a - ceramic products with sludge ashes with additives

8/11/2019 A - Ceramic Products With Sludge Ashes With Additives

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Preparation and characterization of ceramic products by thermaltreatment of sewage sludge ashes mixed with different additives

Ignacio Merino a, *, Luis F. Are valo b , Fernando Romero b

a Instituto de Ensen anza Secundaria Celso D az, Avenida de Numancia 5, 26580 Arnedo, La Rioja, Spainb Departamento de Ingenier a Qu mica y del Medio Ambiente, Universidad del Pa s Vasco, Escuela Superior de Ingenieros,

Alameda de Urquijo s/n, 48013 Bilbao, Spain

Accepted 16 October 2006Available online 5 December 2006

Abstract

The study of the ceramic characteristics of sludge ashes, alone or mixed with additives (kaolin, montmorillonite, illitic clay, powderedat glass) includes characterization of additives, preparation of probes (dry or wet mixed), thermal treatment (up to 1200 C, except melt-ing or deformation) and control (densities, compressive strengths and water absorption). Thermal treatment increases the density andcompressive strength of probes (both parameters go through maxima, with later decreases) and decreases the absorption of water.The densication is also revealed by the evolution of the ratio of decrease of volume/loss of mass. The maximum values of compressivestrengths were obtained for 25% of illitic clay, montmorillonite and glass powder. Densication concerning probes with sludge ashesalone does not occur with kaolin. Experimental data were adjusted to exponential relationships between compressive strengths and den-sities for every composition, and also to a general equation for all probes. The apparent density obtained was adjusted to a non-lineardependence with temperature, leading to a maximum in density and permitting calculating the temperature of occurrence of this max-imum. The adjustment was not possible for probes containing kaolin, requiring presumably higher temperatures to densify. Waterabsorption has low values for ashes or kaolin probes, intermediate values for illite and powdered at glass probes and high valuesfor montmorillonite probes. Excepting with kaolin, ceramic materials with better characteristics than sludge ashes without additives wereobtained at lower treatment temperatures.

2006 Elsevier Ltd. All rights reserved.

1. Introduction

A vast amount of sludge is generated every day in awastewater treatment plant. Sludge disposal by landllingmay no longer be appropriate due to the scarcity of land

and increasingly stringent environmental controls; inciner-ation may be an alternative solution, but substantialamounts of ash are produced after the ring process, whichmust be disposed by other means ( Tay, 1987 ).

The inuence of sewage sludge ashes on the strength of cement mortars has been studied ( Monzo et al., 1999), withan enhancement of compressive strength attributed to poz-

zolanic properties of the ashes. Actually, two Europeanstandards prohibit the use of ash from co-ring of coaland municipal sewage sludge as additive to cement or con-crete (Cenni et al., 2001). Ashes can be converted into light-weight aggregates after being pelletized and red ( Kato and

Takesue, 1984 ), into asphaltic paving mixes ( Al Sayedet al., 1995) or into water permeable paving bricks ( Kaishaand Ltd., 2005 ).

The use of ash from the combustion of wastewatersludge for obtaining ceramic materials has recently beensuggested (Endo et al., 1997). The production of tiles(Lin et al., 2005) and bricks by mixing sludge ash and clayhas also been studied ( Grehl and Muller, 1998; Anderson,2002). Glass-ceramic has been prepared by melting a blendof limestone and ash with further reheating at lower tem-peratures ( Suzuki et al., 1997). When mixed with ashes

0956-053X/$ - see front matter 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.wasman.2006.10.008

* Corresponding author. Tel.: +34 941383312; fax: +34 941383358.E-mail address: [email protected] (I. Merino).

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and/or clay minerals, nely ground glass may act as a uxlowering the ring temperature during the ceramic-makingprocess (Smith, 2004) and reducing the leaching by inerti-zation of hazardous constituents ( Li et al., 2003).

The treatment of wastewater from the Bilbao area in theGalindo plant ( Concha and Henze, 1992; Consorcio de

Aguas/Uren Partzuergoa, 1994 ) gives rise to nearly12,000 tons of sewage sludge ashes per year, rich in ironoxides, as a consequence of the use of ferric salts as coag-ulant ( Eizaguirre and Lueje, 1993 ). The characterizationand possible uses of these ashes have been studied by theauthors in a previous paper ( Merino et al., 2005 ), wherethe possibility of obtaining ceramic materials for industrialpurposes with the use of only sludge ashes was shown.

This investigation tries to deepen the preparation of ceramic products using these sludge ashes with severaladditives, after thermal treatment. The following aims wereintended: (i) characterization of ashes and additives byphysico-chemical methods; (ii) characterization of pre-pared test probes of ceramic products by density, compres-sive strength and water absorption to evaluate theirquality; (iii) comparative analysis of the ceramic productsprepared taking into account the composition, treatmenttemperature and type of additive; (iv) nding a use of sludge ashes avoiding both the transfer to landll and theenvironmental problems derived from the leaching becauseof the inertization of the more soluble constituents duringthe thermal treatment. A detailed investigation on theleaching by water or chemical agents of probes preparedwith sludge ash was not intended and will be the aim of future investigations.

2. Methods

2.1. Sampling of ashes and selection of additives

In this research the following substances have been used:

Ashes from the incineration of sludge were obtainedfrom the Galindo Wastewater Treatment Plant. Thetemperature of combustion is regulated at 850 C. Asheswere taken mainly from the electrostatic precipitatorwith a small portion from the bottom of the boiler.

Three different types of clays of industrial applicationwere used as additives, namely a kaolin-rich sample(from Industrias Caobar SA, Taracena, Guadalajara,Spain), a montmorillonite-rich sample (called bentoniteLos Trancos, from Industrias Minas Gador SA,Almera, Spain) and an illite-rich sample (from a claypit near Pradejon, La Rioja, Spain).

Powdered at glass (from Guardian Glass Espan a, Llo-dio, A lava, Spain), as a uxing agent with additionaleffect on the inertization of soluble constituents.

Samples, dried at room temperature, were homogen-ised according to NLT-101/72 standard ( CEDEX,

1992). Additives (clay minerals and powdered at glass)

were also sieved (100 ASTM) rejecting the fractionhigher than 149 l m. To avoid humidication, the sam-ples were stored in polyethylene vessels inside a desicca-tor provided with anhydrous calcium chloride until theiruse.

