Yiannis 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 CizerResearcher, 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 dose to stainless steel converter slags was

mixed with 22, 30 and 38 wt% fly ash (FA) from lignite combustion, heated up to 1550°C 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

blendcd Portland ccmcnts, It is produced by water granulatioll of

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

component is a calcium-magnesium aluminosilicate glass (Wang

and Scrivener, 2003). GGBFS has latent hydraulic properties,

implying that the slag reacts with water to givc a cementitious

material once activated in the presence of Portland cement, limc

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

alkali, calcium or magncsium (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 he accelerated at high pH

values in the pore solution, therc is a tcndency to use Portland

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

produce blended cement containing GGBFS (Bcllmann and

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

confirmcd that GGBFS is an economical, environmentally

friendly and highly chemically resistant component of building

materials (Mozgawa and Deja, 2(09). The European standard

EN 197-1: 2000 (CEN, 2(00) reflects the above: CEM Ill/C can

contain up to 95 wt'y') GGBFS, which delivers a hydraulic binder

with a particularly low carbon dioxide fbotprint.

Stainless steel slags are typically used as aggregates and only a

few, higher value applicatiolls, such as fertiliser, are practised

worldwide (Engstrom et ai" 20 II), Considering that GCIBFS is a

material with a relatively high added value compared with other

slags, it is worthwhile to investigate if stainless steel could

be converted to OGBFS-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-technieal' barriers often cncountercd when new building

materials are introduced (van Deventer et aI., 20 I0).

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

synthesising a material with similar properties as CJGBFS, after

modifying a stainless steel converter with additions of a

silicon. aluminium-rich industrial waste (i.e< fly In the

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

GOBI'S in terms of performance, could be applied in blended

cements, mortars, pre-cast concrete or even lI1c.rg;al1lc p,oly'm(~rs,

21

Advances in Cement ResearchVolume 25 Issue 1

Experimental methodA mixture of analytical grade oxides and carbonates with anelemental composition close to typical stainless steel converterslags was mixed with 22, 30 and 38 wt% of industrially producedfly ash (FA) from lignite combustion (Table I). The FA used isclassified as type F according to ASTM C618-0 I (ASTM, 2008)with quartz (SiOz), anorthite (CaAIzSizOs), magnetite (Fe)04),

anhydrite (CaS04) and gehlenite (CazAlzSi07) identified as themain 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 andmelted in a bottom loading furnace (AGNI ELT 160-02), atI550°C, with a heating rate of 5°C/min. After an isothermal stepat maximum temperature for 1·5 h, the melt was quenched inwater. The final product, vitreous in nature, was dried in air andmilled for 2 h in a bead mill (Oispermat SL-12-CI, VMA) at

5000 rpm. The particle size distribution was determined by laserscattering technique (MasterSizer Micro Plus, Malvern). Eachpowder was measured three times and the average values arereported. The final slag mixtures with 22, 30 and 38 wt% FA arenamed FA22, FA30 and FA38 respectively and have compositionsthat fit into the range of GGBFS (Bhatty et al., 2004) (see TableI). An industrially produced GGBFS was also integrated in theresearch programme and used as a reference material in the studyof the hydraulic behaviour of the slag mixtures.

The mineralogical composition and the amorphous content weredctermined by X-ray powder diffraction (XRPD, 0500 Siemens)and Rietveld analysis using Topas Academic software. Materialswere mixed with 10 wt% of zinc oxide and measured over a 28

range of 10-70° using CuKa radiation of 40 kV and 40 mA, witha 0·02° step size and step time of 4 s.

The reactivity of the synthetic slags was studied after alkaliactivation with analytical grade solutions of sodium hydroxide

On a new hydraulic binder from stainlesssteel converter slagPontikes, Kriskova, eizer, Jones and Blanpain

(NaOH (NH», sodium carbonate (NazCO) (NC) and sodiumsilicate, with a nominal molecular formula Na20·3·4Si02 (NS).In all cases, the sodium oxide to slag ratio was equal to 8 wt%,while keeping the solid to liquid equal to I. The paste sampleswere subjected to isothermal conduction calorimetry (TAM Airdevice, TA Instruments) at 20°C to monitor heat release duringthe reaction.

