Advances in Cement Research, 2011, 23(6), 265–275

http://dx.doi.org/10.1680/adcr.2011.23.6.265

Paper 900041

Received 26/09/2009; revised 09/08/2010; accepted 04/11/2010

Thomas Telford Ltd & 2011

Advances in Cement ResearchVolume 23 Issue 6

Quantification of hydration phases insupersulfated cements: review and newapproachesGruskovnjak, Lothenbach, Winnefeld et al.

Quantification of hydrationphases in supersulfated cements:review and new approachesAstrid GruskovnjakEmpa, Swiss Federal Laboratories for Materials Science and Technology,Department for Concrete and Construction Chemistry, Dubendorf,Switzerland

Barbara LothenbachEmpa, Swiss Federal Laboratories for Materials Science and Technology,Department for Concrete and Construction Chemistry, Dubendorf,Switzerland

Frank WinnefeldEmpa, Swiss Federal Laboratories for Materials Science and Technology,Department for Concrete and Construction Chemistry, Dubendorf,Switzerland

Beat MunchEmpa, Swiss Federal Laboratories for Materials Science and Technology,Department for Concrete and Construction Chemistry, Dubendorf,Switzerland

Renato FigiEmpa, Swiss Federal Laboratories for Materials Science and Technology,Laboratory for Analytical Chemistry, Dubendorf, Switzerland

Suz-Chung KoHolcim Group Support Ltd, Product Innovation and Support, Holderbank,Switzerland

Michael AdlerHolcim Group Support Ltd, Product Innovation and Support, Holderbank,Switzerland

Urs MaderUniversity of Bern, Institute of Geological Sciences, Rock-Water-InteractionGroup, Bern, Switzerland

Quantification of the progress of hydration of supersulfated cements (SSC) may be approached in two ways: (a) from

recording the increasing dissolution of the slag particles directly, and (b) indirectly from quantifying the formation of

the hydration phases. Image analysis based on backscattered electron imaging in a scanning electron microscope

(SEM), the dissolution of hydrates (EDTA), differential thermal analysis (DTA) and sulfide concentration (SP) were

used to quantify the dissolution of the slag particles; selective extraction of hydrates by sodium carbonate (SE), X-ray

diffraction (XRD) with Rietveld analysis and thermogravimetric (TGA) refinement methods were used to quantify the

amount of hydration products formed. SEM-based image analysis was found to be a direct and promising way for

the quantification of slag particles. With the help of selective extraction by sodium carbonate (SE), it was possible to

quantify the amorphous C–S–H phase in SSC. Mass balance calculations constrained by thermodynamic stability were

used to calculate the amount of reacted slag in the system. XRD Rietveld and TGA methods were used to assess the

amounts of specific hydration products formed in SSC but did not allow an absolute quantification of the amount of

slag reacted. Other methods such as the dissolution of the hydrates by EDTA and DTA were not found to be reliable

due to intrinsic problems.

IntroductionThe activation of ground granulated blast furnace slag (GGBFS)

by sulfates was first described by Kuhl in 1909 (Kuhl and

Schleicher, 1952). Generally, the supersulfated cements (SSC)

consist of 80–85 wt% of slag mixed with 10-15 wt% of

anhydrite and an alkaline activator such as Portland cement

clinker (Taylor, 1997). SSC shows an increased resistance to

sulfate attack and exhibits a lower heat of hydration compared to

ordinary Portland cement (OPC). The main hydration products

are C–S–H phase, ettringite, gypsum and some hydrotalcite can

be observed as well.

Quantification of phases in cementitious systems is mainly

focused on raw materials (Taylor, 1997). There are some common

methods for the quantification of the phase composition in

clinker: quantitative X-ray diffraction analysis by Rietveld refine-

ment (Walenta and Fullmann, 2004), optical microscopy using

point counting, the Bogue method and chemical or physical

methods for the separation of phases (Taylor, 1997).