2.2. Chemical analysis

Most of the analytical techniques used have alreadybeen described (Merino et al., 2005 ), including previousdrying of samples at 110 C and determination of mostof their constituents (Si, Al, Fe, Ti, P, Ca, Mg, Na, K,Mn and Cr, expressed as oxides) with ARL 3520 ICP(Inductively Coupled Plasma) equipment after meltingof samples at 1000 C with lithium metaborate in graph-ite crucibles, and dissolving in water with a small quan-tity of nitric acid. The others (C, N, H and S) weremeasured with LECO equipment. Carbon and hydrogenwere determined with a LECO CHN-600, according tothe ASTM D5373 standard, by infrared absorption, afterthe conversion into CO 2 and H 2O by combustion at950 C in an oxygen atmosphere. The apparatus alsomeasured nitrogen, present as molecular nitrogen aftercombustion, with a thermal conductivity detector, withresults lower than 0.05%. The carbonate contents in sam-ples were determined by evolution of CO 2, after reactionwith diluted hydrochloric acid, in a Bernard calcimeter,according to NLT-116/91 standard ( CEDEX, 1992 ).Organic carbon was calculated by difference. The totalcontent of sulphur was measured with LECO SC-132equipment according to ASTM D4239 and ASTM

D1552 standards, based on combustion of samples at1370 C and measuring in an infrared cell as SO 2. Twoother methods were used to determine sulphides and sul-phates, namely gravimetry as barium sulphate (NLT-120/72 standard ( CEDEX, 1992 )), and determination of theresidual content of sulphur, with the LECO SC-132equipment, after treatment of the sample with hydrochlo-ric acid to eliminate sulphides. Both methods led to sim-ilar results.

For additives: (a) weight loss was measured at 1000 C,both in an inert atmosphere (nitrogen) and in air, with dif-ferences lower than 0.5% in weight, mean values of thesedeterminations are reported, (b) sulphur was not detected,(c) the nitrogen contents were lower than 0.05% and (d)carbonates were only found in the illitic clay.

2.3. Size analysis

Size analysis of the sludge ashes has already beenreported ( Merino et al., 2005 ). An arithmetic mean diameterof particles equal to 62.2 l m was calculated. Ashes can bethen classied as a very ne material, with a small percentageof accumulated rejection (6.8%) on sieve ASTM 100(149 l mmesh). To have a size similar to that of the ash and to obtainhomogeneous mixes, additives were previously sieved,

rejecting particles that were higher than 149 l m.

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2.4. True density

True densities of samples were determined with 60 gportions by displacement of an organic solvent (toluene)in a Le Chatelier volumeter, according basically to ASTMC188-84 (ASTM, 1985a ). Mean values of two independent

determinations are included. 2.5. Thermal analysis

The thermal behaviour of these sludge ashes by ther-mogravimetric and differential thermal analyses hasalready been reported in a previous study. For additives,thermogravimetric analysis with LECO TGA-500 equip-ment was used. Crucibles with 1 g samples were driedat 110 C and then heated at a rate of 2 C/min up to1000 C, with continuous record of weight. Data, calcu-lated as percentages of loss of weight, referred to110 C dry samples, are shown as a function of temperature.

2.6. X-ray diffraction

Besides data previously reported for sludge ashes, X-raydiffraction was used to assess the nature of the illitic clay. Adiagram record was made with a goniometer, whichexplores the interval of 2 h angles from 2.01 to 61.99 .

2.7. Characterization of the sludge ashes, alone or mixed with additives, as ceramic material: preparation, thermal treatment and tests of probes

A set of probes was prepared with sludge ashes and theselected additives. The probes were thermally treated andtested according to the following steps:

Dosage and mixing . Test probes were prepared bymoulding samples, obtained in dry or wet conditions(with a small contribution of water near to 7%) to ndan adequate method for preparing probes with eachadditive and to search the differences in workabilitybetween both conditions. Besides samples containingonly sludge ashes, the additives were dosed at 12.5%,25%, 50%, 75% and 100% in weight, when possible,to have wide information for industrial application.Test probes could not be moulded in dry conditionwith contents higher than 50% kaolin or 25% pow-dered at glass. In wet conditions, neither a stableprobe with 100% montmorillonite nor probes withcontents higher than 37.5% powdered at glass couldbe moulded.(1) Mixing in dry conditions includes the following

steps: (a) crumbling of sludge ashes in a dispersionmill for 10 s; (b) incorporation with a spatula of the amount of additive required in each case; (c)homogenization in a horizontal agitator tube dur-

ing 10 min at the rhythm of three oscillations of

200 mm/s. Mixtures were put in the quantityrequired in each case to occupy half of the volumeof the tube.

(2) Mixing in wet conditions includes the followingadditional stages: (d) placing the dry, homoge-nized mass onto a tray and addition of 7% water

by weight; (e) mixing with a spatula and latercrumbling in a dispersion mill during 20 s; (f)homogenisation in the horizontal agitator tubeduring 10 min; (g) maturation in hermetic con-tainer of plastic at room temperature (202 C)during 24 h.

Moulding . High probes of 46 mm were prepared byputting mass in a 23 mm inside diameter mold, lubri-cated with powdered zinc stearate, and applyingslowly and maintaining the compaction pressure for2 min. In these conditions, probes with a height/diam-eter ratio of very close to 2, convenient for mechanicalstrength measurement, were obtained. The quantity of mass used for the preparation of each probe was var-iable, being in the range between 26 and 34 g. Thecompaction pressure also was variable, oscillatingbetween 15 and 59 MPa. Changes in the amount of mass and the compaction pressure were necessary toachieve a good workability in the moulding (absenceof obstructions, vibrations and surface defects inprobes). Practical compaction pressures were generallylower for both higher proportions of additives andsamples mixed in wet conditions. Practical conditionswere xed after several attempts, selecting in each casethe conditions in which the best workability was

achieved. Drying . The slow and gradual drying of test probes,avoiding their breaking, was carried out for the follow-ing cases:(1) Drying of most of the wet mixed probes was made

in several steps from room temperature up to200 C (over 36 h), controlling the weight of theprobes at 50 C and 70 C, as indicated in the pre-vious work. However, it was necessary to make aspecial drying method for preparing wet probeswith 100% kaolin, so that they would not breaksupercially.