To gain more insight into the reaction mechanism and reactionproducts, paste samples were prepared by mixing the slag withselected alkali activators (conditions as above). The pastes were

stored in closed plastic capsules for 3, 7 and 28 days. Sampleswere subsequently crushed into powder and were vacuum dried at0·035 mbar for 2 h (Alpha 1-2 LD, Martin Christ), as suggestedelsewhere (Knapen et al., 2009). Fourier transform infraredspectroscopy (FTlR) (Alpha spectrometer, Bruker) was employedto reveal bond structure information. For the measurement,

approximately 4·5 mg of hand-ground sample was mixed with450 mg of KEr and compressed into pellets. Thermal analysis ofthe dried samples was performed using TGNDSC (STA 409 PCLuxx@, Netzsch). The samples were heated at 5°C/min in a

continuous nitrogen gas flow up to 1000°C. The microstructure ofthe hydrated product was studied by means of a scanning electronmicroscope (SEM XL30, Phillips). For this purpose, bulk samplesof hydrated pastes were dried at 50°C for 2 days.

Finally, blended mortar samples composed of ordinary Portlandcement (OPC, CEM I, 42·5 R) in 70 wt% and slag mixtures in30 wt% were prepared based on EN 196-1 (CEN, 2005). CENstandard sand (0-2 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. Themortar mixtures were cast in 20 mm x 20 mm X 160 mm moulds

and stored at 20°C and relative humidity >95%. Compressive andflexural strength tests were performed using an Instron 4467. Fourmeasurements for compressive strength and two for flexuralstrength per mortar sample and hydration time were performed

and the average values and standard deviations are reported.

Composition: wt%

CaD SiOz MgO AI2 0 3 Fe2 03 503 Other

Typical GGBF5 30-50 27-40 1-10 5-15 <1 0·6-2

GGBFS 41·9 355 9·2 9·5 0-4 0·8 2·7

FA 12·1 429 5-4 22·9 6·6 6·3 3·8

Synthetic slag 56·7 284 6·5 1·3 1·1 6·0FAn 46·9 31·6 6·3 6·1 1·5 1-4 6·2FA30 43·3 327 6·2 7·8 1·9 1·9 6·2FA38 39·8 339 6·1 9·5 2·5 2·4 5·8

Table 1. Chemical composition of typical GGBFS (Bhatty et a/.,2004 and references therein) and the GGBFS, FA, synthetic slag

and the three slag mixtures studied in this work

22

Advances in Cement ResearchVolume 25 Issue 1

On a new hydraulic binder from stainlesssteel converter slagPontikes, Kriskova, elzer, Jones and Blanpain

706040 502fJ: degrees

3020

1 ZnO2 Merwinite3 Bredlgite

GGBFS

FA30

2,31

1\11 ?<~~1 1f-FA_2_2_........._VU~~

~.~ FA38~c

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

zinc oxide is added as internal standard

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 complctely amorphous. The industrial samplc of

GGBFS was also mainly amorphous, with merwinite identified as

the only crystalline phase present.

The quenched slag was glassy and dark brownish colour,

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

produced granules (Figure I(a» could be easily broken by hand

into angular fragments due to the extensive formation of cracks

(Figure I(b».

Results and discussion

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

produced

After milling, all powders showed similar particle size distribu-

tion, with d lO 0·5 pm, dso ~ 3 pm and d90 9 ~lm.

Isothermal conduction calorimetry results of slags activated with

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

effect of an activator depends mainly on its nature, dosage and

characteristics of the activated material (Ben Haha el aI., 20 I Ia,

2011b, 2012; Shi el aI., 2006). Consequently, activation of

different materials results in the formation of differcnt hydration

products with different properties (Shi and Day, 1996; Shi el al.,2(06).

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, thc main peak

occurred after approximately 10 h of hydration, whcreas in the

ease of r!\30 and FA38 the peak occurred faster, implying thc

acceleration of hydration rcactions. According to Shi el al.(2006), NH-activated slags typically have a ealorimctry 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 pcriod, is ascribcd to accelcrated

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

FA22; FA30 showed also two pcaks but thc time interval between

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

showed only a single peak and no peak These data

suggest that the reaction kinetics are enhanced as the FA content

in the synthetic 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

and morcover the cumulative heat release was

smaller. This could be attributed to the slower kinetics of

23

Advances in Cement ResearchVolume 25 Issue 1

On a new hydraulic binder from stainlesssteel converter slagPontikes, Kriskova, eizer, Jones and Blanpain

..• - NH NS NC CHOpen symbols for cumulative heat release

16 150 16

o 10 20 30 40 50 60 70 80 90Time: h

(b)

100

50

200

o

Qj

. 150 ~

~

1<1OJ

.J::OJ>.;:::;

'":JE:::JU

_<,~<_._,p,~.--JJ~<J.-.J-Jf.YJ-~=-n.~,p.t."C=·"'C!-:P

-i.-"~"--

:::Jo 14

J::

