtransport;the moulds are readily available as they are used asstandard for testing concrete;they are the easiest of the moulds to cast;all independent laboratories are familiar with thestandard.
Clarke and Coombs (1995) emphasised the following pointsas those which are most important with respect to testing thestrength of fly ash as an engineered fill:
the time between preparing and testing a specimen mustbe known since the strength of an ash can increase;tests should be carried out on saturated specimens toensure there are no suction pressures. Soaking aspecimen may not be enough;measured 'peak' strengths may not be accurate sincesome ashes show increasing brittleness with age;many ashes show a permanent increase in peak and postpeak strength with age.
4.3.4 Autoclave expansion
Autoclave expansion is of particular concern to users oflignite and subbituminous fly ash because they contain highCaO and MgO contents. Although a concentration of MgOgreater than 5% in Portland cement is reflected as periclasewhich would cause excessive autoclave expansion, MgO infly ash is not present as periclase. However, some fly asheswith high free lime contents will produce false set and rapidheat rises in concrete mixes. Excessive autoclave expansioncaused by free lime is highly significant in cases wherewater-to-mineral admixture and cement ratios are low, forexample, in block or shotcrete mixes. No limit is set foreither free CaO or free lime content in ASTM C618 (Manz,I995b).
4.3.5 Permeability
According to Day and Konecny (1989) the pore structure ofconcrete is often the most important factor which governs theservice life of structures exposed to chlorides, sulphates,acids and various other aggressive agents. Permeability anddiffusivity are associated with porosity and pore structure.Many methods have been devised to measure thepermeability of concrete, mortar or hardened cement paste.However, accurate and reproducible results are difficult toobtain and no single method is recognised as being the best.Day and Konecny (1989) report that it is particularly difficultto obtain consistent results when materials such as fly ash areused as these tend to produce specimens which, at later ages,
57
Classification and specifications
have very low permeabilities. The effect of fly ash on thepermeability of cements, concretes, mortars and engineeredfills was discussed in Section 2.2.2.
4.3.6 LOI
Loss on ignition (LOI; see Section 3.2.3) is sometimeswrongly assumed to represent the carbon in ash valueconsistently. LOT is determined by igniting a sample (1 g) offly ash in an oxidising atmosphere (air) at a chosencontrolled temperature. When ignited at 750°C the LOI isassumed to represent mainly carbon and any moisturepresent. It is assumed that there is no volatilisation of alkalisalts at this temperature. At temperatures greater than 9500Cit is assumed that any oxidisable material present would beoxidised. Neither ASTM or CEN test methods for LOI makeany correction for the evaporation of any moisture or for theinfluence of oxidation. Owens (1995) suggests that thecurrent methods for LOI determination be reviewed.
Brown and Dykstra (1995) have studied the accuracy of theLOI test based on ashing samples for several hours at 725°C(ASTM method d3l74-82). According to studies on ash froma stoker boiler, a pulverised coal boiler, an atmosphericfluidised bed combustion (AFBC) boiler and a pressurisedfluidised bed combustion (PFBC) boiler, dehydration ofportlandite and calcination of the carbonate upon heatingproduce weight losses that are confused with oxidation ofunburnt carbon. Although this was more of a problem in thefly ash from the PFBC which contained portlandite(Ca(OH)2) and carbonate originating from limestone ordolomite injected into the boilers for sulphur control, even flyash from pulverised coal combustion contained enoughcarbonate from naturally occurring minerals in the coal toproduce an unacceptably large error in the LOI analysis (up to75%).
An alternative to LOI measurement may be measurement ofcarbon-in-ash. A carbon-in-ash measurement system (CAM)has been developed to measure carbon-in-ash in an on-linemanner (DiGioia and Kelly, 1995). The CAM systemmeasures fly ash carbon content with microwaves at2.45 Ghz. At this frequency fly ash absorbs little of themicrowaves whereas carbon is a good microwave absorber. Itis a non-contact system which operates at ambienttemperatures and does not destroy the ash. Although thesystem was initially designed to indicate poor combustionperformance to plant operators, it may also be used for theon-line monitoring of fly ash. If the carbon content exceeds apre-specified level then the ash can be directed to a low valuesalesllandfill silo, otherwise the ash is sent to the cementreplacement sales silo. The on-line factor of this systemmeans that any sudden and/or short deviations in the plantoperating conditions can be detected and the low quality ashkept separate from the rest. The CAM system could also beused for skimming good fly ash in units that have variableoperating duty. The CAM system is best sited between theeconomiser and air heater, ahead of the precipitator/baghouseand must be operated isokinetically (particles being collectedat the same speed that they were travelling in the duct). Amulti-point system is used where results from severalsamplers are combined and averaged. A CAM system and the
58
associated adaptive duct sampling system cost under $70,000(DiGioia and Kelly, 1995).
A cylindrical microwave resonant cavity (MRC) can be usedin conjunction with commercial automatic dust samplers tomeasure carbon in ash. For use as an on-line analyser for flyash, a technique must ensure a controlled flow of fly ashthrough the gauge and also ensure that the sample isrepresentative. CSIRO (Commonwealth Scientific andIndustrial Research Organisation) tested the MRC system atthe Electricity Commission of New South Wales Wallawerangpower station. The system was found to measure unbumtcarbon over the range 4-14 wt% with errors of under0.51 wt%. Measurement error was often due to variation inthe bulk density of the fly ash sampled which was in tum aresult of variations in sampling collection rate from the duct.These problems could be reduced by adjustments to thesampling and measurement systems (Sowerby and others, 1992).
Microwave analysis has rapid analysis times, high accuracyat low carbon content, adaptability to small sample sizes andno requirement for bulky radiation shielding. The microwavetechnique is regarded as the most suitable for field trials.Carbon in fly ash can also be determined using neutroninelastic scattering techniques. Neutron inelastic scatteringhas advantages over microwave techniques in that itmeasures carbon directly and is insensitive to variations inthe moisture contents of the samples. However, it hasdisadvantages which include the requirement of samplesgreater than 3 kg and analysis times of 5-10 minutes(Sowerby and others, 1992).
Techniques for the measurement of LOI based on reflectanceare also available. For example, Nottingham University in theUK is developing a technique for the combined automatedand semi-automated petrographic/reflectance analysis of coalsand chars. This method could have the advantage of beingable to analyse the pattern of reflectance from coal samplesto predict unburnt carbon. Jones and others (1995) emphasisethat a wider range of coals have to be studied before thetechnique can be validated.
4.3.7 Other methods of testing
Several other methods used in the determination of fly ashcharacteristics are prone to problems. For example, the lowconcentration of free CaO in most Class C ashes causesproblems in some methods for measuring free lime content,such as ASTM C25. Here samples sometimes have to beadjusted to increase the weight of material studied (Sneddon,1995). Another example is ASTM 422, the method fordetermining grain-size distribution using the hydrometermethod. Water cannot be used as the sedimenting mediumfor Class C fly ash because it has hydraulic cementingproperties and sedimentation is impeded. Methyl alcohol maybe substituted (Sneddon, 1995).
Berube and others (1995) have studied the various ASTMmethods for evaluating the effectiveness of supplementarycementing materials such as fly ash in suppressing expansiondue to alkali-silica reactivity in concrete. These includeASTM C441 - the Pyrex Mortar Bar method, CSA-A23 -
the concrete prism method (a Canadian method), ASTMC227 - the mortar bar method, and ASTM C1260 - theaccelerated mortar bar method. Each method has limitationswhich must be understood and accounted for. The interestedreader is referred to the original reference for more details onthis subject.
4.3.8 Advanced techniques
More and more researchers (for example Vempati and others,1994; Khiinel and others, 1992; Dewey and others, 1994) areturning to advanced techniques such as X-ray diffraction andscanning electron microscopy (SEM) to study fly ash. Deweyand others (1994) stress that techniques such as SEM,electron microprobe analysis, XRD and quantitative digitalimage analysis require further development and testing.However, such systems shows promise and could aid in thedevelopment of a performance based classification schemewhich correlates concrete performance with the results ofmicroscopic and bulk analysis techniques.
4.4 Quality assurance andcertification
Quality assurance for fly ash is a fairly new concept since flyash has only comparatively recently been considered andtreated as a marketable commodity. As discussed inSection 4.2, the majority of fly ash use currently is as a fillerin cement and concrete with only a few basic specificationsto be reached. For more advanced uses in cement andconcrete, fly ash has to be of a higher standard - finer, with alower carbon content. To ensure this standard is met, testmethods such as those discussed in Section 4.3 are applied.In order for the results of these tests to be meaningful, thetests must be performed correctly, at suitable time intervals,on a suitable number of samples and must be statisticallyvalid. Only then can it be said that the quality of an ash isassured.
As mentioned in Section 4.2, individual specifications for flyash nOimally include requirements for demonstratingcompliance, that is - quality assurance procedures. Sneddon(1995) points out that, although rapid simple tests can beused to monitor ash sources for variability, natural variationswithin the same population require that sampling error andtest error must be evaluated. In general, sampling proceduresmust be shown to be statistically valid. For example, thesamples must be drawn randomly from a population ofknown history. The sample must be known to represent abatch of materials which were produced at the same time, bythe same process and under the same systems of control.
Classification and specifications
assessed once a week, as should soundness, if required.Chlorides, sulphuric anhydride and particle density should betested once a month and the activity index measured twice amonth. A minimum of ten samples must be tested for eachproduction period (1-12 months). The minimum sample sizerequired is 1 kg, obtained from quartering a compositesample of at least 4 kg. Samples are normally taken inconnection with loading or discharging fly ash to or from astorage silo. Sampling plans may be operated in one of twoways - continuous inspection by variables or continuousinspection by attributes. Continuous inspection by variablesrequires calculation of the mean and standard deviation of thecomplete set of test results (one result per sample).Acceptability can then be calculated statistically. Inspectionby attributes requires that the number of defective test resultsin the complete series of samples is counted. There is then anacceptability factor which may be calculated (ENEL, 1995).
The requirements of ENV 450 are specified as characteristicvalues within a 10% fractile. Hence some individual fly ashesmay exceed some of the limits listed in Tables 10 and 11. Toreduce the chance of major deviations in standards occurring,major deviations have been defined, as shown in Table 16,which may not be exceeded in any single case. Thefrequency at which these defects must be tested for is relatedto production time rather than production quantity. Fly ashwhich passes the specifications outlined in ENV 450 may bemarked with a CEN guarantee.
