on a new hydraulic binder from stainless steel onverter slag

7/28/2019 On a New Hydraulic Binder From Stainless Steel Onverter Slag

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Advances in Cement Research, 2013, 25(1), 2131

http://dx.doi.org/10.1680/adcr.12.00031

Paper 1200031

Received 09/05/2012; revised 02/08/2012; accepted 10/08/2012

ICE Publishing: All rights reserved

Advances in Cement Research

Volume 25 Issue 1

On a new hydraulic binder from stainless

steel converter slag

Pontikes, Kriskova, Cizer, Jones and Blanpain

On a new hydraulic binder fromstainless steel converter slagYiannis PontikesResearcher, Centre for High Temperature Processes and SustainableMaterials Management, Department of Metallurgy and MaterialsEngineering, KU Leuven, Belgium

Lubica KriskovaResearcher, Centre for High Temperature Processes and SustainableMaterials Management, Department of Metallurgy and MaterialsEngineering, KU Leuven, Belgium

Ozlem Cizer

Researcher, Building Materials and Building Technology Division,Department of Civil Engineering, KU Leuven, Belgium

Peter Tom JonesIOF Research Manager, Centre for High Temperature Processes andSustainable Materials Management, Department of Metallurgy andMaterials Engineering, KU Leuven, Belgium

Bart BlanpainProfessor, Centre for High Temperature Processes and SustainableMaterials Management, Department of Metallurgy and MaterialsEngineering, KU Leuven, Belgium

The aim of this work was to investigate the hydraulic behaviour of a stainless steel converter slag after changing

its chemical composition and cooling path. The target slag was designed to resemble ground granulated blast-

furnace slag (GGBFS). A synthetic slag with a chemical composition close to stainless steel converter slags was

mixed with 22, 30 and 38 wt% fly ash (FA) from lignite combustion, heated up to 15508C and then granulated by

quenching in water; the solidified new slags were named FA22, FA30 and FA38 respectively. Quantitative X-ray

diffraction on FA22 revealed that the amorphous phase was approximately 40 wt%, the rest being bredigite and

merwinite. For FA addition of 30 wt% or more, the amorphous phase reached almost 100 wt%. The resulting slags

showed significant hydraulic activity when mixed with sodium-based activators, with C-S-H, hydrotalcite and

hydrogarnet being the main hydration products formed. The calorimetric behaviour and the mechanical properties

of blended cements with 30 wt% FA30 and FA38 were comparable to a blended cement with GGBFS. Assuming

that FA addition will take place during the liquid state of the slag, the proposed process can result in a new

hydraulic binder.

IntroductionGround granulated blast-furnace slag (GGBFS) is one of the

most widely used supplementary cementitious materials in

blended Portland cements. It is produced by water granulation of

a blast-furnace slag, forming a material in which the principal

component is a calcium magnesium aluminosilicate glass (Wangand Scrivener, 2003). GGBFS has latent hydraulic properties,

implying that the slag reacts with water to give a cementitious

material once activated in the presence of Portland cement, lime

or alkalis such as caustic soda, sodium carbonate or sulfates of

alkali, calcium or magnesium (Lang, 2002). The hydration of

GGBFS is slow when compared with Portland cement clinker,

resulting in lower strength gain at early stages and higher

strength gain at later stages (Taylor, 1990). The hydration of slag

proceeds by way of dissolution of slag particles followed by

precipitation of hydrated phases from the supersaturated pore

solution. Since the dissolution can be accelerated at high pH

values in the pore solution, there is a tendency to use Portland

cement clinker with a higher content of water-soluble alkalis to

produce blended cement containing GGBFS (Bellmann and

Stark, 2009). In terms of applications, a number of studies have

confirmed that GGBFS is an economical, environmentally

friendly and highly chemically resistant component of building

materials (Mozgawa and Deja, 2009). The European standard

EN 197-1: 2000 (CEN, 2000) reflects the above: CEM III/C can

contain up to 95 wt% GGBFS, which delivers a hydraulic binder

with a particularly low carbon dioxide footprint.

Stainless steel slags are typically used as aggregates and only afew, higher value applications, such as fertiliser, are practised

worldwide (Engstrom et al., 2011). Considering that GGBFS is a

material with a relatively high added value compared with other

slags, it is worthwhile to investigate if stainless steel slags could

be converted to GGBFS-like materials. In addition, the fact that

GGBFS has been well studied provides end users with relative

confidence regarding its behaviour and minimises the so-called

non-technical barriers often encountered when new building

materials are introduced (van Deventer et al., 2010).