(2) The drying of dry mixed probes with montmoril-lonite (as well as those mixed wet with 75% of this additive) was made by heating in a stovefrom room temperature up to 200 C at a con-stant rhythm during 6 h, with continuous air cir-culation. This temperature was maintainedovernight (11 h) before the thermal treatmentwas made.

Thermal treatment . This process was made with probes(lots of six) in independent nickel crucibles. Heating from200 C to the maximum temperatures (900, 1000, 1100,1125, 1150, 1175 and 1200 C, if possible) was performedat a rate close to 2 C/min. Maximum temperatures were

maintained for 60 min. Cooling down to 500 C, at a rate

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close to 2 C/min, was followed by cooling to 110 Covernight and nal cooling to room temperature insidea desiccator with silica gel.

Characterization as ceramic material. This stage includedcontrol of dimensions, preparation of probes and mea-surement of compressive strength and water absorption.

For initial (in green) probes, mass, height and diameter(within 0.01 mm accuracy), volume and apparent den-sity were measured. For treated probes, the followingtests were conducted at their suitable temperatures: (a)mass, height and diameter (including superior and infe-rior if necessary, with the same accuracy), volume andapparent density; (b) break surfaces and ratio of height/diameter, after mechanical facing of probes if necessary because of a lack of parallelism between thebases especially in probes treated at the highest temper-atures; facing was made when deviations in height werehigher than 0.15 mm; (c) compressive strength was mea-sured with SUZPECAR equipment, operating at a rateof 0.5 MPa/s (The apparatus gives a digital record of the force. Compressive strength is obtained as the quo-tient between this force and the area of the base of theprobe. When the ratio of height/diameter did not havethe value 2, the strength was corrected according toNLT-250/91 standard ( CEDEX, 1992 )); (d) percentagesof variation of density, loss of mass and decrease of vol-ume; (e) quotients between percentages of decrease involume and loss of mass, which we will designate asratio DV /Dm; and (f) measure of water absorption by24 h submersion method according to ASTM C 67(ASTM, 1985b ), using pieces of probes that had been

previously broken in the test of compressive strength.

3. Results and discussion

3.1. Chemical composition

Results of the chemical analyses of sludge ashes andadditives are shown in Table 1 . Data are expressed as oxi-des, except loss of ignition (LOI), which is mainly due to

the elimination of water, organic carbon, carbonates andsulphur compounds, as included in Table 2 . Based on thedata of chemical composition the following commentscan be made:

Sludge ashes . They have: (a) high iron and calcium con-

tents, attributable to the coagulation process used in thewastewater treatment plant (addition of ferric salts andlime for pH adjustment); (b) considerable contents of sil-ica and alumina, originally contained in the wastewater;(c) high percentages of phosphates, possibly due todomestic detergents and (d) loss of ignition (LOI), whichis mainly due to the elimination of water, organic car-bon, carbonates and sulphur compounds.

Kaolin. This additive is a rather pure clay (it containsonly small amounts of Fe, K, Ti, Ca and Na), whoseanalytical data are concordant with typical composi-tions (Singer and Singer, 1979; Bonneau and Souchier,1987). The loss of ignition is basically due to the elimi-nation of the structural water formed from OH ions,although a small quantity may proceed from organicmatter. The molar relationship H 2O/Al 2O3 has the valueof 1.95, quite close to the theoretical value of 2, deducedfrom the ideal formula Si 2O5Al2(OH) 4. However, thecontent of silica is somehow higher than the valuededuced from this formula. The value of 2.22 calculatedfor the molar SiO 2/Al 2O3 ratio suggests the probableexistence of a small proportion of free silica.

Montmorillonite . Analogously, the contents of most of the elements t quite well the theoretical formula ( Singerand Singer, 1979) of a montmorillonite, namely

Si16 {Al6, (Mg,Ca) 3}O40 (OH) 8 nH 2O, which involvesthe substitution in octahedral positions of three Mg 2+

or Ca 2+ ions for two Al 3+ ions. In our case, is also pos-sible a low incorporation of Fe 2+ in octahedral posi-tions. By heating, the elimination of water must bepartially due to structural water, formed from OHions, and the rest to interlayer water. We shall insiston this subject after exposing data of thermogravimetricanalysis. For the formula included, n is close to 4 for theinterlayer water, which remains at 110 C after drying.

Illitic clay . The illitic clay used as additive is quite com-plex in composition. Major constituents are Si and Al,but it contains also many minor (Na, K, Ca, Mg andFe) and trace elements (Ti, P and Mn), as well as a largeamount of water and CO 2 as LOI. The theoretical com-

Table 1

Chemical composition of sludge ashes and additivesOxides Sludge

ashes (wt%)Kaolin(wt%)

Montmorillonite(wt%)

Illitic clay(wt%)

Flat glass(wt%)

SiO2 25.40 48.60 58.90 53.80 70.10Al2O3 7.64 37.15 20.50 14.72 0.73Fe 2O3 20.00 0.47 2.40 4.85 0.75CaO 21.05 0.08 2.50 8.58 8.50P2O5 14.20 0.05 0.14 MgO 1.63 5.40 2.30 4.40Na 2O 0.48 0.04 0.13 0.74 15.05K 2O 0.78 0.34 0.10 2.64 TiO 2 0.29 0.11 0.03 0.39 MnO 0.03 0.01 0.06 Cr2O3 0.02

LOI 8.36 13.08 10.12 11.60 0.39

Table 2Constituents of the loss of ignition (LOI) of sludge ashes and additives

LOI Sludge ashes(wt%)

Kaolin(wt%)

Montmorillonite(wt%)

Illitic clay(wt%)

C (organic) 0.49 0.08 0.02 0.08CO2 (carbonates) 1.47 7.41S (sulphides) 0.80 SO3 (sulphates) 2.63

H 2O 2.07 12.78 9.63 3.96



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position of illite is (Si8 x ,Al x )(Al4)O20 (OH) 4K x (Singerand Singer, 1979 ), in which x varies from 1 to 1.5 andwhere several substitutions in the tetrahedral and octa-hedral constituents, as well in the interlayer cation(Na + for K + ), are possible. The illitic clay used is alsoa mixture, whose mineralogical composition is discussed

later with X-ray diffraction data. Powdered at glass . This additive has the typical compo-sition of a sodocalcic at glass ( Pfaender, 1996 ). In thisglass type, the combined addition of Na and Ca is madein such proportions that the softening point is reducedto about 730 C, while Mg is used to prevent devitrica-tion (Smith, 1994). The theoretical melting point of thisadditive, obtained starting from the diagram of phasesof the system formed by SiO 2 CaONa 2O, (Vian,1976), is close to 925 C.