As a further example of quality assurance withinspecifications, Table 17 shows the test requirements andfrequency of testing required under the Australian standardAS3582 for use with Portland cement. Individual samples arenormally grab samples taken at the time of loading whereascomposite samples are samples made by mixing individualsamples taken over a selected time period. Table 18 showsthe permissible variations from the standard. In order todemonstrate compliance with the standard, AS 3582.1includes an appendix which outlines the statistical samplingrequirements (AS 1399 and AS 1199). Once a sample haspassed the standard requirements it may be marked with aproduct certification 'Standards Mark' which meets thecriteria of an ISO Type 5 standard scheme. Qualitymanagement systems to meet the quality assurancerequirements agreed between customer and supplier arecovered by AS 3900 and AS 3904 (Standards Australia,1991). Similar quality assurance schemes are run in othercountries such as Germany (Backes, 1994).
Table 16 Major deviations from ENV 450 (vom Berg, 1993)
Most, if not all, of the specifications for fly ash use in cementor concrete discussed in Section 4.2 contained details ofquality assurance procedures. For example, within ENV 450autocontrol is defined as continuous statistical quality controlof the fly ash based on the testing of samples taken by thesupplier or his agent at points of release from the fly ashgenerating facility. Sampling and testing frequencies are alsospecified within ENV 450. Lor and fineness should be testeddaily during production. Free calcium oxide should be
Property
Loss on ignitionFinenessFinenessChloridesSulphuric anhydrideSoundnessActivity indexFree calcium oxide
Limit for major defect
+ 2.0 percentage points+ 5.0 percentage points±5.0 percentage points+ 0.0 1 percentage points+ 0.5 percentage points+ 1.0mm- 5.0 percentage points+ 0.1 percentage points
59
Classification and specifications
Table 17 Frequency of testing in Australian Standard 3582 (Standards Australia, 1991)
Property
Fineness indexMoisture contentLoss on ignitionSulphuric anhydrideMagnesia (MgO) contentChloride ion contentAvailable alkaliAutoclave expansionRelative densityRelative water requirementRelati ve strength
Type of sample
individualindividualindividualcompositecompositecompositecompositecompositecompositecompositecomposite
Frequency of testing
Every 500 t or daily, whichever is more frequentEvery 500 t or daily, whichever is more frequentEvery 500 t or daily, whichever is more frequent6-monthly6-monthly6-monthly6-monthly6-monthly (only required if MgO >4.0%)6-monthlymonthly6-monthly
Table 18 Permissible variations from Australian Standard3582 (Standards Australia, 1991)
The Netherlands is probably the most advanced country withrespect to the util isation and handling of fly ash. The DutchFly Ash Corporation (Vliegasunie bv) was formed in 1982 bythe Dutch electricity generating companies to optimise themarketing and processing of fly ash. Vliegasunie markets flyash to several areas: the cement industry, the concrete andmortar industry, as asphalt fillers and for artificial gravelproduction. Each of these utilisations has its own specific
Quality assurance procedures for two separate companies aregiven here to demonstrate the approaches taken. Standardpractice for ELSAM in Denmark is the measurement of LOrand sieve residue at least once every day for samples takenfrom the ESP or transport pipes. Some power plants inDenmark have on-line measurement of LOr. Results fromthese tests determine whether the fly ash is stored for sale ordisposed of. Since daily tests of this sort do not preclude lowquality ash passing the selection process, samples from eachtruck leaving the plant are checked again. Only LOr andsieve residue are measured by ELSAM. According to thecompany 'the sale and use of the fly ash is built on arelationship of trust between the customers and the powerplants as the fly ash is neither analysed before it is deliveredto the receiving silos nor even before it is used.' Danaske, thefly ash sales organisation run by ELSAM (see Section 2.1.1),deals with the sale of fly ash to the cement and concreteindustry as well as to some of the industrial cutomers.Danaske carry out extensive analysis of the chemicalcomposition of the fly ash shortly after delivery. Powerplants in Denmark are cUlTently preparing quality assurancemanuals for fly ash delivered to industry. Product and systemcertification is also being set up (Poulsen, 1995).
Property
Moisture contentLoss on ignitionFinenessSulphuric anhydrideMagnesia
Relative densityChloride ion
Permissible variation
0.1 % of moisture content0.2% of mass loss1.0%0.2%0.2% of the magnesia content or5% of the value, whichever is greater0.03%0.01%
quality requirements. Some of the quality requirements aredictated by the standard specifications as outlined inSection 4.2, others are dictated by the relevant industriesthemselves. Vliegasunie has the ability and experience topredict the quality of fly ash produced from its coals.However, samples of fly ash are still taken daily andanalysed chemically and physically. Occasional randomsampling and laboratory testing is performed. Depending onthe client's quality requirements, extra samples can be takenand analysed before and during the loading of the fly ash.For fly ash use in concrete, ENV 450 is adhered to and flyash meeting this specification may be certified. Certificationrequires an internal quality monitoring scheme for thecertification laboratory itself and the scheme must be adheredto under the auspices of the certification institute.
Vliegasunie lists the following as the important elements ofquality control (van den Berg, 1995):
quality control procedures;quality system for laboratories:a quality manual;round robin tests;sampling procedures.
4.5 Summary and commentsCurrently there is a significant amount of confusion over theclassification of fly ash and several schemes are available.The most commonly used scheme, set by the ASTM in theUSA, is not regarded as perfect. This classification scheme,like others, separates fly ash according to characteristicswhich make it suitable as either a pozzolan or a mineraladmixture in cement and concrete.
Although specifications for fly ash in various applicationslargely rely upon chemical and physical characteristics of flyash which are predicted to be beneficial to the end product,compliance with these specifications cannot be used as aguarantee for the quality of the product. The only way todetermine the performance of a fly ash in any application isto prepare a sample and subject it to standard test procedures.However, several CUITent test methods, including that forLOr, are known to have inherent problems and some areregarded as unsuitable for testing fly ash.
60
5 Quality control
Attitudes towards fly ash from pulverised coal combustionare changing. For example, the following excerpt is takenfrom the annual report of the Dutch Fly Ash Corporation(Vleigasunie, 1995):
The composition of the blend coal which is purchasedand blended for the power stations is cruciallyimportant to the quantity and quality of constructionmaterials produced as a by-product of electricitygeneration.
This demonstrates the attitude that fly ash is not a waste but avaluable commodity in its own right, and like any othermarketable commodity, should be subject to quality controland quality assurance procedures.
The quality of fly ash can be controlled at several stages within the power plant itself during formation of the ash; andafter production by various processing, classification andbeneficiation techniques. Quality control techniques arediscussed in detail in the sections to follow. Qualityassurance of the fly ash from such processes is discussedwhere possible but does not normally go beyond optimisingeach process itself. Quality assurance is more associated withthe classification and certification of ash as discussed inChapter 4.
5.1 Operating conditions of the plantOne of the reasons for the limited use of fly ash in manycountries is the large variation in properties between differentashes produced from different power plants. As discussed inChapter 3, the chemical and physical properties of fly ashvary with the composition of the coal burned, type of boilerused and boiler operating conditions (Liskowitz and others,1995). Pollution control equipment can also affect fly ashcharacteristics, especially the unburnt carbon content.
Producing power is the first priority of any power plant.
Complying with emission standards for pollutants is also ahigh priority. Adjusting plant operating conditions to enhancethe characteristics of the fly ash is a new and, in most cases,alien concept. The following sections discuss the effects ofthe different operating conditions of a plant on the coal ashand, where possible, indicate how the coal ash characteristicscan be optimised.
5.1.1 Coal type
Coal type is one of the most important factors affecting thecharacteristics of ash. According to Jones and others (1995),the amount of unburnt carbon is fly ash is more dependent oncoal quality than on combustion conditions in mostpulverised coal fired power plants.
As discussed in Chapter 3, there are many different chemicaland physical characteristics of fly ash which relate back tothe coal from which it is produced. Relating the properties ofthe coal to the properties of the ash is not simple, althoughpredictive computer models are being developed (seeSection 2.1.2).
It is commonly believed that Class F ashes arise fromanthracite or bituminous coal and that Class C ashes arisefrom lignite or subbituminous coals, although this is notalways the case (see Chapter 4). For example, Class F fly ashcan be produced from subbituminous Alberta coals. Somecoals simply produce more carbon in ash than others. Forexample, EI Cerrejon is a borderline high volatile bituminousColombian coal which produces carbon in ash levels 50%higher than other coals, such as Tyne Blend and Ashlandused by PowerGen, UK. The high carbon is thought to bedue to the intrinsic char reactivity or the porosity of the char(Jones and others, 1995).
Ghafoori (1995) compared two types of bottom ash from twoseparate power plants in Indiana and Ohio. One bottom ashhad a low calcium content, from bituminous coal, and the
61
Quality control
other had a high calcium content, from lignite. Concretesmade from either of the ashes showed long-term chemicaland physical resistance to, for example, sulphate attack.However, the high calcium ash from the lignite-fired plantcaused the concrete to have a 50% higher permeation rate forchloride ions. The bituminous bottom ash concrete had alower resistance to abrasion than the reference mixture butthe lignite bottom ash produced the most abrasion resistantconcrete of the three samples.
The blending of coal prior to combustion is one option forreducing coal costs. However, the addition of a new coal willcause a change in the resulting ash (see the IEA CR report oncoal blending (Carpenter, 1995». The combustion of lowsulphur subbituminous Wyoming coal produces a Class Chigh-calcium fly ash which is used widely in Oklahoma,primarily in highway construction. However, recently it hasbecome economically mandatory to blend the Wyoming coalwith local lower quality, high sulphur, bituminous Oklahomacoals. To test the effect of this coal blending on the resultingfly ash, Laguros and Gollahali (1992) have performedpreliminary tests burning both coals in a laboratory-scalecombustor. The Oklahoma coal was found to produce a flyash which hydrates upon the addition of water but does notyield a network of crystal formation. In the fly ash fromWyoming and Oklahoma coal blends the crystal formationincreased with the higher amounts of Wyoming coal. Fly ashobtained from the combustion of Oklahoma coal alone wouldnot be particularly suitable for soil-aggregate stabilisation androller compacted concrete where cementation, bonding andstrength are important. Further work is required to determinethe maximum amount of Oklahoma coal which may beblended without lowering the applicability of the Wyomingcoal.