The aim of this work was thus to evaluate the potential of

synthesising a material with similar properties as GGBFS, after

modifying a stainless steel converter slag with additions of a

silicon, aluminium-rich industrial waste (i.e. fly ash). In the

envisaged process, the final end-product, if proven similar to

GGBFS in terms of performance, could be applied in blended

cements, mortars, pre-cast concrete or even inorganic polymers.

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Experimental methodA mixture of analytical grade oxides and carbonates with an

elemental composition close to typical stainless steel converter

slags was mixed with 22, 30 and 38 wt% of industrially produced

fly ash (FA) from lignite combustion (Table 1). The FA used is

classified as type F according to ASTM C618-01 (ASTM, 2008)

with quartz (SiO2), anorthite (CaAl2Si2O8), magnetite (Fe3O4),

anhydrite (CaSO4) and gehlenite (Ca2Al2SiO7) identified as the

main crystalline phases. To ensure homogeneity of the powders,

the final compositions were mixed in a Turbula T2C mixer for at

least 12 h.

The resulting material was placed in a platinum crucible and

melted in a bottom loading furnace (AGNI ELT 160-02), at

15508C, with a heating rate of 58C/min. After an isothermal step

at maximum temperature for 1.5 h, the melt was quenched in

water. The final product, vitreous in nature, was dried in air and

milled for 2 h in a bead mill (Dispermat SL-12-C1, VMA) at

5000 rpm. The particle size distribution was determined by laser

scattering technique (MasterSizer Micro Plus, Malvern). Each

powder was measured three times and the average values are

reported. The final slag mixtures with 22, 30 and 38 wt% FA are

named FA22, FA30 and FA38 respectively and have compositions

that fit into the range of GGBFS (Bhatty et al., 2004) (see Table

1). An industrially produced GGBFS was also integrated in the

research programme and used as a reference material in the studyof the hydraulic behaviour of the slag mixtures.

The mineralogical composition and the amorphous content were

determined by X-ray powder diffraction (XRPD, D500 Siemens)

and Rietveld analysis using Topas Academic software. Materials

were mixed with 10 wt% of zinc oxide and measured over a 2

range of 10708 using CuK radiation of 40 kV and 40 mA, with

a 0.028 step size and step time of 4 s.

The reactivity of the synthetic slags was studied after alkali

activation with analytical grade solutions of sodium hydroxide

(NaOH (NH)), sodium carbonate (Na2CO3 (NC)) and sodiumsilicate, with a nominal molecular formula Na2O.3.4SiO2 (NS).

In all cases, the sodium oxide to slag ratio was equal to 8 wt%,

while keeping the solid to liquid equal to 1. The paste samples

were subjected to isothermal conduction calorimetry (TAM Air

device, TA Instruments) at 208C to monitor heat release during

the reaction.

To gain more insight into the reaction mechanism and reaction

products, paste samples were prepared by mixing the slag with

selected alkali activators (conditions as above). The pastes were

stored in closed plastic capsules for 3, 7 and 28 days. Samples

were subsequently crushed into powder and were vacuum dried at

0.035 mbar for 2 h (Alpha 1-2 LD, Martin Christ), as suggested

elsewhere (Knapen et al., 2009). Fourier transform infrared

spectroscopy (FTIR) (Alpha spectrometer, Bruker) was employed

to reveal bond structure information. For the measurement,

approximately 4.5 mg of hand-ground sample was mixed with

450 mg of KBr and compressed into pellets. Thermal analysis of

the dried samples was performed using TGA/DSC (STA 409 PC

Luxx1, Netzsch). The samples were heated at 58C/min in a

continuous nitrogen gas flow up to 10008C. The microstructure of

the hydrated product was studied by means of a scanning electron

microscope (SEM XL30, Phillips). For this purpose, bulk samples

of hydrated pastes were dried at 508C for 2 days.

Finally, blended mortar samples composed of ordinary Portland

cement (OPC, CEM I, 42.5 R) in 70 wt% and slag mixtures in

30 wt% were prepared based on EN 196-1 (CEN, 2005). CEN

standard sand (02 mm particle size) was used in a binder to sand

ratio of 1:3 and in a binder to water ratio of 0 .5 by mass. The

mortar mixtures were cast in 20 mm 3 20 mm 3 160 mm moulds

and stored at 208C and relative humidity .95%. Compressive and

flexural strength tests were performed using an Instron 4467. Four

measurements for compressive strength and two for flexural

strength per mortar sample and hydration time were performed

and the average values and standard deviations are reported.