For a better understanding of the hydration processes in cementi-

tious materials, it is important to quantify the amount of

hydration products formed as a function of time and thus to

determine the degree of hydration. In OPC systems, the progress

of hydration is often monitored by X-ray diffraction (XRD),

measuring directly the consumption of the clinker phases and the

formation of hydrates. In slag systems, the glassy part of the slag

as well as the main hydration product (C–S–H) are XRD-

amorphous, which makes the quantification of the hydration

progress a difficult task.

265

Thus, there is a lack of reliable and efficient methods for the

quantification of the phases in hydrated slag cements, even

though several methods have been applied in a number of studies:

selective dissolution by EDTA for blended cements was briefly

reviewed by Lumley et al. (1996); for supersulfated cements

selective dissolution by EDTA has been used by Matschei et al.

(2005) and Stark et al. (1990). Additionally, the exothermic

reaction from the devitrification process was measured (DTA)

and used to estimate the degree of reaction of slag (Rosina et al.,

1966; Schneider and Meng, 2000; Schramli, 1963).

In the present study, the methods used for slag cement systems

mentioned above were reviewed and compared with each other in

order to determine the degree of reaction in supersulfated cement.

In addition, backscattered electron imaging, Rietveld refinement

of XRD patterns and thermogravimetric analysis have been used

for the quantification of hydration in supersulfated cements.

Refinements of the thermogravimetric analysis by manual and

mathematical fitting, as well as a new selective extraction method

using sodium carbonate and a method based on the sulfide

concentration in the pore solution have been developed and

applied to supersulfated cements.

Not all examined methods have provided useful results for

obtaining an estimation of the amount of formed hydration

products, but the combination of some of them is suitable.

Experimental work

Materials

The experiments were carried out with a SSC composed of

85 wt% of a highly reactive (HR) ground GBFS and 15 wt%

natural anhydrite, activated additionally by 0.5 wt% KOH. A

water/solid (w/s) ratio of 0.315 was applied. The hydration of the

pastes was stopped with isopropanol and the powder was dried

for 3 days at 408C.

Pure hydrate phases were used as reference materials. C–S–H

with different C/S ratios was synthesised by mixing Ca(OH)2

(puriss.; Riedel-de Haen) and silica fume (Aerosil 200) with

water, followed by 1 week of rest and subsequent drying for 3

days at 408C.

C=S ratio of 1:0 > 5:6 g Ca(OH)2

þ 6:0 g SiO2 þ 30 ml H2O

C=S ratio of 1:4 > 3:9 g Ca(OH)2

þ 3:0 g SiO2 þ 20 ml H2O

For the production of Al-ettringite, stoichiometric amounts of

CaO and Al2(SO4)3:16H2O (purum p.a.; Fluka) were mixed with

water. CaO was obtained from burning Ca(OH)2 in a laboratory

furnace overnight (, 14 h) at 10008C. After 1 week reaction

time, the ettringite was dried for 3 days at 408C.

For the synthesis of hydrotalcite, the procedure described in

Reichle (1986) was followed. A detailed description can be found

in Johnson and Glasser (2003).

Commercial pure gypsum was used as reference material

(calcium sulfate dihydrate, Fluka, purum p.a.)

Methods

Measurement parameters

The chemical composition of the raw materials used in the SSC

was analysed by X-ray fluorescence analysis (XRF) using a

Philips PW 2400 instrument.

The mineralogical composition of the samples was determined

with XRD by using a Panalytical X’Pert Pro powder diffract-

ometer equipped with an X’Celerator detector on powders ground

to , 63 �m; samples used for Rietveld analysis had been ground

to , 40 �m. Measuring conditions: 40 kV; 40 mA; 2Ł angles 5 to

808; step size: 0.0178; time/step, 20 s; scan speed, 0.18/s; time,

11.5 min/sample. The software applied for the Rietveld refine-

ment method was X’Pert HighScore Plus V. 2.0a from PANaly-

tical. The single crystal structures of the crystalline phases were

taken from the Inorganic Crystal Structure Database (ICSD,

2006).

TGA analyses were performed in alox (Al2O3) cups under N2

atmosphere with powdered samples ground to , 63 �m at a

heating rate of 5 K/min up to the desired temperature (Mettler

Toledo TGA/SDTA 581).