3.2. True densities

The obtained true densities, in g/cm 3, were as follows:sludge ashes 2.86; kaolin 2.57; montmorillonite 2.60; illiticclay 2.71 and at glass 2.51. The comparison of these datasuggests that:

The sludge ashes and the additives used to mix them forpreparing ceramic probes have similar densities. Thehigher difference occurs for at glass with 87.76% of the density of the ashes.

The sludge ashes have a much reduced percentage of rejection (6.8%) on sieve ASTM 100 (149 l m mesh).

Additives were passed through the same sieve, reject-ing the fractions higher than 149 l m. The similarityof particle sizes and the analogy of the true densitiesof sludge ashes and additives suggest the probable

inexistence of segregations of mixtures, after an appro-priate homogenisation. This hypothesis was practicallyconrmed.

After ring at 1200 C, the true density of ashes was3.15. The thermal treatment of sludge ashes to 1200 Cproduced a densication by sintering with an increase

in density equal to 10.14%, even when the sample atthe same time had an 8.36% loss in weight. Later weshall insist on this densication phenomenon of elabo-rated probes with sludge ashes to estimate their proper-ties as ceramic material.

3.3. Thermal analysis

The thermal behaviour of the sludge ashes was made inthe previous investigation. The following phenomena weredescribed from room temperature to 1250 C: eliminationof hydration and structural water, combustion of organicmatter, conversion of sulphides to oxides, elimination of carbon dioxide from carbonates, reaction of the existentsulphates with more xed anhydrides (SiO 2 and P 2O5)and coalescence from powder into solid by heating andsintering.

The thermogravimetric analyses of the additives areshown in Fig. 1, whose examination, jointly with chemicalanalysis data, suggests that during thermal treatment thefollowing phenomena occurred for each additive:

Kaolin . The important weight loss near 550 C is attrib-utable to the elimination of structural water formed

from OH ions in reticular positions. Montmorillonite . Two types of water (practically uniquecomponent eliminated during the heating) can be distin-guished, namely: (i) hygroscopic water, with interlayercharacter, lost up to 400 C, with a maximum rate of

0100 200 300 400 500 600 700 800 900 1000

Temperature (C)

L o s s o f w e i g h t ( % )

Illite

Montmorillonite

Kaolin

-1

-2

-3

-4

-5

-6

-7

-8

-9

-10

-11

-12

-13

-14

Fig. 1. TGA curves of additives.



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loss close to 200 C; and (ii) structural water due to theelimination of reticular OH ions with a maximum rateof loss close to 600 C. Furthermore, it may be con-cluded that both types of water are present in similarquantities in the sample dried at 110 C. This fact agreeswith the formula given for montmorillonite in Section

3.1. The great hygroscopic character of montmorilloniterequired a careful process of drying for probes, even forthe ones elaborated under dry conditions. The great rateof loss of this additive near to 200 C was taken intoaccount to outline a general drying method in severalsteps for all probes.

Illitic clay . The thermogram of this additive suggeststhat there are three zones of loss: (i) up to about350 C, a much reduced loss, attributable to hygroscopicwater; (ii) in the range from 350 to 650 C, the losses canbe attributed to the elimination of structural water,formed from OH ions of the crystalline lattice of illite,as well as to the elimination of carbon dioxide formedby the decomposition of magnesium carbonate; and(iii) above 650 C, the most important loss should beassigned to the decomposition of calcium carbonate pro-ducing carbon dioxide. The maximum rate for this lossis close to 775 C.

Powdered at glass . The scarce value of its loss of igni-tion suggests that no important thermal phenomenaoccur before melting, close to 925 C, as previouslymentioned.

The behaviour of sludge ashes and clay additives, havingquick weight losses in a wide range of temperatures, sug-

gests the convenience of outlining a general method forthe thermal treatment with a low rate of heating. Finally,the fusibility test of sludge ashes, described in the previouswork, suggests that the thermal treatment of probes, elab-orated with sludge ashes and additives, should not be madeat temperatures higher than 1200 C.

3.4. X-ray diffraction

The X-ray diffraction analysis of illitic clay ( ITGE,1991) identies:

Main constituents: quartz (SiO 2), calcite (CaCO 3) andillite.

Minor constituents: dolomite (MgCO 3 CaCO 3), chlorite(a laminar silicate containing magnesium hydroxide andmicaceous layers), felspars (silicates with frameworkstructures) and andalusite (an aluminosilicate with thetheoretical formula Al 2SiO5 containing discrete tetrahe-drons SiO 44 .

The presence of illite has been conrmed by X-ray dif-fraction, after treatment of the sample with ethyleneglycol.The percentage of illite has been estimated to be lower than40% (ITGE, 1991 ). According to our analysis ( Merino,

1999), the percentage of illite must be in the range from

29.1% to 60.6%. Therefore the real content would be closeto 35%.