5.1.2 Combustion conditions
It is not currently possible to relate every parameter ofcombustion to the effect it will have on the quality of the flyash. Following a review of the appropriate literature, Joshi
and Lohtia (1995) summarised the effect of combustionconditions on coal fly ash. The amount of excess air in thefurnace is one of the most important parameters influencingthe amount of carbon in the fly ash. The fineness of the coalcombusted affects the efficiency of combustion and thereforewill also affect the amount of unburnt carbon. The use of fueloil to enhance furnace efficiency results in oil residue whichadversely affects the quality of the ash for use as a pozzolanor mineral admixture in concrete. The unburnt oil aggravatesproblems of elTatic air entrainment in fly ash concrete. Thecolour of the resulting concrete may also be affected by oilresidue.
According to Owens (1995) the following problems canincrease the LOI of fly ash: increased load, especially atvalues above 90% of the rated capacity of the plant; largerthan average daily fluctuation in loading; double shifting,where shut down followed by start-up produce inefficientconditions; increased wear and tear on pulverisingequipment; significant changes in the homogeneity of thecoal supply; and increased levels of impurities in the coal.Section 4.3.6 gave details of on-line techniques for themeasurement of carbon in ash. Such systems could be usedeither as feed-back monitors to warn when combustionconditions are below the optimum or for classification of ashas it is produced.
In a study based on 257 samples of Iowa fly ash Bergesonand Schlorholtz (1992) observed that the compressivestrength development of hydrated fly ash varied widely. Theyfocused their study on ash produced by the Ottumwagenerating station. Figure 14 shows the compressive strengthdevelopment variability as a function of time. The strengthranges from roughly 1.5-34.5 MPa (a few hundred to nearly5000 psi (34.5 MPa». They found a relationship betweenmaintenance shut-down periods, sodium carbonate feed-rateand compressive strength development. Immediately afterstart-up, when the Na2C03 (sorbent for sulphur capture)feed-rate was low, the compressive strengths of the fly ashpastes was high. As Na2C03 feed-rate increased, the
5000(34.5)
<ilIl.
~ 4000'en (27.6)Q.
.cOJ
3000c~ (207)enQ)
>en
2000(j)Q)
(138)CiE0u>- 1000<1l
CJ (69)r-:..
0
0 1000 1200 1400
Days from 1 January 1983
1600 1800
Figure 14 Seven-day compressive strength trends of Ottumwa fly ash pastes (Bergeson and Schlorholtz, 1992)
62
strengths fell. There appeared to be a direct correlation withthe Na2C03 feed-rate and the sodium oxide content of the flyash. Bergeson and Schlorholtz (1992) recommended that'caution be exercised in utilising ash produced immediatelyprior to shut-down and after start-up from any power plant'.If such ashes were used in Portland cement concrete therecould be effervescence problems or an increase in thepotential for future sulphate attack. Highly expansivereactions could potentially occur in high application rates offly ash for soil or base stabilisation.
In general the aim is to minimise the controllable lossescaused by operating under non-ideal conditions. One way todo this is to base control of the boiler on a rapid on-linedetermination of unbul1lt carbon in fly ash. The neutroninelastic scattering technique can measure elemental carbondirectly. However, microwave techniques have theadvantages of rapid analysis time, higher accuracy at lowcarbon content, adaptability to small sample sizes, lower costand no requirement for bulky radiation shielding (Cutmoreand others, 1992). The measurement of wt% unburnt carbonin fly ash by microwave techniques is based on the real orimaginary part of the complex dielectric constant of unburntcarbon compared to the dielectric properties of the remainingmatrix. The complex dielectric constant is a function of thedielectric properties of the fly ash/air mixture, and thereforeaffected by both the unburnt carbon concentration and thebulk density of the fly ash.
5.1.3 Pollution control systems
According to DiGioia and Kelly (1995), changes in coalsupplies and the installation of low NOx burners haveresulted in higher carbon-in-ash levels from many boilers inthe USA. In Japan, two-stage combustion and low NOx
burners to reduce NOx emissions have been found to increaseunburnt carbon contents of fly ash (Mori and others, 1994).Cochran and others (1995) consider that evidence to datesuggests that most NOx abatement projects cause increases inresidual carbon. These increases can be significant and maydetract from the marketability of the plant's fly ash. By finergrinding of the coal and, if costs allow, operating with higherexcess air, this poor carbon burnout problem may be avoided(DiGioia and Kelly, 1995). According to Jones and others(1995), experience with overfire air would suggest that thedeeper the air staging, that is, the gross movement ofproportions of combustion air away from the primarycombustion zone, the higher the unburnt carbon. However,this effect is not always observed and does not occur wherethe necessary conditions are present for good burnout.
The collection mechanisms for ash can influence the type ofash captured. Ash collected by electrostatic precipitators(ESP) generally have higher specific surface areas comparedto those collected by mechanical precipitators such ascyclones (Joshi and Lohtia, 1995). Fly ash capture in ESPcan be enhanced by injection of sulphur trioxide into the fluegas after the regenerative air-heaters to reduce the ashresistivity. This system is applied successfully at the CastlePeak B power station in Hong Kong (Meijers and Brunskill,1994). No indication was given of any negative effect of thesulphur trioxide on the fly ash produced.
Quality control
Fineness is one of the most important parameters for manyapplications and therefore quality control often concentrateson guaranteeing a 'fine' fly ash. Fly ash of varying finenesscan be obtained directly from various steps in the ESP(Monzo and others, 1994). Fineness can also be controlled inthe laboratory or in an ash preparation plant by differentphysical separation processes. These processes are discussedin more detail in Section 5.2.
Wet lime/limestone scrubbers are the most commonly usedflue gas desulphurisation systems (FGD) on coal-fired powerplants (lEA Coal Research, 1995) and do not normally affectthe fly ash because they are sited downstream of particulatecollection systems. Sorbent injection systems and spray dryscrubbers give rise to mixed residues of fly ash and sorbentwhich are not considered here. FGD residues were dealt within a previous report by lEA Coal Research (Clarke, 1993a).
5.2 Storage and transportFly ash can be absorptive and reactive and therefore the wayit is handled can either improve or lower its marketability.
5.2.1 Storage
In modern coal-fired power plants, fly ash is transferred to astorage silo by air or mechanical conveyors. From here thefly ash can be supplied in a dry fOim. However, in mostpower stations there is a limited amount of silo space inwhich fly ash can be stored and therefore it must be soldquickly in a dry form or stored in a wet form. For example,in Iowa, USA, the silo storage capacity of many power plantsis normally only 1-2 weeks of overall generating capacityand it can be difficult to meet the demand for fly ash duringpeak construction months (Kilgour and others, 1989).
Coal ash may be used for temporary or seasonal uses such asroad or dam projects which require large volumes for shortperiods of times. Vliegasunie in the Netherlands have foundthat supply and demand is very much a seasonal problem. Inthe winter the power stations produce more fly ash but thedemand for products such as Betonas (fly ash concrete) ishighest in the summer, when building activity reaches itspeak (Duos, 1994). The Castle Peak B power station in HongKong demonstrates the wide range of fly ash 'disposal'methods. Fly ash can be transported off site in either wet ordry form on lorries or road tankers, dumped into a slurrymixing tank then pumped to offsite lagoons, blown to heavyload berth silos for loading into barges, or blown to theadjacent cement production plant (Meijers and Brunskill, 1994).
In the dry ash handling system at the 2,400 MWe Labadiepower station near St Louis, MO, USA, the ash is blown over900 m through the piping from the ESP to the silos. The flyash control system is controlled by computers linked to videocontrol panels. The capacity of the system is almost 200 t/hwhich is more than the 44-62 t/h that the station normallyproduces. The high cost of the ash handling system is offsetin part by the contract to sell the ash to a major cementmanufacturer. The cement company is building twoadditional silos on the power plant property along with aloading facility (Shadduck and Stillman, 1995).
63
Quality control
Lagoon
1.13'" 1 12E .~1.11
~1.10
.~ 109i!5 1082:' 1070 1.06
24 25 26 27 28 29 30 31 32
Moisture % (dry basis)
Typical BS compaction test results onLagoon PFA:
Optimum moisture: typically 24-33%* Max dry density: typically 1.0-1.4 tlm 3
* obtained during compaction
Conditioned
1.38'" 1 36E .~1.34
~1.32
.~ 1.30i!5 1282:' 1260 124
12 13 14 15 16 17 18 19 20 21 22 23 24
Moisture % (dry basis)
Typical BS compaction test results onconditioned PFA:
Optimum moisture: typically 15-22%* Max dry density: typically 1.25-1.5 tlm 3
Figure 15 Moisture content versus dry density curve for typical lagoon and conditioned ash (PowerGen, 1995)
Since storage of dry fly ash is problematic and expensive, flyash is more commonly available from power stations in oneof three wet forms:
conditioned fly ash, ash taken directly from the silos towhich water is added to assist with delivery andsubsequent compaction on site;stockpiled ash, previously conditioned ash which hasbeen placed temporarily on a stockpile before delivery tosite;lagoon ash, fly ash which has been taken from the ashsilos, pumped to storage lagoons and allowed to settleand drain prior to re-excavation and delivery to site.
Figure 15 demonstrates the differences in optimum moisturecontents between conditioned ash (including stockpiled ash)and lagoon ash. The average dry density is much lower forlagoon ash and the percentage moisture is much higher(PowerGen, 1995).
High calcium (Class C) fly ashes are self-cementitious ifexposed to water (in rain). Once this has occurred the ashesare generally not economically reclaimable (Kilgour andothers, 1989). Even when stored dry, it has been reported thatpulverised coal ash shows a decrease in maximum drydensity and an increase in porosity with storage time. Nosignificant differences in chemical properties were noticedbetween fresh and stored samples (Nagataki and others,1995). Fly ash may be agglomerated into pellets which canwithstand the vibration, attrition, compression and similarforces encountered during stockpiling. The pellets can thenbe re-ground into fly ash and used in the cement industry(Kilgour and others, 1989). This is discussed in greater detailin Section 5.3.3.
5.2.2 Transport
Since coal ash is not commonly used for high-valueapplications, it is not normally economic to transport ashover great distances. This is a particular problem in the USwhere many areas with low population densities areproducing significant quantities of coal byproducts (US DOE,
64
1995). Boyd and others (1994) note that concrete marketsclose to high-quality ash sources in the USA are becomingsaturated, whereas a significant number of regional concretemarkets use limited amounts of fly ash because local sourcesdo not meet the required standards of quality.