~ 12

g 10

~ f\/-.'" 8 .•.•W II' r

~ 6 t,,>:'~ !.I \~

J:: 4 I' / .......a L~.,' ..~ 2 ~ "' ::!~ 0 ".. ...... . .....-.-._.._.::::::--••

QjVl

100 ~

~­'"OJ.J::OJ

50 'E.!!?:::J

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80706020 30 40 50Time: h

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~ 12g

10~:ll 8~'rU 6OJ

.J:: 415~ 2ce

200 g

50

QjVl

150 ~

~-100 ~J::

~.;:::;.!!?:::J

E:::JU

9080o 10 20 30 40 50 60 70Time: h

(d)

16

6 14J::

~ 12

glOQj~ 8OJ

~ 6­'"l! 415~

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150 Si'"OJ

~-100 ~.J::OJ>.;:::;

.!!?50 :::J

E:::JU

200

10 20 30 40

f\.~./......2 .•

~~~~~o50 60 70 80 90 100 110Time: h

(c)

16

15~roce 0

o

Figure 3. Isothermal conduction calorimetry of slags activated

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

(d) GGBFS

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

hallll was e!Tective only in the case of FA30 and FA38. Similarly

to NS, the peak in NC-activated samples occurred at a later time

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

hasicity, higher polymerisation in the glass matrix of thc slag).

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

giving a hydration curve starting with a double peak before the

i11lhll:tion period and one peak atter the induction period (Shi et

al.. 200(,), this was not apparcnt in the current study.

Activation with CH gm'e no peaks of hcat release and resulted in

comparatively low cumulative heats for all samples tcsted (Figure

3). This n:lkets the slower hydration kinetics and is attributed

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

acti\'ation. which suhsequently defines a slower dissolution rate

fix the slag (l3ellmann and Stark. 2(09).

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 withsodium hydroxideThe 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 28 region of 25-38° slightly

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

diffuse peak at about 28 = 29S 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 Mg6AI2C03(OH)16.4H20, JCPDS­

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

Ca.1AI2(OHh2, 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

24

Advances in Cement ResearchVolume 25 Issue 1

On a new hydraulic binder from stainlesssteel converter slagPontikes. Kriskova. Cizer. Jones and Blanpaln

4 Hydrotalcite1 ZnO1 2 CaC03 5 Hydrogarnet

1.31

3 C-S-H90 days

2 3\ ! 1 1 1 155 4 :~; 5

5-.J..1-...,J.,J----JL:...t-.,::_i~ "

28 days A J ~7 days J".- IM! 1 LL-.h3 days

A J ~lI.Original ........ k I ~

C:::J

:~

4 C-S-H5 Hydrotaleite6 Hydrogarnet

1 ZnO2 Mervvinite3 CaC03

28 days

665

3 days

Original

7 days

90 days

20 30 40 50 60 7020: degrees

(b)

1 ZnO 4 Hydrotalcite2 CaC03 5 Mervvinite

.~3 C-S-H

c1:::J 1 1 1c

J~L__L\j.~:e .JAro

i' 7 days'VicOJ 3 days....E

Original

20 3D 40 50 60 7020: degrees

(d)

70

70

60

60

40 5020: degrees

(a)

40 5020: degrees

(e)

30

30

20

20

90 days

1 1 ZnO 4 Hydrotaleite2 CaC03 5 Hydrogarnet

1,3 1 3C-S-H

2.3 1 1 1 1

[ ~~~ ~~,..Jl---,J.._..Jl2J-,-:0 28 days I I I lro ~I '"-'I''''~-'-'~l-.J'---.JI.A.

7 days '" I I I 11:-'--"' ...... I~---.-.-.J~'---"-"Jo...3 days ~-1 :. ~ it· ~Original ~_ ~

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

to the slightly different chemistry of the synthetic slags and in

particular the higher iron content (Table I). The main peak ofcalcite calcium carbonate (CaC03 ) at 2fJ = 29·4° (JCPDS-ICDD #

5-586) is very close to the C-S-H region and most probably calcite

is 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 thefirst 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 85-105°C 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 atapproximately 135°C, an indication of AFm type phases (e.g.

C4AH I3; see Taylor (1990». Additional peaks are clearly ob-

served in the DTG curves between approximately 250 and 380°C.

Most probably, the peak at approximately 260°C is due to thedecomposition of hydrogarnet (Passaglia and Rinaldi, 1984;

Rivas-Mercury et aI., 2008) and the main peak between 300°C

and 335°C is due to the decomposition of hydrotalcite (Hickey el

at., 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 comparablewith 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 3and 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 forthe rest of the period studied. The total weight loss after 28 days