Composition: wt%

CaO SiO2 MgO Al2O3 Fe2O3 SO3 Other

Typical GGBFS 30 50 27 40 1 10 5 15 ,1 0.62

GGBFS 41.9 35.5 9.2 9.5 0.4 0.8 2.7

FA 12.1 42.9 5.4 22.9 6.6 6.3 3.8

Synthetic slag 56.7 28.4 6.5 1.3 1.1 6.0

FA22 46.9 31.6 6.3 6.1 1.5 1.4 6.2

FA30 43.3 32.7 6.2 7.8 1.9 1.9 6.2

FA38 39.8 33.9 6.1 9.5 2.5 2.4 5.8

Table 1. Chemical composition of typical GGBFS (Bhatty et al.,

2004 and references therein) and the GGBFS, FA, synthetic slag

and the three slag mixtures studied in this work

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Results and discussion

Material characterisation

The quenched slag was glassy and dark brownish colour,

primarily due to the presence of iron (Figure 1). The originally

produced granules (Figure 1(a)) could be easily broken by hand

into angular fragments due to the extensive formation of cracks

(Figure 1(b)).

Results from X-ray diffraction (XRD) (see Figure 2) and Rietveld

analysis revealed that with an addition of 22 wt% FA to the

synthetic slag, the quenched material contained approximately

30 wt% merwinite, 30 wt% bredigite, the rest being an amor-

phous phase. For 30 wt% and 38 wt% addition of FA, the material

was almost completely amorphous. The industrial sample of

GGBFS was also mainly amorphous, with merwinite identified as

the only crystalline phase present.

After milling, all powders showed similar particle size distribu-

tion, with d10 , 0.5 m, d50 , 3 m andd90 , 9 m.

Reactivity with alkalis

Isothermal conduction calorimetry results of slags activated with

different alkali solutions are presented in Figures 3 (a)(d). The

effect of an activator depends mainly on its nature, dosage andcharacteristics of the activated material (Ben Haha et al., 2011a,

2011b, 2012; Shi et al., 2006). Consequently, activation of

different materials results in the formation of different hydration

products with different properties (Shi and Day, 1996; Shi et al.,

2006).

Activation with NH resulted in the highest recorded heat release

among all activators, for the time investigated, with the exception

of GGBFS and activation by way of NC where the detected heat

release was slightly higher. Moreover, NH was the only activator

that gave a clear peak for all three slags. For FA22, the main peak

occurred after approximately 10 h of hydration, whereas in thecase of FA30 and FA38 the peak occurred faster, implying the

acceleration of hydration reactions. According to Shi et al.

(2006), NH-activated slags typically have a calorimetry curve

consisting of two peaks: the first peak, before the induction

period, is attributed to wetting and dissolution, and the second

peak, after the induction period, is ascribed to accelerated

hydration. However, a double peak was clearly visible only in

FA22; FA30 showed also two peaks but the time interval between

was ,3 h, whereas FA38 and GGBFS reacted even faster and

showed only a single peak and no peak respectively. These data

suggest that the reaction kinetics are enhanced as the FA content

in the synthetic slags increases but do not reach that of GGBFS.

Regarding NS, a clear peak of heat release was observed for

FA22 and FA30, unlike FA38, where there was no distinct peak

and moreover the cumulative heat release was substantially

smaller. This could be attributed to the slower kinetics of

(b)

1 cm

(a)

1 mm

Figure 1. Water-quenched material (a) detailed view and (b) as

produced

20 30 40 50 60 70

2 : degrees

Intensity:arbitraryunits

FA22

FA30

FA38

GGBFS

1

2, 3

1

1

2, 3 2, 31 1 1 1

2

1 ZnO2 Merwinite3 Bredigite

Figure 2. XRD patterns of FA22, FA30, FA38 and GGBFS; 10 wt%

zinc oxide is added as internal standard

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NS-activated systems (Ben Haha et al., 2011a). NC, on the other

hand, was effective only in the case of FA30 and FA38. Similarlyto NS, the peak in NC-activated samples occurred at a later time

for an increasing FA content in the original slag (i.e. lower

basicity, higher polymerisation in the glass matrix of the slag).

Finally, even though both NS and NC belong to the activators

giving a hydration curve starting with a double peak before the

induction period and one peak after the induction period (Shi et

al., 2006), this was not apparent in the current study.

Activation with CH gave no peaks of heat release and resulted in

comparatively low cumulative heats for all samples tested (Figure

3). This reflects the slower hydration kinetics and is attributed

mainly to the lower pH in the pore solution compared with alkali

activation, which subsequently defines a slower dissolution rate

for the slag (Bellmann and Stark, 2009).