The cost of transport of any product must be borne by thevalue of the final product. Bulk materials such as ash aresensitive to transportation costs to the point that they becomeunmarketable over a certain distance. For example, for theHouston Lighting and Power Company the normal truckingrange for coal ash is about 100 km. In addition, problemsarise due to the consistency of the ash itself. The lightpowder blows like dust when its is not contained andpresents a significant environmental problem duringtransportation. SEEC Inc, a Minnesota-based company, hasdeveloped an air-tight container which is ideally suited to thetransport of coal ash. The container, developed though a USDepartment uf Energy funded project, is collapsible andmade of rubber coated nylon with other heavy duty materials.The containers measure about 2.7 m by 3.04 m and hold upto 23 t of coal ash. The containers can be transported by railon coal trains thus maximising on the backhaul capacities ofa power station site (Hansen, 1995).
5.3 Processing and beneficiationFly ash can be processed in several ways. Firstly, there aresimple treatment processes to ease the handling and storageof the fly ash, for example, dewatering of previouslylagooned ash, ash blending and agglomeration of ash intodiscrete pellets for easier handling. Secondly, there areprocesses to enhance the fineness of ash. These includegrinding, sieving, and air classification. Next there areprocesses such as flotation and fluidised beds which separateash particles according to density. More advanced processessuch as carbon burnout and electrostatic separationconcentrate on removing the carbon from the ash whereasmagnetic processes remove fractions such as iron. Slakingand other chemical treatments are more specialised processeswhich aim either to remediate particular problems of certainashes or treat ashes to suit one specific application.
Quality control
Combined processes are those which use more than one ofthe above methods to improve the quality of fly ash. Thefollowing sections discuss examples of these treatmentswhich are most commonly used and for which information isavailable.
5.3.1 Dewatering
transferred to a holding tank for further dewatering. Threedifferent dewatering processes were tested: a frame pressurefilter; a solid bowl centrifuge; and a vacuum belt filter. Allthree dewatering processes were found to be effective butduring the course of the study it was not possible todetermine which system would be the most economical1yviable (Oussa and Yang, 1995).
As mentioned in Section 5.2, although fly ash is moremarketable in a dry form, storage limitations often mean thatash has to be stored in a wet form in ponds or lagoons. Thisash may require dewatering if it is to be utilised.
The Yallourn W power station in Victoria, Australia, burnsbrown coal from the Latrobe valley which has a highmoisture content (up to 70%) and a low ash content (up to2%). This is a relatively low grade fuel but one which can beused in modern coal-fired power stations (with specialcombustion technologies). The ash handling system wasfound to be incapable of transporting all the ash to ashsettling ponds and this was resulting in contamination of anadjacent river (Oussa and Yang, 1995). An ash dewateringsystem was necessary to reduce the amount of wet ash whichrequired storage. Figure 16 shows a systematic diagram ofthe dewatering system developed. The ash is delivered to theplant by a feedline and passes over a I mm screen to separatesolid particles from the ash water before feeding to theclarifier. The solids are removed manually from the screen.Around 68% of the solid particles collected on the screen areunburnt carbon. This high carbon content is due to the natureof brown coal and its inefficient combustion. Around 4-5%of the total product was ash and 60--65% was moisture.Further drying can reduce this moisture content to 20--30%.If the screen mesh size were reduced to 0.5 mm it may bepossible to collect approximately 93% of the unburnt carbonwhich could then be available for future utilisation. Theunburnt carbon could be used as low grade activated carbonor could be processed further to produce high grade activatedcarbon.
The other product from the dewatering plant is 'ash water'.This water is thickened and clarified with an anionicpolymer. The ash particles are flocculated and allowed tosettle in a slurry. The slurry (10--30% solids) is periodical1y
Surschiste SA, an French company set up to promote shaleand ash utilisation in France, have opted for ash drying as ameans to utilise ash which has been stored in a wet form.The dried ash is reported to have at least the samecharacteristics as the fly ash taken directly from the ESP.Only ash from ash piles is dried, ash from ponds cannot bedried due to the large amount of water to be evaporated andthe presence of contamination from the water andsurrounding run-off. Ash drying must be profitable in orderto be viable. Surschiste SA concluded that the cost of energyfor ash drying would nomla]]y be prohibitive but that the useof methane from abandoned mines would provide cheaperenergy. The first ash drying facility was set up at theHornaing power plant to recover silica-alumina ashes fromheaps with an average moisture content of 18% (maximum24%). The dryer has an output of 50 tJh of product with amoisture content lower than 1%. The dryer is fitted with agyratory miJJ, pneumatic ash removal and a bag filter to trapdust emissions. The dryer consumes 630 MJ/t of dry ash asheat and 6kWh/t of dry ash as electricity and is operated byone person. A second ash drying plant has been set up at theCarling power plant in France and has an output of 80 tJh(Grandjean, 1993).
5.3.2 Blending
Fly ashes can be blended in the same way that coal can beblended. Fly ash which has a very low carbon content can bemixed with fly ash of a carbon content too high to beacceptable for many uses thus making a greater quantity ofuseful ash. For example, although ELSAM, in Denmark,rarely process fly ash as it is consistently of acceptablequality (LOI <4%, seldom exceeding 6-7%), fly ash is oftenblended in order to achieve the right composition (Poulsen,1995). This type of ash treatment may be quite common butis not well documented.
feed tank
ash water
ash slurry --1.~
dewatering
;--. / I,
clean I
water Ioverflow I
~/ , "
OJ c: j (0) (0)
• • • • • •water solids water solids water solids
frame filter centrifuge vacuum filter
ash waterclarification
polymer dosing
----. carbon\..,............
carbonseparation
ash water feedfrom station
•I I
;1··· ....screen (1 mm)
..
Figure 16 Systematic diagram of a pilot ash dewatering plant (Oussa and Yang, 1995)
65
Quality control
At the Maasvlakte power plant in the Netherlands fly ash issieved. Here up to 8 different fly ashes can be blended at thetime of sieving to maximise the quality of the product. TheMaasvlakte plant is discussed in more detail in Section 5.3.5.
Fly ash from pulverised coal combustion can also be blendedwith fly ash from f1uidised bed combustion (FBC) (Nagatakiand others, 1995). Hemmings and others (1995) describemethods to produce no-cement concretes from blendingpulverised coal fly ash with AFBC residues. The pulverisedcoal ash provides the aluminosilicate glass and the AFBCresidues provide the calcium oxide/hydroxide, calciumsulphate (anhydrite) and dehydroxylated clays. Thepulverised fly ash and the AFBC residues can be selected tocover a wide range of potential compositions.
5.3.3 Agglomeration
Agglomeration of fly ash is a method of concentrating theash into discrete pellets. Agglomerated fly ash is easier tostore and stockpile. The pellets may potentially be used inroad construction. Following agglomeration and subsequentregrinding, the fly ash, although reduced in quality, may stillbe used in some other applications.
Bergeson and others (1990; also Kilgour and others, 1990)used field-scale commcrcial agglomeration equipment toagglomerate two different, high calcium (26% and 30%) flyashes from two power stations in Wyoming (Ottumwa andLansing). The agglomeration medium used was simply water.Three different agglomerating methods were used:
A rotary pan continuous agglomerating system. Here thefly ash is fed slowly onto a large rotating disc and theagglomerating liquid (water) is sprayed on as thematerial rotates. The pellets produced are spherical andtheir formation and size are controlled largely by thespeed of rotation, angle of inclination, amount of binderadded and time.An auger type continuous agglomerating system. Thissystem is similar to a pugmill on an asphalt plant. Speedand binder addition rates are controllable. The pelletsproduced are irregular in shape.A batch turbine system. Paddles are mounted in acircular, flat bottomed mixer and mixing time and binderaddition are controllable. The pellets produced areirregular in shape.
With the rotary pan, the optimum moisture content for the flyash was around 10%. Higher moisture contents resulted infaster agglomeration and weaker agglomerates whichcollapsed. Coalescence and pair formation were also aproblem. At lower moisture contents the fly ash adhered tothe surface of the drum. Insufficient data was obtained forany conclusions to be made about the auger system. Thebatch turbine system tended to produce larger agglomerateswhich broke down with extended mixing. Agglomerates ofwidely varying size, strength and pore matrix could beproduced with both the continuous and batch systems.Agglomerates can therefore be produced with thecharacteristics essential for their proposed end use. All threesystems required several trial runs to establish the ideal
66
conditions for the required size and shape of pellets. Therotary pan agglomeration system demonstrated greaterflexibility in the characteristics of the materials produced andwas therefore selected for further research.
Compressive strength testing of the agglomerates indicated atrend of decreasing strength with increasing size. Pelletsgreater than 37.5 mm in diameter showed strengths of50--700 psi (0.35-4.8 MPa). Pellets of 19 mm diameter hadstrengths of 250--800 psi (1.7-5.5 MPa) and those of 9.5 mmhad strengths of 500--1000 psi (3.5-6.9 MPa).
To test the re-usability of the pellets, Bergeson and others(1990) micronised the pellets in a microniser produced by theMicroFuel Corporation, Iowa, USA. The microniser usedcentrifugal force to grind the pellets without the use ofmechanical force. The system recycles larger particles andthe final fly ashes were below 75 fLm in diameter. Both rawand micronised fly ashes from the Ottumwa power plant weretested at 15 and 30% replacement levels in concrete andfound to exhibit strength and durability factors greater than orequal to the control concrete. Raw fly ash from the Lansingpower plant showed higher compressive strengths but lowerdurability factors than the control concrete. The micronisedpellets from the Lansing fly ash produced a concrete whichwas much lower in compressive strength due tocontamination with dolomite fines during the micronisationprocess. Bcrgeson and others noted that the micronised flyashes exhibited lower compressive strengths than the rawashes but that the durability factors were not affectedappreciably. It was concluded that reground agglomeratedClass C fly ash behaves more like a Class F fly ash inconcrete and also in soil stabilisation applications where anactivator is required to initiate the pozzolanic reactions.
5.3.4 Grinding
Grinding of fly ash is a way of obtaining finer fly ash.Grinding will reduce most of the fly ash to the finenessrequired and then sieving can be used to separate out anyremaining fly ash which has not been reduced to a smallenough size. Fly ash obtained from power plants with lowNOx burners is commonly coarser than that from other plants(see Section 5.1.3). In order to remediate this effect, grindingand separation may be used (Eymael and Cornelissen, 1995).