was 24·8 wt%. Sample FA38 reacted similarly to FA22 during the

25

Advances in Cement ResearchVolume 25 Issue 1

On a new hydraulic binder from stainlesssteel converter slagPontikes, Kriskova, Cizer, Jones and Blanpain

u°QJc-Ol

~E

19fo­o

1009896·9492908886848280/87674

3 days

7 days

28 days

3 days

7 daY5

28 day5

200 400 600 800 1000Temperature: "C

uo

n;c­O>

-E0>

E

~o

3 days

7 days

28 days

3 days

7 days

.. _, __~_~~~I~,200 400 600 800 1000

Temperature: O(

~n;c­O>

~E19fo­o

200 400 600 800 1000

Temperature: °c(a)

3 days

7 days

28 days

10098go

9492

~~ 3 days8684828078 28 days

/6 L74~~_~

200 400 600 800 1000Temperature: °C

200 400 600 800 1000

Temperature: DC(e)

200 400 600 800 1000Temperature: °C

(b)

10098959492

'if. 90E 88~ 86~ 84

8280787674"-- -_

200 400 600 800 1000Temperature: O(

200 400 600 800 1000Temperature: DC

(d)

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

tirst J days. Atlcr thc third day, the reactions slowed down. Thewcight loss bctwcen day 3 and day 7 of hydration was almost as

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

atler 28 days of hydration was 24·3 wt%.

Figure (, presents the FTIR spectra of all the slags before and after

90 days of hydration. The band at 500 em -I is assigned to T-0-T

or 0-T-0 (T = Si. AI) bcnding vibration (McMillan, 2001)

whereas th.: band at 700 em -I is assign.:d to symmetric stretchingvibration of Si-O-T bonds (e.g. Peehar and RykL 1983). Themain band at 950 em J is an assemblage of symmetric Si-Ostr.:tehing vihrations of tctrahcdral silicate fomls with one, two.

26

three and four non-bridged oxygen atoms per silicon atom (NBO/Si) (McMillan, 200 I). The band at 1415-1430 cm- 1 is assigned to

C-O stretching vibration (e.g. Tatzber et aI., 2007) whereas peaksat about 3450 cm- J and 1640 cm- I are assigned to O-H stretch­

ing and H-O-H bending respectively (e.g. Pechar and Ryk1,

1983). After 90 days, the main broad peak at 950 cm- I became

significantly smaller and narrower, whereas new peaks appeared orwere different in their intensities. In detail, the typical bands of

C-S-H gel are detected as peaks appearing

• at 950-970 cm- I , corresponding to the Si-O asymmetricstretching bands in Q2 units

Advances in Cement ResearchVolume 25 Issue 1

On a new hydraulic binder from stainlesssteel converter slagPontikes, Kriskova, eizer, Jones and Blanpain

:~c:::J

~

~i'imQjuc.~EVlc

'"~

3500/

1500

(a)

1000 500 3500

90 days

Original

1500

(b)

1000 500

90~II,,,,

i

3500 1500 1000

Wave number: cm- 1

(c)

500 3500 1500 1000

Wave number: cm- 1

(d)

500

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

27

Advances in Cement ResearchVolume 25 Issue 1

• at about 815 -820 cm-I, ascribed to the Si-0 symmetric

vibrations in QI units

• in the region of 500-400 cm ~ 1, notably new peaks at490 cm- I , 450 em-I and 422 em-I, which are associated

with vibrations of the O-Si-0 bonds (Mozgawa and Deja,

2009; Ping et al., 1999; Puertas and Fernandez-Jimenez,2003).

The peak at 670 cm- I is associated with the vibrations ofSi-O-AI

bridges (Mozgawa and Deja, 2009). Both characteristic peaks atabout 3450 em-I and 1640cm- l , assigned to O-H and H-O-H

vibrations respectively, have higher intensity in the hydrated

samples. The presence of carbonate groups [C03f- is evidenced

by peaks at about 1433 cm- I . This is in agreement with the XRDand TGA data supporting the presence of hydrotalcite and CaC03 .

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 particlesarc 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 morph­ology 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

arc most probably AFm phases (Wang and Scrivener, 1995,20(3). This is in line with the DTG results (Figure 5) and thediscrete peaks detected at approximately 135°C. At 28 days of

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

particles of less than 1 flI11 and an extensive cohesive networkwas 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 elsewherefor blast-furnace slag finely milled and after 28 days of hydration(Kumar ef aI., 2(05). At 90 days of hydration (Figure 7(d)), theplate-like AFm crystals were well intercalated into the dense

matrix. Regarding hydrotalcite, the characteristic platelets were

not detected in the hydrated microstructurc, probably as the result