DTA measurements were performed under Argon atmosphere

with powdered samples ground to , 63 �m at a heating rate of

20 K/min up to 11008C (TGA Netzsch STA 409 CD). Alox

(Al2O3) was used as reference material.

Pore fluid of the hardened samples was extracted using the steel

die method (Barneyback and Diamond, 1981) with pressures up

to 530 N/mm2: The solution was stabilised and measured accord-

ing to the procedure described in Gruskovnjak et al. (2008).

Polished and carbon-coated samples were examined by scanning

electron microscopy (SEM) (Philips ESEM FEG XL 30) using

backscattered electron images (BSE) with SEM settings at low

voltage (5 kV) and 100 �m aperture.

Thermodynamic modelling

The Gibbs free energy minimisation program GEMS (Kulik,

2005) was used to calculate the equilibrium speciation of

dissolved species, as well as to predict the kind and amount of

solids precipitating as a function of the amount of dissolved slag.

A description of the basic assumptions and the modelling set-up

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Quantification of hydration phases insupersulfated cements: review and newapproachesGruskovnjak, Lothenbach, Winnefeld et al.

can be found in Lothenbach and Gruskovnjak (2007). The

modelling results are used as a thermodynamically constrained

mass balance approach for the progress of hydration. The solids

precipitated have been calculated as a function of percentage of

hydrated slag.

Quantification methods

Image analysis (SEM method). The same method of backscattered

image analyses based on different grey scale levels was used in

this study as described in Scrivener (2004), but adapted for SSC

pastes in order to identify the amount of anhydrous slag particles.

The magnification used in this study: 10003 and 20003.

Thermogravimetric analysis (TGA method). When analysed with

TGA, overlapping signals occur for the hydration phases C–S–H

and ettringite. Thus, TGA and differential thermal analysis

(DTG) signals were quantified by using reference materials

(Figure 1), similar to the full pattern quantification in XRD.

Synthesised and pure phases were used as reference materials

(see section on ‘Experimental work – Materials’). The TGA and

DTG signals of the reference materials were combined in various

proportions, and these were adjusted until the resulting curve

matched the measured curve of the hydrated SSC samples at the

best. Matching was performed: (a) manually and (b) mathemati-

cally. The mathematical method is based on a Matlab program,

which identifies the local minima in an n-dimensional space

(n ¼ numbers of phases) with the help of the Nelder–Mead

algorithm (Nelder and Mead, 1965). The minimisation criterion

was the mean square difference of the target TGA signal and the

superposed synthetic function.

Selective extraction (SE) method with sodium carbonate. This

method was used previously for the identification of thaumasite in

the presence of ettringite in OPC systems. Van Aardt and Visser

(1975) mentioned a treatment with a 5% sodium carbonate

solution, which decomposed ettringite and left thaumasite un-

changed but the authors gave no further details such as reaction

time used, ratio cement/solution or the fineness of the sample.

This method was applied in this study for the isolation of the C–

S–H phase in supersulfated slag systems (overlapping signals with

ettringite in TGA). It was optimized for this study as follows: 5 g

of the sample (ground to , 40 �m) was stirred in 100 ml of a 5%

sodium carbonate solution for 10 min. The residue was filtered,

stopped with distilled water and dried for 3 days at 408C. The

treated and untreated samples have then been analysed with TGA.

X-ray diffraction analysis (XRD method). Rietveld refinement of

the XRD patterns was first applied to quantify clinker and

unhydrated cements (Rietveld, 1969). Scrivener et al. (2004)

extended the technique to hydrated samples of a typical Portland

cement.

For SSC samples, an internal standard (10 wt% rutile) was added

to determine the mass fraction of the amorphous component. The

amorphous content is composed of the slag particles, the C–S–H

phase and possibly a hydrotalcite-like phase.