For blended fly ash cement the fly ash may be ground on itsown or may be co-ground with clinker. In the latter case, inaddition to reducing the particle sizes, grinding also has theeffect of levelling out the chemical variations between the flyash and the clinker. Further, the fly ash acts as a grinding aidduring the grinding of clinker. Under normal circumstancesthe specific gravity of fly ash can be increased by 5% bygrinding and may be increased by up to 21 % in some cases.This specific gravity corresponds to a decrease in air voidcontent which leads to increased cement strength. Lightgrinding of fly ash gives a significant increase in strengths ofconcretes due to the decrease in air void content (increase inspecific gravity). Ground fly ashes show less agglomeration(Stoltenberg-Hansson, 1989).
Supplementary cementing materials with particle sizes in the
Quality control
original fly ash
•order of 1 f.lm are in great demand for high performanceconcretes. Eymael and Cornelissen (1995) have shown that itis possible, with a newly patented process, to grind('micronise') fly ash particles to under 5 f.lm. The resultingfine fly ash gave higher strength in comparison with other flyashes when used in concrete. However, Eymael andCornelissen emphasise that it would be a challenge toproduce such fine fly ash economically.
Fly ash treated with quicklime can be used as a sorbent inspray dry scrubber systems for flue gas desulphurisation (see
Section 2.2.4). Meyers and Keener (1995) demonstrated thatgrinding the fly ash in a ball mill for 48 hours increased thesurface area of the fly ash and thus its efficiency as a sorbent.
5.3.5 Sievingpass400 mesh
no 200 mesh
no 300 mesh
no 400 mesh
motor
Fineness is one of the most important parameters for manyapplications of fly ash and therefore quality control oftenconcentrates on guaranteeing a 'fine' product. Monzo and
Table 19 Comparison of compressive strength (MPa) ofordinary Portland cement and fly ash ofdifferent fineness (Kruger, 1995)
Age, Ordinary Portland cement containingdays Portland Fly ash Fly ash
cement <45 f.1m <10 f.1m
I 8.9 7.0 7.03 26.7 21.7 26.07 38.3 31.6 40.528 53.8 54.0 63.556 59.6 61.5 71.590 66.8 70.0 80.0
Figure 17 Three-dimensional vibration sieving machine(Sheu and others, 1990)
others (1994) found that when the fineness of fly ash wasincreased by sieving at laboratory-scale the pozzolanic effectof the fly ash was increased. The enhancement ofcompressive strength was found to rely on particles below10 f.lm. Table 19 shows the comparison of strengths betweenordinary Portland cement and that made with fly ash ofdifferent fineness (Kruger, 1995).
Class F fly ash from the Shin-Ta power plant in Taiwan wassieved using a three-dimensional vibrating sieving machine(Figure 17; Sheu and others, 1990). Three sieves were used No 200 (74 f.lm), No 300 (48 f.lm) and No 400 (37 f.lm).From these, four grades of dry sieved fly ash were obtained.Of the 100 kg of ash sieved, 5.6 kg was retained on the 200mesh, 16.2 kg was retained on the 300 mesh and 5.8 kg wasretained on the 400 mesh. A total of 72.4 kg of ash passedthrough all three meshes. Table 20 shows the chemical
Table 20 Chemical composition and pozzolanic activity index of sieved classified fly ash (Sheu and others, 1990)
ltems Original Fly ash Fly ash Fly ash Fly ash CNS 3036 CNS 3036
fly ash retained passing passing passing Class F Class C
200 mesh 200 mesh 300 mesh 400 mesh
Chemical compostion, %19nition loss 7.01 20.0 6.24 5.24 3.00 :5: 12.0 :5: 12.0
SiOz 47.33 44.37 47.65 48.14 48.76 {s,o, + AhO, SiOz + Ab03 }AI203 24.88 16.77 23.35 28.77 25.78 + Fe203 + Fez03
Fe203 7.14 6.99 7.91 8.26 6.84 2:70.0 2:50.0
CaO 8.21 7.99 9.44 8.25 7.75
MgO 1.45 1.25 1.56 1.47 1.24 :5:5.0 :5:5.0
KzO + Na20 1.21 0.60 1.26 0.99 1.14 :5:1.50 :5: 1.50
TiOz 1.34 0.50 1.01 1.40 0.63
SO] 0.46 0.81 0.38 0.45 1.35 :5:5.0 :5:5.0
Pozzolanic Activity IndexCompressive strength
compared to control, 81 83 84 93 2:75 2:75
FIC mix, 28 days, %Compressive strength F/L mix, 7 days,KPa 82,284 85,593 92,125 103,125 2:5,500 2:5,500
psi 1200 1245 1340 1500 2:800 2:800
Compressive strength of control group = 4750 psi
67
Quality control
500(49)
<UQ
2 400;: (39.2)E-2Ol.Y
.r::.OJc~
(j) 300.~ (29.4)(f)(f)Q)
CiEou
200(19.6)
1 FA < 400 mesh2 FA < 300 mesh3 original fly ash4 control group
. watermass ratio II h = 0.35
cement + yas
with 20% fly ash replacement
100(9.8)
3 7 14 28 90
Curing age, days
180 360 720
Figure 18 Fineness of ash and its effect on compressive strength of mortars (Sheu and others, 1990)
composition and pozzolanic activity index of the sieved ash.The results indicate that, in general, the finer the ash thelower the LOr. In addition, the concentration of Aha} ishighest in the finest ash. The sieved ash gave highercompressive strengths than the original coarse ash. Figure 18shows the relationship between fineness of the fly ash andthe compressive strength of the concrete produced (20%replacement). Further research indicated that 30%replacement of fly ash in the concrete gave the best results.The finer the ash, the greater the long-term compressivestrength and the lower the water demand. Also, thosecements made with the finer fly ash showed greaterresistance to sulphate attack. However, samples made withfiner fly ash were found to have greater shrinkage than thosemade with the original un-sieved ash.
Vliegasunie in the Netherlands are developing a fly ashstorage and upgrading plant at Maasvlakte with an overallcapacity of over 225 kt of fly ash per year (Duos, 1994).Construction of the plant started in 1993 and the plant wasoperational by the end of 1995. The plant consists of fourlarge concrete silos with a storage capacity of almost 8 kteach. The concrete terminal building has a further nine silosof almost I kt each for storing incoming deliveries of fly ashand upgraded ash to be dispatched (van den Berg, 1995).There is also the building containing the sieving systems. Thefly ash is sieved into a coarse fraction, containing most of theunburnt carbon, and a fine fraction, which is suitable forutilisation. The coarse fraction can be mixed with coal and
68
reburned in the power plant. The high carbon fly ash can alsobe used for clinker in the cement industry. After sieving, upto three different fly ashes with varying compositions can bemixed. By varying the amounts of the different fly ashes, ahigh quality product can be maintained. Around 75% of thefly ash processed in the Maasvlakte plant is delivered directlyfrom the Maasvlaktecentrale power station by a closedconveyor pipe. The remaining 25% of the ash arrives fromother power plants in the Netherlands (Duos, 1994).
5.3.6 Air classification
A similar but alternative separation process to sieving is airclassification. Air classification separates particles primarilyas a result of different aerodynamic properties.
Minoux and others (1995) investigated the principles of airclassification in two Condux pneumatic separators, one atlaboratory-scale and one at pilot-scale, based on verticallypositioned turbines. In the laboratory-scale separator fly ashis introduced to the separator by a screw and the air flow isinjected through the spiral cover. The counter current draughttravels upwards and disperses the ash for separation. Fineparticles are removed in the overflow section and coarseparticles fall to the lower zone. Minoux and others (1995)studied in detail the different dimensions and parameters ofthe separator which affected the distribution of the fly ash.The most important parameters were the rotation speed andcut diameter. The recovery of fine particles depended on the
Quality control
r inlet screw
(1 )
(5) overflow
b:~~~g~- (2) air, spiral cover
(6) discharge
process are shown in Figure 19. Ash is fed into the separatorat 6 t/h and the air flow rate is 7000 m3/h. Studies on thepilot-scale facility confirmed the importance of the rotationspeed of the turbine. An increase in turbine speed induces anoverflow and reduces the quality of separation of the fineparticles. Increases in air flow and ash feeding speed favourthe extraction of fine particles. The fine particles leaving theturbine through the overflow are further separated by cyclonefilters. Through this process, four sizes of fly ash can beobtained - <10, <30, <50 and <80 /-Lm.
KEMA (NV Tot Keuring van Elektrotechnische Materialen)in the Netherlands has a pilot air classification plant with acapacity of almost 2 t/h. A diagram of the air classifier isshown in Figure 20 and the resulting particle size distributionis shown in Figure 21. Fly ash from power plants fitted withlow NOx burners, which is known to be coarser than otherfly ash, was separated in the air classifier. Removal of thecoarser fractions from the fly ash gave a significantimprovement in concrete properties. However, the airclassifier is not suited for carbon removal because of thedistinct density and aerodynamic behaviour of the carbon incomparison to the fly ash particles (Eymael and Cornelissen,1995).
~ultra fine
fan
rotatingdisc
t coarse
fine
-
Figure 20 The KEMA air classifier (Eymael and Cornelissen,1995)
Air classification of fly ash in an aerodynamic tunnel with ahorizontal air current was used to separate four size fractionsof Class F fly ash from a coal-fired power plant in Andorra,Spain. However, no differences in compressive strength werefound between the separated fractions and the original fly ashexcept in the finest fraction which produced a strongercement. No measurements were made of the actual particlessizes in each fraction (Paya and others, 1995b).
5.3.7 Flotation
The simplest form of density separation is probably theskimming of cenospheres from the surface of lagoons. Since
filters
ultra-fine
fluidisationGalilee number
fine
probability ofpassing
stocking hopper ..--
coarse
gravityReynolds number
The Condux size classifier
transport ofselected particles
feed
The selection process
Scheme of the industrial installation
Figure 19 A pilot-scale air classification process (Minouxand others, 1995)
air flow and the ash feeding speed. Following work with thelaboratory-scale separator, a pilot-scale unit was built at apower plant in France (unnamed). Diagrams of the plant and
69
Quality control
100010010
Particle size distribution, JlITl
0,1
o
~0
<Ii fly ashN'(jj 50 1 inputQj"D 2 classifiedc::J 3 ground
4 silica fume
100
Figure 21 Particle size distributions of classified and ground fly ashes in comparison with the input fly ashes hJom)(Eymael and Cornelissen, 1995)
the cenospheres have a specific gravity of less than one, theyfloat to the surface of the lagoon and may be skimmed offusing special boats (PowerGen, 1995). A more advancedversion of this is froth flotation.