3.5. Characterization as a ceramic material

With sludge ashes and additives, probes were prepared

using a metallic mold, pressed, thermally treated andtested. These steps are described successively:

Preparation of probes. For the elaboration of probes, itwas necessary to work at the most appropriate condi-tions (absence of obstructions, vibrations and surfacedefects in probes). The workability for the different com-positions of probes, prepared in dry or wet conditions, isshown in Table 3 . In turn, it was attempted to get similarinitial compactions for similar compositions. The prepa-ration of probes was not possible in the following cases:(i) with kaolin for dry mixed contents higher than 50%in weight, due to obstructions in the mold; (ii) withwet mixed 100% montmorillonite, because of ssuresduring the drying step; (iii) with powdered at glassfor contents higher than 25% (dry mixed) or 37.5%(wet mixed), because of lack of cohesion of probes. Ingeneral, wet mixed probes required lower compactionpressures than the dry ones. The compaction pressurediminished also when increasing the percentage of addi-tive. In the case of illitic clay for dry mixed probes with50% or more in this additive, the compaction pressurescould be higher, but included data were selected to get

Table 3Compaction pressures to elaboration by moulding of probes (workability)Constituents Symbols (wt%) Compaction pressure (MPa)

Dry mix Wet mix

Sludge ashesASH 100 59 39

KaolinKAO-12.5 12.5 44 32KAO-25 25 25 25KAO-50 50 15 25KAO-75 75 17KAO-100 100 17

Illitic clayILL-12.5 12.5 59 37ILL-25 25 54 34ILL-50 50 37 27ILL-75 75 22 17ILL-100 100 17 15

MontmorilloniteMON-12.5 12.5 51 34MON-25 25 51 29MON-50 50 51 29MON-75 75 47 29MON-100 100 39

Powdered at glassPFG-12.5 12.5 56 39PFG-25 25 54 37

PFG-37.5 37.5 29



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initial densities close to the rest of the probes. This situ-ation occurred also for wet mixed probes with high kao-lin percentages.

Initial apparent densities and compressive strengths .Depending on the proportion of additive used, the typeof mixing and the workability in the moulding of

probes, the mean values and variability coefficients(CV) of initial apparent densities are shown in Table4. In most of the cases, it may be observed that apparentdensities obtained by dry mixing are somehow biggerthan the densities obtained by wet mixing. Data werecalculated for groups with 2030 probes for each com-position and mixing (a total close to 900 probes). Thelow values of CV imply an acceptable experimentalreproducibility. For the compositions with illitic clay,an increase in the initial apparent density was observedfor both dry and wet probes with higher proportions of clay. Initial apparent densities obtained by dry mixingcould be even higher, but lower compaction pressureswere used to get densities close to the rest of the probes.For wet mixing illitic clay compositions, no problemswere found in the compaction step, but very strongvibrations occurred in the phase of demoulding. The ini-tial (in green) compressive strengths of dry or wet mixedprobes have similar mean values (3.7 MPa and 3.1 MPa)and variability coefficients (34.5 and 44.2), respectively.

Drying of the probes . The wet mixed probes were dried inseveral steps, already mentioned in Section 2.7. The ini-tial mean humidities (calculated from the water used inthe mixing) and the humidities after the drying processesat 50 C and 70 C were 7.25%, 2.03% and 0.69%,

respectively, with variability coefficients of 6.8%, 68.5%and 39.9%. These data, which do not include wet mixedsamples with special drying (KAO-100 and MON-75),

enable us to make the following comments: (i) Thescarce content in humidity after the drying at 70 C sug-gests that the drying was performed effectively and itwas able to avoid ssures in the probes, when heatedat higher temperatures, as conrmed experimentally.(ii) The high dispersion of the data for drying at 50 C

must be attributed to the wet samples MON-25 andMON-50, with humidities close to 5%, due to theirstrong hygroscopic character.

Results of the specic thermal treatment: Densication,compressive strength, and water absorption . The probeshad specic thermal treatments at temperatures of 900 C, 1000 C, 1100 C, 1125 C, 1150 C, 1175 Cand 1200 C, when possible if melting or excessive defor-mation do not occur. A summary of the data is shown inTable 5 , where mean values of apparent densities ( d) ,compressive strengths (CS) and water absorption(WA), obtained after thermal treatment at the two besttemperatures, are included for all compositions studiedwith wet or dry mixing. From these data the followingcomments can be made:(1) Variability coefficients (VC), not included in Table

5, were generally lower than 4% for densities,lower than 20% for compressive strengths andlower than 15% for water absorption, with theexceptions of some compositions with montmoril-lonite (MON-50-wet with VC of 8.63% in density,32.08% in CS and 23.02% in WA; MON-50-drywith 8.35% for density; MON-75-dry with40.04% for CS and 24.13% in WA). These excep-tions are due to materials with low densication

and very poor mechanical properties.(2) During the thermal treatment, besides the changesof mass, dimensions, compressive strength andwater absorption, the probes had appreciablequalitative variations. These affect the color andsometimes the brightness and the form of theprobes (in this case mechanical facing was neces-sary for the compressive strength test). It wasobserved that probes became darker when heatedmainly at temperatures higher than 1100 C,except in those with high contents in kaolin.

(3) The thermal treatment made above 900 C leads toan increase of the apparent density of probes, aswell as to a considerable increment of compressivestrength. In some cases, especially with powderedglass as additive, maximum values for densityand compressive strength are present beforeintense deformation and melting occur at evenhigher temperatures. This phenomenon is pre-sented in Fig. 2 for mean values of dry mixedprobes with 25% of powdered at glass and triedthermally up to 1150 C.

(4) Furthermore, densication also becomes evidentthrough the evolution, with treatment tempera-ture, of the ratio DV /Dm whose values higher than

one implies densication. This evolution, for the

Table 4Initial apparent densities of probes (without thermal treatment)

Constituents Dry mix Wet mix

Mean (g/cm 3) CV(%) Mean (g/cm 3) CV(%)

ASH 1.404 0.31 1.330 0.89KAO-12.5 1.430 0.48 1.326 0.50KAO-25 1.356 0.65 1.332 0.66KAO-50 1.406 0.58 1.460 0.62KAO-75 1.483 0.56KAO-100 1.499 0.35ILL-12.5 1.457 0.61 1.345 0.56ILL-25 1.506 0.48 1.363 0.69ILL-50 1.562 0.32 1.511 0.41ILL-75 1.634 0.26 1.580 0.43ILL-100 1.762 0.45 1.852 0.24MON-12.5 1.406 0.38 1.329 0.54MON-25 1.394 0.69 1.331 0.64MON-50 1.409 0.54 1.398 0.55MON-75 1.412 0.39 1.401 0.42MON-100 1.365 0.46 PFG-12.5 1.435 0.74 1.345 0.48PFG-25 1.424 0.47 1.362 0.60

PFG-37.5 1.339 0.59



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Table 5Apparent densities ( d ), compressive strengths (CS) and water absorption (WA) of probes after thermal treatmentConstituents Thermal treatment at 1175 C Thermal treatment at 1200 C