Sulfide concentration (SP method). The measured sulfide concen-

trations after 8 h, 1, 7 and 28 days were compared with the

calculated concentrations from the thermodynamic modelling and

used as indicators of the progress of slag dissolution (Gruskovn-

jak et al., 2006). Sulfide is present exclusively in the slag and is

released as the slag dissolves. Small amounts oxidise to thiosul-

fate (S2O32�) and sulfite (SO3

2�). The main fraction, however,

remains in solution and its concentration increases as hydration

proceeds (Gruskovnjak et al., 2008).

7306305304303302301300

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Hydrotalcite

C–S–H (1:1)

Hydrotalcite

Ettringite

Ettringite

C–S–H (1:1)

Figure 1. TG and DTG curves of the reference materials (C–S–H,

ettringite and hydrotalcite) are shown. The hydrotalcite

Mg4Al2(OH)14:4H2O exhibits a two-stage mass loss pattern

267

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Quantification of hydration phases insupersulfated cements: review and newapproachesGruskovnjak, Lothenbach, Winnefeld et al.

Selective dissolution by EDTA (EDTA method). EDTA extraction

(Erntroy, 1987) is a method for the determination of slag content

in cements by selective dissolution, which is based on the fast

dissolution of the hydration products (C–S–H, ettringite) and

calcium sulfates. The set-up of the experiment is fully described

in Lumley et al. (1996). The calculations were carried out as

described in Stark et al. (1990).

Differential thermal analysis (DTA method). This method is based

on the principle of recrystallisation of the remaining slag

(Schneider and Meng, 2000) in unhydrated and hydrated samples

during heating. The devitrification process liberates heat, which

can be used for the quantification of the unhydrated slag compo-

nents (Lommatzsch, 1956). For the calculation, the unhydrated

sample is used as reference and compared with the hydrated

samples.

Corrections and assumptions

The hydration products analysed by TGA, XRD and SE method

have been corrected for the loss on ignition taken from TG

analyses; ettringite calculated from TGA was additionally cor-

rected for hydrotalcite (Mg4Al2(CO3)(OH)12:4H2O) as the two

peaks overlap to a certain extent. Thermogravimetric analysis of

the hydrotalcite shows a two-stage mass loss pattern (Figure 1).

The first mass loss is due to the dehydration of water molecules

in the interlayer, while the second mass loss is attributed to the

dehydroxylation of sheets (Kannan, 2004). Corrections have been

performed by the weight loss of the hydrotalcite phase between

30 and 2708C which was subtracted from the weight loss of the

C–S–H phase in the same range.

The amount of reacted slag can be calculated from the corrected

amount of formed ettringite assuming that (a) the slag dissolves

congruently, (b) all magnesium is used for the formation of

hydrotalcite, and (c) the remaining aluminium is completely

incorporated into the hydration phase ettringite (it is assumed that

Al incorporated into the C–S–H phase in SSC is negligible). The

amount of reacted slag can also be calculated from the corrected

amount of C–S–H phase formed assuming that (a) the slag

dissolves congruently and (b) all Si is used for the formation of

C–S–H.

For the selective extraction method, the calculations for the

amount of C–S–H phase are based on the assumptions that the

C/S ratio is 1 and that after the treatment 1.1 H2O per Si is

present.

Results and discussion

Raw material

The chemical composition of the HR-slag and the natural

anhydrite used in the experiments is given in Table 1. The

negative loss of ignition of the slag is due to the oxidation of

sulfide. XRD and TGA analyses have shown that the slag consists

of a glassy phase with no detectable crystalline components, and

that the natural anhydrite contains significant amounts of gypsum

(24 wt%), calcite (12 wt%) and dolomite (3 wt%).

Hydration products

During the hydration of SSC the formation of C–S–H, ettringite

and hydrotalcite is observed (Figures 2 and 3). Calcite, dolomite,

anhydrite and gypsum originating from the natural anhydrite

added are identified by XRD (Figure 3). The amount of ettringite

and C–S–H increases and in correspondence the amount of

gypsum decreases with hydration time. The amount of calcite and

dolomite (Figure 2) is rather constant during the first 28 days.

The changes in the hydrate assemblage during hydration were

also calculated by thermodynamic modelling (Figure 4). The

calculation indicates the formation of increasing amounts of

ettringite and C–S–H phase, minor amounts of hydrotalcite,

decreasing amounts of gypsum and traces of syngenite and FeS

during the hydration of the slag.