Froth flotation is the most common method used in themineral processing industry. The process is aphysico-chemical one taking place in a solid slurried withwater. The col1ector selectively coats the surfaces of themineral particles causing selected components to becomehydrophobic and therefore they become attached to airbubbles. A second reagent, known as a frother, is used tostabilise the air bubbles. Separation is achieved as themineral-laden bubbles rise to the surface as a froth whichflows over a weir. The success of froth flotation dependsupon the capability for selective differentiation between thematerials in the raw mix. Kerosene can be used as a col1ectorfor carbon from ash and pine oil or a polypropylene glycolmay be used as a frother. Froth flotation for fly ashbeneficiation has become more attractive with the increasedproblem of carbon in ash due to NOx abatement technologiesand also due to the fact that such wet processes can beapplicable to wet ashes such as those stored in ponds orlagoons. However, the disadvantages of froth flotationinclude the fact that the product is wet and requiresdewatering and drying. Residence times are high as arereagent costs.
A new system has been developed based on froth flotationwhich separates low LOI ash from a high carbon productwhich can be upgraded and reburned. It was presumed thatthe carbon in fly ash would behave similarly to oxidised coaland therefore flotation reagents which are suitable forcleaning oxidised coal were used. Fly ashes were cleaned toless than 0.5% carbon with typical carbon reductions of 90%.The froth product had a sufficiently high carbon content toallow it to be recycled to the furnace. A 'TREE' procedurefor the flotation was adopted whereby a separation occurs ina sequence of rougher, cleaner and scavenger flotation stages
to generate a variety of products with varying grades.Emulsification of the collector eliminates the need for costlypreconditioning steps. The wet products were dried byincorporating a gravity thickener into the dewatering circuit.The fly ash dewaters easily to moisture levels of 17-19%with vacuum filtration cycle times as short as 1.5 minutes.The dewatered ash could easily be handled at such moisturecontents or could be dried further with conventional methods.The whole system, as shown in Figure 22 was developed toprocess 25 tlh fly ash. Two principal products are produced a low LOI fly ash and a high carbon product. The recoveryof cenospheres and spherical magnetic material could also beincorporated into the process. All of the equipment used inthe design is currently commercial1y available (Groppo andothers, 1995). A combined process for fly ash beneficiationwhich includes froth flotation as well as magnetics removal isdiscussed in Section 5.3.13.
5.3.8 Fluidised beds
Density separation of fly ash is distinct from particle sizeseparation (for example, sieving) because different mineralshave different bulk densities (Vempati and others, 1994). TheJapanese Industrial Standards (nS) set an upper limit of 5%unburnt carbon in fly ash. In order to increase the amount offly ash that is acceptable, Mori and others (1994) at NagoyaUniversity, Japan, have developed a 'reforming' processbased on f1uidised beds. The process separates fly ash byparticle size and by density and the unburnt carbon isremoved.
In the first stage of the process the carbon rich coarserfraction (>100 j.Lm) is separated from the finer fraction byeither a fluidised bed or an air separator. The air separator issimpler to use and has been adopted for the process. Thefiner fraction is then fed into a newly developedvibro f1uidised bed where the carbon rich fraction rises to thebed surface and is removed by suction nozzles, as shown inFigure 23. Fine fly ashes do not distribute uniformly in the
70
Quality control
......................................... .L..-_,....----I .
010
~fly ashfeed
hopper
bulk reagentstorage
..8 .collector
80
81.331.3
31.3
mixingtank
1
o 0
1093.7 1093.7
Level 3
dryer
31.3 10
1125 1175
25 97
43.1
low carbon product
31
106.3
Level 2 I ~~,====:=j........................................L,.
227 237
o 0
220.7 220.7
25
898
10
936 ~~~
25 ~
~ 65
diskfilter
Level 1
~I:~~ ~
...........~ .....~...clarifiedwatersump
------------------------------
--::-1153~high
carbonproduct
--------------Dlegend
TPH dry wt%solids solids
GPM GPMwater slurry
Figure 22 Conceptual flow-sheet and solid balance for a 25 t/h fly ash processing facility (Groppo and others, 1995)
71
Quality control
low gas velocity of a normal fluidised bed and therefore thenew horizontal vibro-fluidised bed was developed. Thevertical vibration is generated by two vibro-motors set at25Hz and 0.5 mm amplitude. Figure 24 shows the reductionin unburnt carbon with time for various raw fly ashes. Theoperation is only started after the fluidised bed has been filledwith fly ash. In the initial period of operation the unburntcarbon rises up into the discharge tube and overflows causingthe increase in carbon at the beginning of the operation. After
this the carbon content gradually decreases. If the gasvelocity is too high solid mixing is promoted and the carboncannot be separated. The plant, which has only been tested atbench-scale, can result in 60% of the raw fly ash beingcleaned to below 5% carbon. It is estimated that a pilot plantwould operate at around 1000 kg/m2h. Assuming across-sectional area of 5 m2, around 120 t of fl y ash could betreated in a day.
Figure 23 Vibro fluidised bed (Mori and others. 1994)
Meijers and Brunskill (1994) describe the state-of-the-artcentrifugal classification system for fly-ash which is locatedat the China Light and Power Company's Castle Peak Bpower station in Hong Kong. Ash is fed through a fluidsolids pump into the classifier at 150 tlh and is classified intotwo fractions -smaller or greater than 45fLm. The fractiongreater than 45 fLm is rejected. A diagram of the classifier isshown in Figure 25. The ash classifier works by feeding theash from the two inlets onto a distribution plate whichachieves homogeneous distribution before dispersion. In thedispersion zone agglomerates and larger particles areimpacted against the wall. Agglomerates are thusdecomposed and are dispersed into the air flow along withthe fine material. From the dispersion zone the ash enters theseparation zone circulating at a high velocity. The separationof fine and coarse particles is achieved due to the balancebetween centrifugal and drag forces. The air flow enteringthe separation zone via the rotating cage causes the dragforces on the particles. The coarse materials fa]] though theseparation zone into the outlet cone. The fine materialemerges with the separation air through the rotating cage andis discharged through one or two discharge pipes. Separationof the fine materials from the separation air is accomplishedwith two fine material cyclones downstream of the separator.Essentia]]y the fineness of the ash can be adjusted bychanging the speed of the rotating cage. The classificationplant is highly sophisticated and only requires four operators.The cost of the plant, which was designed and built byBateman Material Handling Ltd, South Africa, wi]] be offsetwithin five years by the sale of the by-product.
CD entrance@ bed@ overflow
@ withdraw
® suction nozzlel@ vibro-motor(J) distributor
carbon-t-----. rich ash
®
CD
20 6. B U02 =22cm/s, Wf2 =1.7kg/h
... E UOz=2.0 , Wfz =2.0
• F UOz=2.2 , Wfz = 1.6• H U02 =2.0 , Wfz =1.8
o I UOz=2.0 , Wf2 =3.1
oo 80 120
Operation time, min
180 240
Figure 24 Reductions in unburnt carbon content in product fly ash from various raw fly ashes (Mori and others, 1994)
72
Quality control
connecting point for grease
~\---------(3 dispersion zone
L....C:::=.---\--6,L------:---------{2 distribution plate,
I11"':1!---~---++------{4 separation zone
---~--
I~!-+I---L---+!------{5 rotati ng cage
l..:::::::==='--~=:::±::::=~lL..lL-j:!:==F=~IJ
7}----------f+--..
discharge pipes
~L-------------___{6 outlet cone
Figure 25 Typical high efficiency centrifugal fly ash classification system (Meijers and Brunskill, 1994)
5.3.9 Carbon burnout
It has been estimated that a reduction of 3% in carbon-in-ashwould reduce costs at a 325 MWe unit by $900,000 per year.This saving would be as a result of reduced disposal costs,increased sales revenues and the improvement in the unit heatrate. A carbon-in-ash measurement system (CAM) has beendeveloped to measured carbon-in-ash in an on-line manner.This was discussed in more detail in Section 4.3.6.
According to Cochran and others (1995), the improvement offly ash by size separation is based on the assumption that thelarger particles are carbon. Cochran and others argue that,although this is often the case, the larger particles are only asmall percentage of the whole and therefore their removalonly marginally reduces the carbon content of the fly ash. Acarbon burnout process (CBO) based on a fluidised bed hasbeen developed jointly by Electric Power Research Institute(EPRI), The Duke Power Company, and the Florida Powercompany to combust the remaining carbon from the fly ashafter it has left the boiler. In order to achieve maximumburnout there must be excess air (> 12.5 Ib/lb of carbon for10% excess). Fluidised bed combustion can provide thisexcess air and will also supply the long residence time whilstminimising particle entrainment.
Figure 26 is a flow diagram of the CBO. Fly ash received inthe silo is metered into the fluidised bed. Any particles thatare entrained into the gas flow are collected in a collectiondevice and recycled to the bed. The cleaned ash productoverflows at the discharge end into a product cooler. Sincethe fly ash itself is inert, no other bed material is required.The cooler may also be a heat recovery device, depending on,among other things, the specific application. The operatingtemperature of the bed is 1200-1 500°F (650-8l5°C).Optimum bed temperatures are determined by initial pilotplant evaluations and vary from one fly ash to another.
The I t/h CBO pilot plant has been tested over the past twoyears burning fly ashes with carbon contents ranging from4.5% to 18%. Continuous combustion of the fly ash for over63 hours without supplemental fuel has been demonstrated.Within 3 hours of a cold start-up, fly ash with Lor below2-3% is produced. The CBO process also results in areduction in the mean particle size, further enhancing thequality of the fly ash, and has a tendency to produce concretewith a greater strength. Plans are now under way fordemonstration plants processing 100 kt per year each.Cochran and others (1995) recommend that the CBO couldbe used in conjunction with low NOx burners to reduce thehigher Lor ash typically associated with these burners.