Dry mix Wet mix Dry mix

d (g/cm3) CS (MPa) WA (%) d (g/cm3) CS (MPa) WA (%) d (g/cm3) CS (MPa) WA (%)

ASH 2.039 74.3 8.51 1.930 64.9 11.23 2.361 170.5 1.89 KAO-12.5 2.030 79.5 8.31 1.926 52.9 12.36 2.423 157.3 0.59 KAO-25 1.835 64.3 13.31 1.907 69.5 11.33 2.248 100.1 3.75 KAO-50 2.004 97.6 7.81 1.925 91.9 9.56 1.991 102.3 7.85 KAO-75 1.719 60.4 16.66 KAO-100 1.549 15.4 25.80 ILL-12.5 2.165 104.0 4.88 2.023 65.0 9.50 2.378 146.0 1.12 ILL-25 2.341 203.1 3.63 2.256 147.7 3.01 2.464 227.5 3.25 ILL-50 2.029 91.8 7.63 2.151 100.9 5.22 2.345 213.7 4.19

ILL-100 1.708 63.6 16.71 2.145 99.9 5.35 MON-12.5 2.271 140.8 2.72 2.210 123.8 3.72 2.409 229.5 4.27 MON-100 1.839 66.0 13.08 1.831 78.8 12.33 PFG-12.5 2.524 249.0 3.94 2.337 204.9 3.79 2.376 189.1 2.38

At 1150 C At 1175 CILL-75 1.990 98.7 8.01 1.603 32.2 23.06 2.291 198.4 4.14 ILL-100 1.821 77.7 12.64 MON-25 2.393 211.9 3.34 2.267 113.5 3.03 2.535 308.4 9.60 MON-75 1.788 44.1 16.27 1.631 20.9 22.59 1.925 75.0 10.57

At 1125 C At 1150 CMON-50 1.572 17.6 24.86 1.565 13.4 25.08 1.673 30.5 20.72 PFG-25 2.480 255.8 5.24 2.443 211.3 2.52 2.162 81.9 5.99

At 1100 C At 1125 C

PFG-37.5 2.360 189.8 2.66


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cases of dry mixed probes with 25% of additivesand the own sludge ashes, is shown in Fig. 3,whose observation also suggests that there is a ten-dency for the densication to reach a maximumvalue with the treatment temperature before defor-mation and melting occur.

(5) The evolution of water absorption versus treat-ment temperature, for the cases of dry sludge ashesand dry mixed probes with 25% of additives, isshown in Fig. 4, where a decrease in WA is rstobserved, followed by a slight increase in somecompositions. This effect must be attributed to abloating effect with the subsequent inhomogeneoussolidication due to the presence of many openpores in the red probes ( Pimraksa et al., 2000 ).

Maximum compressive strengths . Moreover the obser-vation of the maximum compressive strengthsachieved by thermal treatment as a function of thepercentage of additive, allows the following particularconsiderations:(1) Kaolin (Fig. 5). Very little difference exists in the

maximum compressive strengths (CS max.) amongthe values obtained for wet or dry mixing (justpossible until 50% in weight). A compressivestrength higher than that of the own sludge asheswas never obtained. This fact can be attributedto the high refractory character of the kaolinwhose melting interval is between 1750 C and1770 C (Singer and Singer, 1979 ). The maximumtemperature used in the thermal treatment

1.0

3.0

900 950 1000 1050 1100 1150 1200

Temperature (C)

Compressive Strength

Density

D e n s i t y ( g / c m

3 )

300

250

200

150

100

50

0

C om pr e s s i v e S t r e n g t h ( MP a )

2.8

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

Fig. 2. Apparent densities and compressive strengths for dry mixed probes with 25% of powdered at glass.

0

1

2

3

4

5

6

7

8

9

10

850 900 950 1000 1050 1100 1150 1200 1250

Temperature (C)

PFG-25-dry

MON-25-dry

ASH-dry

ILL-25-dry

KAO-25-dry

V / m

r a t i o

Fig. 3. Evolution of the DV /Dm ratio with temperature for dry mixed probes with only ash and with 25% of additives.



10/16

(1200 C) must be considered insufficient toachieve an important additional densication withthis additive and consequently an improvement of compressive strength of test probes.

(2) Illitic clay (Fig. 6). Best maximum compressivestrengths were obtained for probes with 25% of additive and higher values for the wet mixing.The probes with 75 and 100% of illitic clay seemto have symptoms of an incipient melting for thetreatment at 1200 C.

(3) Montmorillonite (Fig. 7). The maximum compres-sive strengths have similar values for dry or wetmixed probes. The compressive strengths showmaximum values for 25% of additive and also min-imum values toward 50%. Because the used mont-

morillonite does not melt at 1200 C and the

probes prepared with the 50% and 75% showsymptoms of an incipient melting (as well as verypoor mechanical properties), it must be suggestedthat a eutectic mixture is formed at a compositionclose to 50% of montmorillonite.

(4) Powdered at glass (Fig. 8). The prepared probeshad the maximum compressive strengths for theinterval between 12.5% and 25% of additive, withhigher values for dry mixing. Furthermore, thethermal treatment at higher temperatures leads tolower compressive strength and the subsequentmelting of probes.

Relationship between compressive strength and density .The evolution of the compressive strength and thedensity with the treatment temperature suggests that

an increase in the densication of probes is accom-

0

5

10

15

20

25

30

35

40

45

850 900 950 1000 1050 1100 1150 1200 1250

Temperature (C)

W a t e r a b s o r p t i o n ( % )

PFG-25-dry

MON-25-dry

ASH-dry

ILL-25-dry

KAO-25-dry

Fig. 4. Evolution of the water absorption with temperature for dry mixed probes with only ash and with 25% of additives.

C S m a x . (

M P a )

Composition (%)

350

300

250

200

150

100

50

0

0 25 50 75 100

KAO-dry

KAO-wet

Fig. 5. Maximum compressive strengths using kaolin as additive. Thermal treatment at 1200 C.