Quantification

A number of different quantification methods were applied to

determine the degree of reaction. The SEM, EDTA and the DTA

methods determine the reacted amount of slag while the other

methods (TGA, SE, XRD, SP) determine the amount of slag

indirectly using the quantity of hydrates formed or sulfide

released combined with mass balance considerations.

Image analysis (SEM method)

The amount of unreacted slag as determined by image analysis

shows a steady decrease as a function of time (Table 2). With

image analysis the amount of slag present is determined directly;

the anhydrous slag can be distinguished from the hydration

products by its grey level as the average atomic number of the

hydration products and the anhydrous material differs sufficiently

HR-slag: wt% Natural anhydrite: wt%

LOI �3.1 12.0

SiO2 32.5 3.6

Al2O3 11.6 0.9

Fe2O3 3.6 0.4

CaO 44.0 34.4

MgO 5.0 4.2

SO3 4.5 45.5

K2O 0.3 0.3

Na2O 0.2 0.1

TiO2 0.8 0.0

Mn2O3 0.1 0.0

P2O5 , 0.01 0.0

Cl , 0.01 n.a

Total 99.6 101.4

Table 1. Element oxides in wt% from XRF data for unhydrated

HR-slag and the natural anhydrite used for the SSC mixtures

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Quantification of hydration phases insupersulfated cements: review and newapproachesGruskovnjak, Lothenbach, Winnefeld et al.

(Figure 5). Nevertheless, care has to be taken that the relatively

small difference in density between the slag particles and

anhydrite does not result in an overestimation of the non-reacted

slag content.

A relatively large error results already from a small variation in

the percentage of slag particles determined by BSE imaging and

thus can have a significant impact on the calculated amount of

the reacted slag.

The initial volume of the present slag in an unhydrated SSC was

calculated from the known amount of the phases present (slag,

anhydrite, gypsum and calcite/dolomite), their density, and the

w/s ratio.

Thermogravimetric analysis (TGA method)

The two TGA results, the manually matched (man.) and the

mathematically fitted (math.) ones indicate that during hydration

increasing amounts of ettringite and C–S–H are formed (Table 3).

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Gypsum

C–S–H

Hydrotalcite Calcite

HR-slag_unhydrated

HR-SSC_1d

HR-SSC_7d

HR-SSC_28d

Dolomite

Figure 2. TG and DTG curves of unhydrated HR-slag and HR-SSC

hydrated for 1, 7 and 28 days

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HR-SSC_28d

E

GHt E

E E E

A

G

E AAE

DG

GCG

DA

Figure 3. XRD analyses of HR-SSC hydrated for 1, 7 and 28 days.

E, ettringite; G, gypsum; Ht, hydrotalcite; A, anhydrite; C, calcite;

D, dolomite

Unhydrated slag:

cm2/cm2

Slag reacted:

%

HR-SSC_unhydrated 44 0

HR-SSC_1d 34 � 2 23 � 4

HR-SSC_7d 28 � 2 36 � 4

HR-SSC_28d 24 � 2 46 � 4

Table 2. The amount of unhydrated slag particles from SEM

imaging and the calculated amount of slag reacted after 1, 7 and

28 days of hydration

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g/85

g s

lag

Unhydrate slag

Pore solutionSyngenite

EttringiteHydrotalcite

FeSC–S–H

Gypsum

Figure 4. The amount of pore solution and solids precipitated as a

function of the amount of slag (85 wt%) dissolved based on

thermodynamically constrained mass balance calculations

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Quantification of hydration phases insupersulfated cements: review and newapproachesGruskovnjak, Lothenbach, Winnefeld et al.

The mathematically matched TGA results yield lower amounts of

ettringite, but higher amounts of gypsum and C–S–H phase than

the manually matched.