From the FA addition point of view, FA30 was the only material

that showed clear peaks of heat release for all three sodium-based

activators. FA30 also generated the largest amount of heat during

the hydration when activated with every activator except CH.

Reactivity and hydration products after activation with

sodium hydroxide

The XRD patterns of slags activated with NH having zinc oxide as

internal standard are presented in Figure 4. A broad hump present

in each non-hydrated slag in the 2 region of 25388 slightly

diminished during the first 3 7 days of hydration and a new

diffuse peak at about 2 29.58 appeared. This peak is assigned

to C-S-H phase, JCPDS-ICDD # 45-1480 (Song and Jennings,

1999). C-S-H is generally considered to be poorly crystalline but

its crystallinity in sodium hydroxide-activated slag has already

been reported by Shi et al. (2006). Other crystalline phases such as

hydrotalcite (identified as Mg6Al2CO3(OH)16:

4H2O, JCPDS-

ICDD # 41-1428) and hydrogarnet (identified as katoite

Ca3Al2(OH)12, JCPDS-ICDD # 24-217) were also identified.

Interestingly, hydrogarnet was only detected in the hydrated slag

samples in which FA was incorporated. The latter may be related

NH NS NC CH

Open symbols for cumulative heat release

0

2

4

6

8

10

12

14

16

0

50

100

150

0 10 20 30 40 50 60 70 80 90

Rateofheatrelease:J/gperhour

Time: h(a)

Cumulativeheatrelease:J/g

0

2

4

6

8

10

12

14

16

0

50

100

150

200

0 10 20 30 40 50 60 70 80 90


Time: h(b)


0

2

4

6

8

10

12

14

16

0

50

100

150

200

0 10 20 30 40 50 60 70 80 90 100 110

Rateo

fheatrelease:J/gperhour

Time: h(c)


0

2

4

6

8

10

12

14

16

0

50

100

150

200

0 10 20 30 40 50 60 70 80 90

Rateo

fheatrelease:J/gperhour

Time: h(d)


Figure 3. Isothermal conduction calorimetry of slags activated

with NH, NS, NC and CH: (a) FA22; (b) FA30; (c) FA38;

(d) GGBFS

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to the slightly different chemistry of the synthetic slags and in

particular the higher iron content (Table 1). The main peak of

calcite calcium carbonate (CaCO3) at 2 29.48 (JCPDS-ICDD #

5-586) is very close to the C-S-H region and most probably calciteis present in a small amount; thermogravimetric analysis (TGA)

and FTIR results presented later on corroborate the presence of

carbonates. In general, the above findings are in good agreement

with other published research (e.g. Puertas and Fernandez-Jime-

nez, 2003; Shi et al., 2006; Song and Jennings, 1999).

Thermogravimetric analysis and differential thermogravimetry

(DTG) were used to monitor the hydration progress during the

first 28 days (Figures 5 (a)(d)). For all samples activated with

NH, the first peak observed in the DTG curves (Figures 5 (a)

(d)) was at 851058C and is attributed to C-S-H decomposition

(Hewlett, 1998). The intensity of this peak increased with

increasing hydration time up to 28 days, which is an indication of

increasing C-S-H formation (assuming no variation in the

stoichiometry of C-S-H). Occasionally, a shoulder was detected at

approximately 1358C, an indication of AFm type phases (e.g.

C4AH13; see Taylor (1990)). Additional peaks are clearly ob-

served in the DTG curves between approximately 250 and 3808C.

Most probably, the peak at approximately 2608C is due to the

decomposition of hydrogarnet (Passaglia and Rinaldi, 1984;

Rivas-Mercury et al., 2008) and the main peak between 3008C

and 3358C is due to the decomposition of hydrotalcite (Hickey et

al., 2000; Wang and Scrivener, 1995).