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panied by a higher increment in compressivestrength, as shown in Fig. 2 for probes with 25%of powdered at glass. After several statistical tests(SPSS, 1994), the experimental data could beadjusted as a function of exponential type, which justies that the compressive strengths had biggervariation coefficients than the densities, as it waspreviously said. The mean values for different com-positions and mixings (as well as the individual val-ues corresponding to near 900 probes shown inFig. 9), have been adjusted to an exponential equa-tion of the type:

CS A

e B=density

or using natural logarithms:LnCS Ln A

B

density

An empiric relationship has been described ( Aleixandre,1971) between mechanical resistance ( r ) and porosity(p): r = r 0 e b

p , easily transformable in the formerequation, considering that density and porosity are in-versely related. The adjustment of our data is highly sig-nicant in all the cases and the values of the coefficientsfor the different compositions and mixings are close tothose correspondent to all the probes (ln A = 9.987;B = 11.149; r2 = 0.923), except for the compositions

with 75% or 100% of additive.

1200 C

1200 C

1200 C 1200 C

1200 C

1200 C

1200 C 1175 C

1175 C

1175 C1200 C

1200 C

C S m a x . (

M P a )

Composition (%)

350

300

250

200

150

100

50

0

ILL-dry

ILL-wet

0 25 50 75 100

Fig. 6. Maximum compressive strengths using illitic clay as additive.

1200 C

1200 C

1175 C

1150 C

1200 C

1200 C

1175 C

1150 C

1175 C

1175 C

1150 C

C S

m a x . (

M P a )

Composition (%)

350

300

250

200

150

100

50

0

0 25 50 75 100

MON-dry

MON-wet

Fig. 7. Maximum compressive strengths using montmorillonite as additive.



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Relation between apparent density and temperature of thethermal treatment . The evolution of the density with thetreatment temperature (shown in the Fig. 2 for drymixed probes with 25% of powdered at glass) impliesa quick increase with the temperature of the density thatreaches a maximum close to 1125 C. At somehowhigher treatment temperatures, a diminution of densityoccurs and melting takes place at even higher tempera-tures. The careful examination of available data, for

the probes of sludge ashes and for those containing pow-dered at glass, illitic clay and montmorillonite as addi-tives, suggests that the behaviour during the heatingshould be similar to the case shown in Fig. 2 and thatthe effected thermal treatment has produced a densica-tion close to the maximum possible for the conditions of

initial compaction limited by the workability in themoulding. The decrease of the apparent density withheating, after the maximum densication is achieved,should be attributed to thermal expansion of the mate-rial and/or to expansion by exit of gases by overburning,(Guillem and Guillem, 1984 ). It can also be due to a

bloating effect associated with the formation of a highvolume of approximately spherical pores ( Cheesemanet al., 2003). The melting, not studied in our case, shouldhappen at even higher temperatures. For the elaboratedprobes with kaolin as additive, a similar situation doesnot exist because of its strongly refractory character thatwould require thermal treatment at some higher temper-atures that those used in this study to produce densica-tion. The experimental data, when an evidentdensication occurred as in most of the cases, can beadjusted statistically to the following non linearexpression:

Density d 900 Ax

1 Bx Cx 2

where the density obtained at the treatment temperaturedepends on d 900 (the average density at 900 C, selectedas the base for the adjustment of experimental data) andx, the increment of temperature from 900 C. The rest of the parameters ( A, B and C ) are constants that shouldbe obtained for each additive and composition by nonlinear regression with a statistical program ( SPSS,1994). For this statistical tting it is necessary that thedensity have a nite value and so the expression(1 + Bx + Cx 2) cannot be null. The analysis of the t-

ting equation suggests that the density takes a maximumvalue when x = 1/ C 0.5 and minimum for x = 1/C 0.5 .Moreover, it is possible to estimate the ring tempera-ture at which the maximum densication is reached ineach case: 900 + x = 900 + 1/ C 0.5 . Also the maximumdensity for this temperature can be obtained. A selection

1175 C 1125 C

1200 C

1175 C 1125 C

1100 C1200 C

C S m a x . (

M P a )

Composition (%)

350

300

250

200

150

100

50

0

0 12.5 25 37.5 50

PFG-dry

PFG-wet

Fig. 8. Maximum compressive strengths using powdered at glass asadditive.

0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75

Density -1 (cm 3 /g)

0.80

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0.0

L n C S ( M P a )

y = 9.987 11.149x (r 2 = 0.923)

Fig. 9. Relation between compressive strength and apparent density for all probes.



13/16

of results is shown in Table 6 . Additionally the followingcomments can be made:(1) The calculated tting equations are highly statisti-

cally signicant, in the cases in which an evidentdensication occurs. It was not possible to obtainsatisfactory results of tting with kaolin as addi-

tive, where there is not a clear densication inthe studied interval of treatment temperaturesbecause of the commented refractory character of kaolin. Neither acceptable results for compositionswith 50% or more of montmorillonite wereachieved, whose probes had very poor mechanicalproperties, for their low densications and alsowith symptoms of incipient melting, probably forthe formation of a eutectic mixture.

(2) The equations corresponding to the dry or wetmixed samples lead to similar results, for the sameadditive proportion. This analogy is shown in theFig. 10, which includes experimental mean values

and those calculated (curve of tting) for probeswith 25% of powdered at glass (PFG-25-dryand PFG-25-wet) and 25% of montmorillonite(MON-25-dry and MON-25-wet).

(3) The calculated optimum treatment temperaturesfor the probes with additives are generally lower

than the ones for the probes with the sludge ashesthemselves. This fact implies to get a ceramic mate-rial at lower treatment temperatures. The curves inFig. 10 are suitable to illustrate this importantresult, as well as the data which are included inTable 6 .

(4) It is also suggested that the use of the kaolin as anadditive can give appropriate ceramic products if the thermal treatment is carried out at higher tem-peratures. This would require a complementarystudy in the interval of temperatures between themelting of the sludge ashes (12501300 C) andkaolin (17501770 C).