of the small size (e.g. Ben Haha ef al., 20 II b). On the contrary,

hydrogarnet could be distinguished more easily due to thecharacteristic trapezohedral crystals (Figure 7(e».

Behaviour in blended cements with OPC and comparisonwith GGBFS

To further compare the hydraulic potential of the slags developed

in this work with currently produced GGBFS, blended cementsWeTe de\'eloped with ope and the slags or the reference GGBFS.

Two aspects were c\'aluated - the hydration behaviour by means

of isothermal calorimctry and the mechanical properties.

28

On a new hydraulic binder from stainlesssteel converter slagPontlkes, Kriskova, Cizer, Jones and Blanpain

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 thanseparate 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. Theblends 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 Jig. Interestingly, the cement blend with FA22

evolved heat faster, having its peak between 15 hand 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 thereference 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, yetat 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 implementation

The presented results demonstrate that production of a 'replica'

blast-furnace slag may be a viable option for the valorisation ofsecondary steelmaking slags. The required processing would take

place after slag-metal separation and some recent papers

(Engstrom et al., 20 II; Pontikes et at., 20 I I) 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 or

alkali-activated). Industrial implementation would probably re­

quire substantial investment and secured access to secondary

resources. It is thus likely that local conditions will evenmallydictate how industrially realistic such a process is and whetherindustrial symbiosis could occur.

Conclusions• 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.

• The amount of crystalline phase decreased when the amountof FA increased and, for an addition of 30 wt% FA and above,

the resulting material was almost completely amorphous.

• Activation with sodium hydroxide typically resulted incomparatively fast and substantial heat release. Only the FA30

sample could be activated with all three alkali activators.

Advances in Cement ResearchVolume 25 Issue 1

On a new hydraulic binder from stainlesssteel converter slagPontikes, Knskova, eizer, Jones and Blanpain

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

• The slags activated with sodium hydroxide resulted in thc

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

phases as the main hydration products.

The calorimetric behaviour and mechanical properties of

blended cements with FA30 and FA38 were very similar to a

blended cement with GGBFS.

Production of such by way of 11ot-s'tagg proc:es:sing

could upgradc the currcntly pro,dw;cd scc:ond,u'y

29

Advances in Cement ResearchVolume 25 Issue 1

On a new hydraulic binder from stainlesssteel converter slagPontikes, Kriskova, eizer, Jones and Blanpain

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

and 30 wt% GGBFS, FAn, FA30 or FA38

Figure 10. Compressive strength of blended mortars with

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

AcknowledgementsThe authors gratefully acknowledge IWT 0&0 project 090594

for financial support. Y. Pontikes and 6. Cizer thank the FWO for

post-doctoral fellowships.

steelmaking slags with regard to their applications and

deliver a new binder, engineered specifically for particular

applications.

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.....ro<lJ

-'=

.~.....ro

:5E:::J

U

160

140 ~

120 ~ro<lJ

100 ~

10080

-.- OPC-.- 0·7 OPC + 0·3 GGBFS-e- 0·7 ope + 0·3 FAn- .. - 0·7 ope + 0·3 FA30- ... - 0·7 ope + 0·3 FA38

40 60Time: h

-'<- ope

-.- 0·7 OPC + 0·3 GGBFS-e- 0·7 OPC + 0·3 FAn-A- 0·7 ope + 0·3 FA30- ... - 0·7 ope + 0·3 FA38

20

10 20 30 40 50 60 70 80 90Hydration time: days

O+-~---.--~--r-~-,.-~-,--~--+

o

12

11

10

~ 9:2: 8-5 701CE:! 6~ 5ro

~ 4<lJ

u::: 3

21O+-~.--~-.---~-.---~,---,---,---~.,---~,.-~,--,,------,,------,

o

50

-'=0,c 30

~<lJ>~ 20i:l'Q.Eou 10

6

O+-~"""---'-~----.~--.-~,---~r-.--,~---,-~-.--~

o 10 20 30 40 50 60 70 80 90Hydration time: days

:J

i! 5QjQ.

~ 4<lJVl

Q'l 3~

~ 2-'='0~ro

0::

Figure 8. Rate and cumulative heat release from isothermal

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30

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