In terms of reaction kinetics, the weight loss after 28 days of

hydration was substantial for all studied materials and comparable

with the reference GGBFS for the same activator (Figure 5 (a)

(d)). In detail, FA22 showed a small weight loss of 12.8 wt% after

3 days of hydration. The reactions were very slow between day 3

and day 7, but an acceleration was observed later on, resulting in

a weight loss of 24.3 wt% after 28 days. The above trend is

partially explained by the rather sluggish hydration kinetics, as

also suggested by the calorimetry data (Figure 3). The highest

reaction rate at early stage was observed in FA30, where a

substantial weight loss of 19.9 wt% was recorded after 3 days of

hydration. The reactions continued but at a decreasing rate for

the rest of the period studied. The total weight loss after 28 days

was 24.8 wt%. Sample FA38 reacted similarly to FA22 during the

20 30 40 50 60 702 : degrees

(a)


Original

3 days

7 days

28 days

90 days

6

2 2 2

6 5 63, 4

1, 4

6

1

1

6 61, 2

21 1 1

1 ZnO

3 CaCO2 Merwinite

3

4 C-S-H5 Hydrotalcite6 Hydrogarnet

20 30 40 50 60 702 : degrees

(b)


Original

3 days

7 days

28 days

90 days

1 ZnO2 CaCO33 C-S-H

4 Hydrotalcite5 Hydrogarnet

5 5 4 52, 3

1, 31

1

5 51

3 5

1 15

1

20 30 40 50 60 702 : degrees

(c)


Original

3 days

7 days

28 days

90 days

1 ZnO2 CaCO33 C-S-H

4 Hydrotalcite5 Hydrogarnet

5 5 4 52, 3

1, 31

1

5 51

3

1 15

1

20 30 40 50 60 702 : degrees

(d)


Original

3 days

7 days

28 days

90 days

1 ZnO2 CaCO33 C-S-H

4 Hydrotalcite5 Merwinite

42, 3

1, 31

1

51

31 1 1

Figure 4. XRD patterns of alkali-activated hydrated pastes at 3,

7, 28 and 90 days: (a) FA22 + NH; (b) FA30 + NH; (c) FA38 + NH;

(d) GGBFS + NH

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first 3 days. After the third day, the reactions slowed down. The

weight loss between day 3 and day 7 of hydration was almost as

much as that between day 7 and day 28. The total weight loss

after 28 days of hydration was 24.3 wt%.

Figure 6 presents the FTIR spectra of all the slags before and after

90 days of hydration. The band at 500 cm1 is assigned to TO T

or OTO (T Si, Al) bending vibration (McMillan, 2001)

whereas the band at 700 cm1 is assigned to symmetric stretching

vibration of SiO T bonds (e.g. Pechar and Rykl, 1983). The

main band at 950 cm1 is an assemblage of symmetric SiO

stretching vibrations of tetrahedral silicate forms with one, two,

three and four non-bridged oxygen atoms per silicon atom (NBO/

Si) (McMillan, 2001). The band at 1415 1430 cm1 is assigned to

CO stretching vibration (e.g. Tatzber et al., 2007) whereas peaks

at about 3450 cm1 and 1640 cm1 are assigned to OH stretch-

ing and H O H bending respectively (e.g. Pechar and Rykl,

1983). After 90 days, the main broad peak at 950 cm1 became

significantly smaller and narrower, whereas new peaks appeared or

were different in their intensities. In detail, the typical bands of

C-S-H gel are detected as peaks appearing

j at 950970 cm1, corresponding to the SiO asymmetric

stretching bands in Q2 units

DTG:mg/mgperC

3 days

7 days

28 days

200 400 600 800 1000

Temperature: C(a)

3 days

7 days

28 days

100

200 400 600 800 1000Temperature: C

989694929088868482

80787674

Weight:% D

TG:mg/mgperC

3 days

7 days

28 days

200 400 600 800 1000

Temperature: C(b)

3 days

7 days

28 days

100

200 400 600 800 1000Temperature: C

98969492908886848280787674

Weight:%

DTG:mg/mgper

C

3 days

7 days

28 days

200 400 600 800 1000

Temperature: C

(c)

3 days

7 days

28 days

100

200 400 600 800 1000Temperature: C

98969492908886848280787674

Weight:%

DTG:mg/mgper

C

3 days

7 days

28 days

200 400 600 800 1000

Temperature: C

(d)

3 days

7 days

28 days

100

200 400 600 800 1000

Temperature: C

98969492908886848280787674

Weight:%

Figure 5. DTG and TGA (insets) of hydrated pastes activated with

NH at 3, 7 and 28 days: (a) FA22; (b) FA30; (c) FA38; (d) GGBFS

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Transmitance:arbitraryunits

90 days

Original

3500 1500 1000 500

(a)

90 days

Original

3500 1500 1000 500

(b)

Transmitan

ce:arbitraryunits

90 days

Original

3500 1500 1000 500

Wave number: cm

(c)

1

90 days

Original

3500 1500 1000 500

Wave number: cm1

(d)

Figure 6. FTIR spectra of original slag samples and activated slag

hydrated for 90 days: (a) FA22 + NH; (b) FA30 + NH;

(c) FA38 + NH; (d) GGBFS

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j at about 815820 cm

1, ascribed to the SiO symmetricvibrations in Q1 units

j in the region of 500400 cm1, notably new peaks at

490 cm1, 450 cm1 and 422 cm1, which are associated

with vibrations of the OSiO bonds (Mozgawa and Deja,

2009; Ping et al., 1999; Puertas and Fernandez-Jimenez,

2003).