Table 6Statistical relationships between density and temperature for some compositions

Constituents d 900 (g/cm3) 104 A 103 B 106 C r2 Calculated optimumtemperature ( C)

Calculated maximumdensity (g/cm 3)

ASH-dry 1.381 1.064 6.135 9.700 0.987 1221.1 2.513ASH-wet 1.291 1.154 6.029 9.374 0.980 1226.6 2.513ILL-25-dry 1.446 0.500 6.840 11.852 0.997 1190.5 2.544ILL-25-wet 1.304 0.703 6.752 11.598 0.997 1193.6 2.494MON-25-dry 1.362 0.832 7.452 14.135 0.997 1166.0 2.609MON-25-wet 1.301 1.106 7.277 13.564 0.980 1171.5 2.543PFG-12.5-dry 1.423 0.897 7.025 12.619 0.993 1181.5 2.550PFG-12.5-wet 1.329 0.967 6.969 12.472 0.992 1183.2 2.357PFG-25-dry 1.402 1.614 8.860 20.288 0.995 1122.0 2.486PFG-25-wet 1.345 3.291 8.298 18.463 0.969 1132.7 2.459PFG-37.5-wet 1.332 1.843 9.243 22.114 0.998 1112.7 2.472

1.3

1.5

1.7

1.9

2.1

2.3

2.5

2.7

850 900 950 1000 1050 1100 1150 1200


3 )

Temperature (C)

1.1

PFG-25-dry

PFG-25-wet

MON-25-dry

MON-25-wet

ASH-dry

1250

)Cx Bx(1

Ax d y 2 900 ++

+=

( x = Temperature 900)

(r 2 = 0.995)

(r 2 = 0.969)

(r 2 = 0.997)

(r 2 = 0.980)

(r 2 = 0.987)

Fig. 10. Statistical relationship between density and temperature for calculation of optimal ring temperature treatment.



14/16

Relation of water absorption with density and compressivestrength. The dependences of average values of waterabsorption with the mean values of these parameters,obtained after thermal treatment at the two best ringtemperatures, are shown in Figs. 11 and 12 . The datawere adjusted in both cases to second order relation-

ships. It may be appreciated that water absorption ishigh at low densities, decreases when the density of probes increases up to a value close to 2.35 g/cm 3; sub-sequently WA increases slightly. The behaviour versuscompressive strength is quite similar, with a minimumvalue of WA close to 200 MPa. This effect must be againattributed to the bloating effect previously described.

Comparison among the different additives . As a summaryof the possibilities of preparation of ceramic materialsfrom sludge ashes and the additives used in this work,Fig. 13 shows comparatively the mean values of maxi-

mum compressive strengths and apparent densities jointly with their water absorption. Among the additivesstudied, the best results for density and compressivestrength were obtained with 25% of montmorilloniteand wet mixing, for the initial conditions of workabilityin the moulding and for the effected thermal treatment.

The scarce increase in density with kaolin should beattributed to the necessity to make the treatment athigher temperature to produce the densication of theprobes, not searched in this study. Satisfactory resultswere obtained for compositions with 25% of illitic clayor powdered at glass, in this case with the lower tem-perature of treatment, 1125 C. Water absorption waslow for ashes or kaolin probes, has intermediate valuesfor illite or powdered at glass probes and high valuesfor montmorillonite probes, which is attributed to thedescribed bloating effect.

0

5

10

15

20

25

30

1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6

W a t e r A b s o r p t i o n ( % )

Density (g/cm 3)

y = 36.364x 2 170.35x + 203.18 (r 2 = 0.956)

Fig. 11. Relationship between water absorption and density for the best red probes.

0 50 100 150 200 250 300 350

30

25

20

15

10

5

0

W a t e r A b s o r p t i o n ( % )

Compressive Strength (MPa)

y = 0.0007x 2 0.2769x + 27.686 (r 2 = 0.919)

Fig. 12. Relationship between water absorption and compressive strength for the best red probes.



15/16

4. Conclusions

As a result of this research the following conclusionshave been established:

1. Ceramic materials of engineering and constructionapplications can be obtained with the sludge ashes them-selves, which may be improved with some of the studiedadditives requiring even thermal treatments at lowertemperatures.

2. The best compressive strengths were obtained with com-positions of 25% of montmorillonite, illitic clay or pow-dered at glass. Improvements were not possible whenadding kaolin, because of its refractory character.

3. A densication of the material occurs during the thermaltreatment of the probes, at the same time as the com-pressive strength increases considerably. Both magni-tudes go to a maximum value and then diminish byoverburning or/and a bloating effect before the meltingof the material. It has been possible, in most of the cases,to nd a statistical non linear relationship between den-sity and treatment temperature, predict the maximumdensity and estimate the optimum temperature of thethermal treatment, for the conditions of initial compac-tion limited by the workability.

4. Furthermore, the compressive strengths and apparentdensities have been related by a logarithmic relationshipembracing all studied additives, compositions and typeof mixing.

5. Water absorption is related with compressive strengthand density and show minima close to the best valuesof the former parameters. The later increase of water absorption must be ascribed to a bloating effectwith the subsequent inhomogeneous solidication dueto the presence of many open pores in the red

probes.

6. The present study allows the development of a techniquefor use of the sludge ashes from the Galindo wastewatertreatment plant for the production of ceramic productsfor engineering and industrial purposes, manufacturedwith the ashes themselves or with additions up to 25%of powdered at glass or clayish minerals for favoringdensication at lower treatment temperature.

7. The use of sludge ashes for the production of ceramicmaterials brings about a reduction in the cost of thetreatment of sludge, avoiding the transfer of ashes to

landll and the environmental problems derived fromthe leaching of their more soluble constituents. Also, areduction in costs seems possible for the companies thatmanufacture ceramic products for the inclusion of sludge ashes and even a very interesting use of the resi-dues in the production of at glass.

Acknowledgements

This paper is a part of the Doctoral Thesis of IgnacioMerino, one of the authors, directed by the other two part-ners. The authors thank: Consorcio de Aguas/Uren Part-

zuergoa of Bilbao (Spain) for the supply of sludge ashesand operational data of the Galindo wastewater treatmentplant in this research, as well as for the permission for thispublication; Industrias Caobar SA, Industrias Minas Ga-dor SA and Guardian Glass Espan a for the supply of addi-tives and Instituto Tecnolo gico Geominero de Espana forX-ray diffraction results.

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ILLITE25-wet1200C

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16/16

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http://www.kakoki.co.jp/http://www.kakoki.co.jp/http://www.kakoki.co.jp/http://www.kakoki.co.jp/

a - ceramic products with sludge ashes with additives

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