on a new hydraulic binder from stainless steel onverter slag
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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
Rateofheatrelease:J/gperhour
Time: h(b)
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 90 100 110
Rateo
fheatrelease:J/gperhour
Time: h(c)
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
Rateo
fheatrelease:J/gperhour
Time: h(d)
Cumulativeheatrelease:J/g
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)
Intensity:arbitraryunits
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)
Intensity:arbitraryunits
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)
Intensity:arbitraryunits
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)
Intensity:arbitraryunits
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
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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.
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