The peak at 670 cm1 is associated with the vibrations of Si O Al

bridges (Mozgawa and Deja, 2009). Both characteristic peaks at

about 3450 cm1 and 1640 cm1, assigned to OH and HOH

vibrations respectively, have higher intensity in the hydrated

samples. The presence of carbonate groups [CO3]2 is evidenced

by peaks at about 1433 cm1: This is in agreement with the XRD

and TGA data supporting the presence of hydrotalcite and CaCO 3:

Microstructural analysis

The microstructural development during hydration depended on

the chosen activator. FA30 and FA38 behaved similarly for the

same activator. Figure 7 shows the SEM results for FA30

activated with NH during the first 90 days.

The microstructure at 3 days of hydration (Figure 7(a)) was

characteristic of C-S-H crystal growth. The original slag particles

are covered with fibrillar crystals and a reticular network with

distinctive bridges started to form. Porosity remained substan-tial, however; even at 7 days of hydration, a more compact

structure had evolved (Figure 7(b)). The fibrillar C-S-H morph-

ology was still apparent, yet the crystals were well developed,

forming densified isles. Plate-like crystals were occasionally

detected; based on their size and characteristic morphology, they

are most probably AFm phases (Wang and Scrivener, 1995,

2003). This is in line with the DTG results (Figure 5) and the

discrete peaks detected at approximately 1358C. At 28 days of

hydration (Figure 7(c)), former slag grains were reduced to

particles of less than 1 m and an extensive cohesive network

was formed in the inter-particle space. Clusters of elongated

crystals could be seen occasionally; these are attributed to C-S-H.Similar C-S-H morphologies have also been detected elsewhere

for blast-furnace slag finely milled and after 28 days of hydration

(Kumar et al., 2005). At 90 days of hydration (Figure 7(d)), the

plate-like AFm crystals were well intercalated into the dense

matrix. Regarding hydrotalcite, the characteristic platelets were

not detected in the hydrated microstructure, probably as the result

of the small size (e.g. Ben Haha et al., 2011b). On the contrary,

hydrogarnet could be distinguished more easily due to the

characteristic trapezohedral crystals (Figure 7(e)).

Behaviour in blended cements with OPC and comparison

with GGBFS

To further compare the hydraulic potential of the slags developed

in this work with currently produced GGBFS, blended cements

were developed with OPC and the slags or the reference GGBFS.

Two aspects were evaluated the hydration behaviour by means

of isothermal calorimetry and the mechanical properties.

In terms of heat release (Figure 8), the blends with GGBFS,FA30 and FA38 behaved almost identically. All blended cements

showed the characteristic double peak more as a shoulder than

separate peaks. Similar calorimetry curves were reported by

Meinhard and Lackner (2008) who investigated the hydration of

GGBFS and OPC blends at room and elevated temperature. The

blends showed the peak of the heat release rate at approximately

25 h whereas, at 100 h, the evolved cumulative heat release is

slightly above 120 J/g. Interestingly, the cement blend with FA22

evolved heat faster, having its peak between 15 h and 25 h

approximately. It is possible that the presence of crystals

(merwinite, bredigite) in FA22 affected the hydration kinetics by

providing additional nucleation sites.

The results of flexural and compressive strength up to 90 days

shown in Figures 9 and 10 respectively reveal that the blends

with FA30 and FA38 behave similarly and are very close to the

reference mixture with GGBFS. The blend with FA22 had a

slower strength gain and a small overall strength at 90 days.

Comparison of all blended cements with CEM I showed that

strength gain is slower, as expected for cements with GGBFS, yet

at 28 and 90 days there was no notable difference. With respect

to flexural strength, no clear trend was observed as all blends

presented comparable values for a chosen hydration day.

Considerations for industrial implementationThe presented results demonstrate that production of a replica

blast-furnace slag may be a viable option for the valorisation of

secondary steelmaking slags. The required processing would take

place after slag metal separation and some recent papers

(Engstrom et al., 2011; Pontikes et al., 2011) clearly demonstrate

that there is know-how to perform such an operation. This new

slag could be produced from various waste materials that

currently find limited or no use (e.g. high-carbon FA or secondary

aluminas), by effectively controlling the chemistry. As a result,

this slag becomes a product itself and not a by-product or residue.