73
Quality control
.----.....-------1~ heat recovery gas system
feed ash
Figure 26 Process flow diagram for carbon burnout (Cochran and others, 1995)
Figure 27 Triboelectrostatic particle separation (Ban andothers, 1995a and b)
Table 21 Analysis of ash fractions from triboelectrostaticseparation (Ban and others, 1995b)
feeder
copper plates
1-------- -15 kV
Negative Centre Positive
electrode electrode
7.73 31.25 61.0150.18 38.25 2.9749.79 66.19 97.23
Feed
100.0023.3977.05
co-flow
charger
Weight, %Carbon, wt%Ash, wt %
Differences in physical and chemical properties betweencarbon and ash mean that they can be electrically charged toopposite polarities and thus separated by passing through anexternal electric field. Figure 27 shows the electrostaticseparation principle (Ban and others, 1995a and b).Electrostatic separation is a fairly new method for separatingcarbon from fly ash and can only be performed on dry ash.For this reason, electrostatic separation has the advantageover other processes such as froth flotation andagglomeration in that it does not require excessive amountsof water, which may have a contamination effect on the ash,and the product does not require dewatering.
With electrostatic separation, fly ash is selected on electricalsusceptibility. This method of separation is far more sensitiveto chemical and mineralogical composition than methodssuch as air classification (Kruger, 1995).
Ban and others (I995a and b) have tested fly ashbeneficiation with electrostatic separation (triboelectricseparation) at laboratory-scale. The system processed 109 ofash sample at a time. Over 65% of the ash sample, from theESP of two pulverised coal fired boilers, was recovered witha carbon content of less than 3%. About 50% of the carbonin the ash was recovered when the carbon content of the ashwas greater than 35%. The wide size distribution of fly ashparticles «25 microns to >150 microns) presents a challengedue to an order of magnitude range in aerodynamic drag andgravitational forces. Table 21 shows the separation results fora fly ash sample which had not responded well to wetprocessing attempts to separate carbon from the ash. Studieswith ash from a plant burning intermediate to high sulphurcoal indicated that two-stage separation increased the ashrecovery by about 15%. Further work is being peIi'ormed toevaluate the effects of ash properties on separation with thegoal of optimising the beneficiation process.
A commercial company, Separation Technologies Inc, MA,USA, has developed a triboelectric separation process whichimproves upon normal electrostatic separation by four ormore orders of magnitude (Thompson and others, 1995).
5.3.10 Electrostatic separation
74
Quality control
positive electrode
belt
gap 8 mm
negativelychargedparticles
negative electrode
length 6.6 metres
belt speed 3-20 m/sec
Width of machines
6 inches-2 tons/h20 inches-12 tons/h40 inches-25 tons/h
positivelychargedparticles
Figure 28 Process diagram for electrostatic separation of fly ash (Thompson and others, 1995)
Figure 28 is a diagram of the separation process. Fly ash isfed into the thin gap between the two pm'allel plate electrodesand the particles are swept up by the moving open mesh belt.The moving belt sets up a counter cUlTent plane tlow fieldindependent of the electric field. The particles charge bycontact with neighbouring particles and the electric fieldmoves opposite charged particles in the direction appropriateto their charge. The electric field needs only to move theparticles a tiny fraction of an inch to place it in the con'ectdirection stream on the belt. The counter cun'ent tlowprovides multiple opportunities for separation and ensuresexcellent separation and recovery. The triboelectric systemeffectively separates the ash from the unburnt carbon. At theSalem Harbor plant, and a high LOI carbon product (21.37%LOl). The cleaned ash is sold to local concrete andconstruction markets.
The whole system is simple with the belt and rollers beingthe only moving parts. The system is 6.6 m long but this canbe altered depending on the specific application. Theelectrical consumption is around I kWh/t processed. At fullscale, the processor can process 23 t/h. Currently there aretwo such systems in operation, one in Salem Harbor Powerplant and one in Brayton Point power plant, both in MA,USA.
5.3.11 Magnetics
Fly ash contains metals such as iron which can be removedby magnetic separation. Paya and others (1995a) treatedground ancl unground lly ash under laboratory conditions byhand with an electromagnet to separate magnetic fractionsfrom the remainder of the fly ash. Following such treatment,
Table 22 Chemical composition of magnetic andnon-magnetic fractions of fly ash samples (%)(Paya and others, 1995a)
Sample Si02 Fe02 Ah03 CaO MgO LOI
Fly ash 39.7 11.5 33.3 11.1 2.0 0.3
Fly ash:magnetic fraction 28.7 24.3 34.4 7.4 1.5 0.5
non-magneticfraction 42.5 5.6 31.4 11.7 2.0 1.8
Ground ny ash:magnetic fraction 28.2 31.9 25.5 7.9 1.3 0.7
non-magneticfraction 40.2 7.2 31.0 11.2 1.9 1.9
unground fly ash with an initial iron oxide content (as Fe203)of 11.5% was separated into a magnetic fraction with 24.3%iron oxide and a non-magnetic fraction with 5.6% iron oxide.Lower Si02 and higher CaO contents were also noticed inthe magnetic fraction as shown in Table 22. In general,mortars prepared with non-magnetic tly ash were found tohave similar or higher tlowabiJity than mortars containingmagnetic fly ash fractions. However, cement mortars madewith magnetic fractions had greater tlowability than wouldnormally be expected from the size and morphology of theparticles. When tly ash replaced cement in mortar by 30%the non-magnetic fractions showed greater pozzolanic activityat 20°C and 40°C curing temperatures. The pozzolanicactivity of magnetic fractions could be achieved by grinding.
75
Quality control
5.3.12 Chemical treatment 5.3.13 Combined processes
Figure 29 Separation schematic for a fly ash beneficiationprocess (Hwang and others, 1995a)
silicate spherecomponent
(cleaned ash)
Hwang and others (1995a,b) describe a 'beneficiation'process designed and patented by the Institute of MaterialsProcessing at Michigan Technological University. Theprocess separates unburnt carbon, cenospheres and iron oxidemicrospheres from fly ash by a combination of physical andchemical treatments. A flow diagram of the process is shownin Figure 29. The gravitational separation process separatescenospheres which are composed of low density hollowspheres. A magnetic separation step then separates iron oxideand iron silicate spheres from the ash. Lastly, a froth flotationprocess is used to recover the unburnt carbon. The 'cleaned'fly ash produced then consists of the silicate spherecomponent.
At pilot scale, fly ash is cleaned at a feed rate of about90 kg/h. The ash is mixed with water to form a slurry andpumped into a wet drum magnetic separator (Jeffrey Model510). The magnetic material is collected in a field of5 kilogauss, then filtered and dried. The remainingnon-magnetic material is fed into a settling tank to capturethe cenospheres. This settling tank provides a constant feedto the flotation circuit where the cenosphere fraction floats tothe top of the tank and is skimmed off, filtered and dried.The underflow material is pumped into a conditioning tank.In the conditioning tank reagents are added and mixed withthe slurry which is then pumped to a rougher flotation circuitconsisting of four Wemco No 12 flotation cells. The carbonfrom the rougher circuit flows to the cleaner floatation circuitand clean fly ash is collected in the cells. The cleaned fly ashis thickened, filtered, weighed and then dried. Hwang andothers (1995a) recommend that, if the received ash is verycoarse, a screen separation should be used as the first step ofthe process. Likewise, if Class C ash is used no carbonseparation is required as the LOI will already be low.
cenosphere component
carbon component
iron sphere component
fly ash
igravitationalseparation
+magnetic
separation
+froth flotation -.separation
•
Improvement of fly ash used in land reclamation or otheragricultural uses depends on the intended use of the land.According to National Ash (1995) there are solutions formost of the constraints. For example, to reduce the solublesalt content, fly ash may be lagooned or weathered forbetween 1 and 3 years. High pH and high boronconcentrations can be also be reduced by weathering orleaching or by choosing plant species most suited for the pHof the ash. If the fly ash becomes cemented upon applicationthen deep ripping is required or incorporation of soilimprovers. To reduce potential erosion the surface may bestabilised by water sprays or with soil cover or suitabledrainage can be provided. To compensate for any lack ofnutrients in the fly ash, ferti'i,ers may be applied ornitrogen-fixing plant species introduced.
One of the main reasons why concrete manufacturers requirelow LOI ash is that the unburnt carbon absorbs air-entrainingagents causing problems in controlling the air content of theconcrete mix. Fly ash with LOI below 3% has little or noeffect on the air entraining agents. However, Keenan (1995)suggests that fly ash with an LOI above 5% can be used tomake concrete as long as the fly ash is supplied at aconsistent LOI for the air-entrainment/concrete mix to beadjusted to compensate. The effect of LOI on air entrainmenthas been found to vary with different air entrainment agentsand therefore it may be possible to develop an agent which isnot sensitive to carbon in ash. This would mean that 'lowerquality' ash could be used on concrete manufacture.
Cerchar, the coal research centre of the French CdF Group(Charbonnages de France), has performed several studies onthe utilisation of ash from the 600 MWe Gardanne coal-firedunit. The coal supplied to Gardanne (42-44% volatile matter)has a high sulphur content (3.8%) and an exceptionally highnatural CaiS ratio (1.6-2.6). Crushed limestone is injectedinto the furnace along with the coal and this results in fly ashwith high calcium and sulphate content. The lime contentinduces mortar swelling by hydration. In 1990 Cercharpatented a new process to allow selective hydration of quicklime contained in fly ashes. The patented slaking process hastwo steps. Firstly the ash is moistened in a controlled mannerwith cold water. Secondly quick lime hydration is allowed ata heightened temperature. The resulting ashes are effectivelydry and have the same size distribution range as Portlandcement. A full-scale plant was due to start processing in 1993but no further information has been found (Grandjean, 1993).
It has been reported (Shi and Day, 1995) that the addition ofsmall amounts of Na2S04 and CaCh can increase thepozzolanic activity of both low calcium subbituminous ashand high calcium subbituminous ash. This results in asignificant improvement in strength. The two types of fly ashshow different degrees of sensitivity to the chemicalactivators due to differences in the chemistry and mineralogyof the ashes. The use of CaCh activator produced higheststrengths in the lime-pastes with low calcium ash. For pasteswith high calcium ash Na2S04 was the more efficientactivator.
76
Quality control
5.4 Summary and comments
Table 23 Summary of properties of Class F fly ashfollowing beneficiation (Hwang and others, 1995a)
viable filler in the plastics industry too as it has improvedbrightness, oil absorption, surface control properties and lesscenosphere breakage than uncleaned fly ash. The cleaned flyash could also be used at higher loading rates in PVc.However, the PVC containing cleaned fly ash showed lowerultimate tensile and elongation values which may be a resultof particle surface changes and coarser particle size comparedwith most commercial fillers (Huang and others, 1995).