The latter is important as it appears to be possible to design

tailor-made binders for specific applications (blended cement oralkali-activated). Industrial implementation would probably re-

quire substantial investment and secured access to secondary

resources. It is thus likely that local conditions will eventually

dictate how industrially realistic such a process is and whether

industrial symbiosis could occur.

Conclusionsj Addition of fly ash (FA) to a synthetic slag close in

composition to stainless steel converter slag resulted in a

comparable material to blast-furnace slag in terms of

chemistry and mineralogy.

j The amount of crystalline phase decreased when the amount

of FA increased and, for an addition of 30 wt% FA and above,

the resulting material was almost completely amorphous.

j Activation with sodium hydroxide typically resulted in

comparatively fast and substantial heat release. Only the FA30

sample could be activated with all three alkali activators.

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j The slags activated with sodium hydroxide resulted in the

formation of C-S-H, hydrotalcite, hydrogarnet and AFm

phases as the main hydration products.

j The calorimetric behaviour and mechanical properties of

blended cements with FA30 and FA38 were very similar to a

blended cement with GGBFS.

j Production of such slags by way of hot-stage processing

could upgrade the currently produced secondary

(a) (b)

(c) (d)

(e)

Figure 7. SEM images of FA38 hydrated for (a) 3 days, (b) 7 days,

(c) 28 days and (d, e) 90 days. The arrows in (e) point to

hydrogarnet crystals

29


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steelmaking slags with regard to their applications anddeliver a new binder, engineered specifically for particular

applications.

AcknowledgementsThe authors gratefully acknowledge IWT O&O project 090594

for financial support. Y. Pontikes and O. Cizer thank the FWO for

post-doctoral fellowships.

REFERENCES

ASTM (2008) ASTM C618-01: Standard specification for coal fly

ash and raw or calcined natural pozzolan for use as a mineral

admixture in concrete. ASTM, West Conshohocken, PA,

USA.

Bellmann F and Stark J (2009) Activation of blast furnace slag by

a new method. Cement and Concrete Research 39(8): 644

650.

Ben Haha M, Le Saout G, Winnefeld F and Lothenbach B

(2011a) Influence of activator type on hydration kinetics,

hydrate assemblage and microstructural development of alkali

activated blast-furnace slags. Cement and Concrete Research

41(3): 301310.

Ben Haha M, Lothenbach B, Le Saout G and Winnefeld F

(2011b) Influence of slag chemistry on the hydration of

alkali-activated blast-furnace slag Part I: Effect of MgO.Cement and Concrete Research 41(9): 955963.

Ben Haha M, Lothenbach B, Le Saout G and Winnefeld F (2012)

Influence of slag chemistry on the hydration of alkali-

activated blast-furnace slag Part II: Effect of Al2O3:

Cement and Concrete Research 42(1): 74 83.

Bhatty JI, Miller FM and Kosmatka SH (eds) (2004) Innovations in

Portland Cement Manufacturing, 1st edn. Portland Cement

Association, Skokie, IL, USA.

CEN (Comite Europeen de Normalisation) (2000) EN 197-1:

2000. Cement part 1. Composition, specifications and

conformity criteria for common cements. CEN, Brussels,

Belgium.CEN (2005) EN 196-1. Methods of testing cement part 1.

Determination of strength. CEN, Brussels, Belgium.

Engstrom F, Pontikes Y, Geysen D et al. (2011) Review: hot stage

engineering to improve slag valorisation options. Proceedings

of the 2nd International Slag Valorisation Symposium,

Leuven, Belgium, pp. 231 251.

Hewlett PC (ed.) (1998) Leas Chemistry of Cement and

Concrete. Butterworth-Heinemann.

Hickey L, Kloprogge JT and Frost RL (2000) The effects of

various hydrothermal treatments on magnesiumaluminium

hydrotalcites. Journal of Materials Science 35(17): 4347

4355.

Knapen E, Cizer O, Van Balen K and Van Gemert D (2009) Effect

of free water removal from early-age hydrated cement pastes

on thermal analysis. Construction and Building Materials

23(11): 34313438.