Hwang and others (1994) estimate capital costs of thisprocess at from $9.7/t for an on-site plant processing 13.6t offly ash per day to $1.76/t for an on-site plant processing635 t/day. Operating costs range from $122/t for the13.6t/day plant to $11.80/t for the 635 t/day plant. Filtering,filtration and drying make up over 70% of the capital cost ofthe smaller plant, and nearly 58% of the larger plant. Thesecosts apply to a plant annexed to an existing power plant anddo not allow for transport and handling costs.
ADTM C 618(Class F ash)
70 (minimum)
6.0 (maximum)5.0 (maximum)
3.0 (maximum)34 (maximum)
Chemical composition As-received Clean ashash
Si02, wt% 54.8 57.5Ah03, wt% 28.8 30.2
Fe203, wt% 2.5 3.2
Total Si02 + Ab03+ Fe203, wt% 87.1 90.9
CaO, wt% 0.74 0.77
MgO, wt% 0.83 0.87
Na20, wt% 0.29 0.30
K20, wt% 2.70 2.80
P20S, wt% 0.16 0.17LOI, wt% 7.25 0.61
S03, wt% 0.31 0.32
Total, wt% 99.11 96.42
Physical test resultsmoisture content, % 0.28 0.13325 mesh retained, % 9.01 4.9\specific gravity, g/cm3 2.22 2.22
Fly ash may be beneficiated in many ways ranging fromsimple dewatering or sieving to very advanced processesinvolving physical and chemical treatments. Most of themore advanced techniques are at laboratory- or pilot-scaleand little information is available on how feasible they wouldbe economically. Although beneficiated ash may providematerials to specialised markets, this has to be weightedagainst the mass market available for the relatively simpleapplication of fly ash in concrete or as a simple bulk fill. It islikely to be the common factors such as transport costs, localmarkets and availability of alternative materials (seeSection 2.1) which detennine whether fly ash beneficiation iseconomical.
Hwang and others (\ 995b) also tested the cleaned fly ash as asubstitute for alumina in the production of mullite(3Ah03.2Si02). Mullite is an expensive material($1500-1900/ton) used for furnace linings, ceramiccomposites, protective coatings and infrared transmittingwindows. Class F and C fly ashes cannot normally be used toproduce mullite. However, Class F fly ash beneficiated by theabove process was used to synthesise mullite and theresulting material was comparable to commercial mullite.Even after beneficiation, Class C ash remained unsuitable dueto the high CaO content.
Class C ash from Detroit Edison Power Station, MI,USA. This ash has a high sulphur and soluble saltcontent.Class F ash from Consumers Power Company, MI, USA.The ash had a high LOI value of 7.2%.A mixture of 80% Class F and 20% Class C fromDetroit Edison. The ash mixture has an LOI of 4.4%which meets the ASTM C618 requirement (6.0%maximum) but which exceeds the 3% maximumrequirement of many concrete contractors.
The 'cleaned' fly ash is reported to have a consistent lowLOI value and meets ASTM C618 specifications. Hwang andothers (1 995a) studied the effect of the beneficiation processon three samples:
Table 23 shows the improvement in the properties of theClass F ashes following cleaning. Concrete prepared with thecleaned Class F ash had many advantages including lowerwater to cement ratios to achieve the same slump, higher aircontents, excellent reproducibility and high 28-daycompressive strengths. The cleaned Class C ash had lowersulphate content and the major efflorescence producing saltswere removed. The concrete produced from the cleaned ClassC ash also had improved slump and air content as did theconcrete produced from the cleaned Class C and F mixture.
A subsequent experiment by the same authors (Uu andothers, 1995) showed that the cleaning process reduced theLOI of ESP ash from the Warden power station from 6% to1.7%. Types 35S and 30S concretes containing 20% of thecleaned ash met the compressive strength requirements of3500 psi (24 MPa) and 3000 psi (20.7 MPa) at 28 days for35S and 30S concretes respectively. Dust collector ash fromthe same plant had an LOI of 29.8% and 62% of this ash wasretained on a 325 mesh screen. Following the cleaningprocess with an added screening step the LOI was reduced to2.5% and only 23% was retained on the 325 mesh screen.The compressive strength requirements were met when usedat 20% in 35S and 30S concretes. Lui and others (1995)concluded that the patented cleaning process could be used toclean ashes with a wide range of LOI and particle sizedistributions and that the resultant ash is suitable for use inconcrete applications.
Fly ash cleaned by the above process was found to be a
77
6 Conclusions
Fly ash from coal combustion has been recognised as acommodity by the cement and concrete industry in mostcountries for many years. However, the fu]] potential of thishuge resource is currently underestimated in many areas dueto a lack of understanding and a lack of positive marketing.
Fly ash is a potential commodity in a number of markets engineered fi]], cement and concrete, artificial aggregates,bricks and tiles, zeolites, waste stabilisation, water treatment,FGD sorbents, agriculture, fisheries, and materials recovery.Large tonnages of coal ash are used in many countries.However, the fly ash utilisation rate is often below 50% ofthat produced and is above 90% in few countries (seeTable I). This lack of success is largely due to marketingbarriers. The product, fly ash, is not we]] understood and,since other pozzolans and admixtures are available, is not aunique product. Fly ash has been regarded as a waste formany years and, in some places, removing this image sti]]requires active marketing. Transport is a particular problemsince many power plants are remote. However, theseproblems may be overcome. Vliegasunie in the Netherlandsis an exce]]ent demonstration of how fly ash can be marketedat a profit. Fly ash from several power plants are 'pooled' tobe sieved, certified and sold. The fly ash is we]] characterisedand computer-based models even allow the company topredict the quality of a fly ash before it is produced so thatmarkets can be identified in advance.
Fly ash can be used either as an active pozzolanic agent orsimply as a cheap admixture to provide bulk in engineeringmaterials. Fly ashes are low density, fine particles, which canenhance many of the properties of cement and concrete.Since fly ash characteristics vary considerably withparameters such as coal type and combustion conditions,grading or classification systems are necessary in order toensure that the correct fly ash is used for a specific purpose.Fly ash can be graded simply by whether it is coarse or fineor by whether it can be delivered wet or dry. Several morespecialised classification systems, based on chemical and
78
physical tests, have been proposed, the most popular systembeing the ASTM system from the USA. In the ASTMsystem, fly ashes are split into two classes: Class F arepozzolanic, normally derived from anthracite or bituminouscoals; and Class C are cementitious, norma]]y derived fromsubbituminous coals or lignite. However, many doubts havebeen raised over the applicability and usefulness of theseclassification systems. Since fly ash is so variable and sinceso many of its characteristics are important, classificationsystems can result in the over-simplification of fly ashproperties. Most of the current systems aim to classify fly ashaccording to its suitability for use in cement and concrete.These do not apply to other end uses. Thus an overa]] ratingsystem would be desirable for a]] potential uses of fly ash.
Fly ash to be used in most engineering work must passcertain specifications. Most countries have defined their ownspecifications but these are usually similar. Specifications forfly ash use in cement and concrete list criteria such asfineness, LOI, and pozzolanicity. However, simply because afly ash meets a certain specification does not mean that itwill necessarily pelform well in the final product. There ismuch debate in the USA at the moment over whetherspecifications should be prescription based (based on theproperties of the fly ash itself) or performance based (basedon the end performance of the product). Test methods usedfor the specification of fly ash can be used to determine thepotential strength and pozzolanicity of a fly ash. Althoughthese test methods are well established, they are not ideal.There is no widely accepted 'real' test for pozzolanicity.There is debate over the accuracy of certain fineness tests.Even the standard test for LOI is prone to artefacts which, itis alleged, could lead to inaccuracies of up to 75%. Tests forstrength pose a particular problem - strength can bedetermined by a number of means including compressivestrength tests, shear box tests, triaxial tests, and plate bearingtests. The test used depends upon the application in which thefly ash is to be used. However, many of these tests weredesigned for the strength determination of other materials,
most commonly soil, and there is concern that they are notapplicable to fly ash.
Specifications for fly ash use are normally accompanied bythe list of appropriate test methods to be used and by qualityassurance criteria. Tests have to be applied in a particularmanner and at a particular frequency. The results must fallwithin a certain range of permissible variation. If thesecriteria are met then, in many countries, the fly ash may becertified and sold as a guaranteed product. The primaryrequirement for marketing fly ash is consistent quality.Technologies to process fly ash into a consistent product anda guarantee of consistent quality increase the marketability offly ash.
Although much untreated fly ash can be sold as a pozzolan oradmixture for cement and concrete, there is potential for flyash beneficiation, resulting in greater profit. Fly ash whichwould previously have been rejected can be upgraded andsome fly ash can even be processed for more advanced enduses. Fly ash can be upgraded by simple process such asdewatering, blending (to use a low carbon ash to bring a highcarbon ash down to an acceptable average), grinding, sievingand air classification. More advanced processes can be usedto lower the carbon content of the fly ash. Froth flotation is arelatively expensive method of skimming off carbon from ashand results in a wet product which then requires dewatering.Fluidised beds can be used to separate high carbon fractionsby density or can be used to burn off excess carbon.
Conclusions
Electrostatic separation is a relatively new and specialisedprocess for removing carbon which can then be soldseparately. Magnetic fractions, such as iron, can also beremoved from fly ash. A patented process developed by theInstitute of Materials Processing at the MichiganTechnological University combines several beneficiationprocesses to separate cenospheres, iron and unburnt carbonleaving a high quality ash product (see Section 5.3.13).
The commercial cost of advanced beneficiation processes isnot often known as many are run either at pilot scale or areco-funded by the purchaser of the fly ash. The cost ofbeneficiation processes must be evaluated with respect to thepotential sale price of the product, the management costs andpotential customers. It is likely that advanced beneficiationprocesses will only be successful at certain sites. However,continued work on fly ash upgrading and beneficiation willincrease the return of revenue to power plant operators. Itwill reduce the costs incurred by fly ash disposal and theamount of natural resources required in many basicengineering applications.
The improvement and creation of markets for pulverised coalash depend on improved knowledge of their characteristicsand wider appreciation of their potential in specificapplications. Similar guarantees to those widely available forcement and concrete would promote the use of coal ash inother applications.
79
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