Kumar R, Kumar S, Badjena S and Mehrotra S (2005) Hydration

0

1

2

3

4

5

6

0

20

80

120

160

0 20 40 60 80 100


Time: h

Cumulativeheatrelease:J/g140

100

60

40

07 OPC 03 GGBFS07 OPC 03 FA2207 OPC 03 FA3007 OPC 03 FA38

Open symbols for cumulative

Figure 8. Rate and cumulative heat release from isothermal

calorimetry measurements for blended cements with 70 wt%

OPC and 30 wt% GGBFS, FA22, FA30 or FA38

0

1

2

3

45

6

7

8

9

10

11

12

0 10 20 30 40 50 60 70 80 90

Flexuralstrength:MPa

Hydration time: days

OPC

07 OPC 03 GGBFS

07 OPC 03 FA22

07 OPC 03 FA30

07 OPC 03 FA38

Figure 9. Flexural strength of blended mortars with 70 wt% OPC

and 30 wt% GGBFS, FA22, FA30 or FA38

0

10

20

30

40

50

0 10 20 30 40 50 60 70 80 90

Compressivestrength:MPa

Hydration time: days

OPC

07 OPC 03 GGBFS

07 OPC 03 FA22

07 OPC 03 FA30

07 OPC 03 FA38

Figure 10. Compressive strength of blended mortars with

70 wt% OPC and 30 wt% GGBFS, FA22, FA30 or FA38

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of mechanically activated granulated blast furnace slag.Metallurgical and Materials Transactions B 36(6): 873883.

Lang E (2002) Blastfurnace Cements, 2nd edn. Spon Press, New

York, NY, USA.

McMillan PF (2001) Amorphous materials: vibrational

spectroscopy. In Encyclopedia of Materials: Science and

Technology. (Buschow KHJ, Chan RW, Flemings MC (eds)).

Elsevier, pp. 251256.

Meinhard K and Lackner R (2008) Multi-phase hydration model

for prediction of hydration-heat release of blended cements.

Cement and Concrete Research 38(6): 794802.

Mozgawa W and Deja J (2009) Spectroscopic studies of alkaline

activated slag geopolymers. Journal of Molecular Structure

924926: 434441.

Passaglia E and Rinaldi R (1984) Katoite, a new member of the

Ca3Al2(SiO4)3Ca3Al2(SiO4)3(OH)12 series and a new

nomenclature for the hydrogrossular group of minerals.

Bulletin de Mineralogie 107(5): 605618.

Pechar F and Rykl D (1983) Study of the complex vibrational

spectra of natural zeolite mordenites. Zeolites 3(4): 329332.

Ping Y, Kirkpatrick RJ, Brent P, McMillan PF and Cong X (1999)

Structure of calcium silicate hydrate (C-S-H): Near-, mid-,

and far-infrared spectroscopy. Journal of the American

Ceramic Society 82(3): 742748.

Pontikes Y, Kriskova L, Wang X et al. (2011) Additions of

industrial residues for hot stage engineering of stainless steelslags. Proceedings of the 2nd International Slag Valorisation

Symposium, Leuven, Belgium, pp. 313 326.

Puertas F and Fernandez-Jimenez A (2003) Mineralogical and

microstructural characterisation of alkali-activated fly

ash/slag pastes. Cement and Concrete Composites 25(3):287292.

Rivas-Mercury JM, Pena P, de Aza AH and Turrillas X (2008)

Dehydration of Ca3Al2(SiO4)y(OH)4(3-y) (0 , y , 0.176)

studied by neutron thermodiffractometry. Journal of the

European Ceramic Society 28(9): 17371748.

Shi C and Day RL (1996) Selectivity of alkaline activators for the

activation of slags. Cement, Concrete and Aggregates 18(1):

814.

Shi C, Krivenko PK and Roy D (2006) Alkali-Activated

Cements and Concretes. Taylor & Francis, New York, NY,

USA.

Song S and Jennings HM (1999) Pore solution chemistry of

alkali-activated ground granulated blast-furnace slag. Cement

and Concrete Research 29(2): 159170.

Tatzber M, Stemmer M, Spiegel H et al. (2007) An alternative

method to measure carbonate in soils by FT-IR spectroscopy.

Environmental Chemistry Letters 5(1): 912.

Taylor HFW (ed.) (1990) Cement Chemistry, 2nd edn. Academic

Press, London, UK.

van Deventer J, Provis J, Duxson P and Brice D (2010) Chemical

research and climate change as drivers in the commercial

adoption of alkali activated materials. Waste and Biomass

Valorization 1(1): 145155.

Wang SD and Scrivener KL (1995) Hydration products of alkali

activated slag cement. Cement and Concrete Research 25(3):561571.

Wang SD and Scrivener KL (2003) 29Si and 27Al NMR study of

alkali-activated slag. Cement and Concrete Research 33(5):

769774.

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