The obtained quantities of ettringite and C–S–H have been used,

after corrections (see subsection on ‘Corrections and assump-

tions’), to calculate the amount of slag reacted (Table 4). An

increasing reaction of the slag is observed. The amount of slag

reacted is in the case of C–S–H similar to the results obtained by

SEM, while the amount of slag reacted based on ettringite is, at

longer hydration times, considerably smaller (Table 4).

The TGA method can be associated with significant systematic

errors as the presence of four different overlapping hydrates

(ettringite, C–S–H, gypsum and hydrotalcite) observed between

30 and 2008C makes the deconvolution difficult. In contrast to the

XRD patterns there are not several peaks of one phase present,

which could be used for a more precise refinement.

Selective extraction (SE) by sodium carbonate

The SE method uses again TGA to quantify the amount of C–S–H

formed but uses a selective extraction by sodium carbonate to

completely dissolve ettringite and gypsum leaving C–S–H phase

and hydrotalcite-like phases unchanged as verified by XRD and

TGA, while some additional calcite precipitates (Figures 6(a)–6(c)

and 7(a)–7(c)). The selective extraction enables a more reliable

quantification of the C–S–H phase of SSC, as the TGA signal of

C–S–H does not overlap with ettringite and gypsum any more

although the TGA still has to be corrected for the amount of

hydrotalcite present (Table 4). The results of the SE method are

comparable with the SEM method. However, the data may be

associated with a systematic error, as the incorporated water of the

C–S–H may be sensitive to (a) the C/S ratio of the C–S–H

(Figure 1), (b) the replacement of Si by Al in C–S–H and (c) the

drying method.

Slag particles

Anhydrite

Anhydrite

Figure 5. BSE image of HR-SSC after 1 day of hydration. The

bright particles are slag and anhydrite; the grey area belongs to

the hydration phases and the black area to the pores

Phases HR-SSC_1d HR-SSC_7d HR-SSC_28d

man. /math.: wt% man. /math.: wt% man. /math.: wt%

Ettringite 13/12 � 1 15/11 � 1 17/10 � 1

Gypsum 2/3 � 0.5 2/6 � 0.5 2/7 � 0.5

C–S–H 13/14 � 3 23/29 � 3 25/36 � 3

Table 3. The amount of hydration products (ettringite, C–S–H

and gypsum) from TG and DTG curves in wt% obtained by the

manual (man.) and mathematical (math.) method after 1, 7 and

28 days of hydration

TGA-method TGA-method SE-method SE-method

slag reacted (from

ettringite): wt%

slag reacted (from C–S–H):

wt%

amount of C–S–H:

wt%

slag reacted (from C–S–H):

wt%

HR-SSC_1d 21 � 2 23 � 5 18 � 3 27 � 5

HR-SSC_7d 25 � 2 41 � 6 28 � 3 41 � 6

HR-SSC_28d 28 � 2 45 � 6 34 � 3 52 � 6

Table 4. The results only from the manual method were

corrected and the percentage of slag reacted from the amount

of ettringite and C–S–H was calculated. The corrected amount

of C–S–H in wt% from the SE method was used for the

calculation of the percentage of slag reacted

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X-ray diffraction analysis (XRD method)

The amount of slag reacted is calculated based on the amount of

ettringite determined by XRD, as described above for the TGA

method. The amount of unreacted slag cannot be determined

directly by XRD as a distinction between amorphous C–S–H

phases and the glassy phase of the slag is not possible.

The amount of slag reacted calculated on the basis of the amount

of ettringite formed can be found in Table 5. The amount of

ettringte and of slag reacted corresponds with the results of the

TGA (ettringite) method and at longer reaction times it is

considerably lower than the SEM or SE results.

Sulfide concentration (SP method)

The measured sulfide concentrations (Table 6) can be used to

calculate the amount of slag reacted as only the slag contains

sulfide. The comparison of the measured concentrations with the

modelled concentrations in Figure 8 (dotted vertical lines) shows

that after 1 day of hydration, already 35 wt% of slag has reacted

(Table 6). The further reaction of the slag seems to proceed only

slowly.

However, there are some uncertainties associated with the SP

method: (a) it is not clear whether the sulfide incorporated in the

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Hydrotalcite

Figure 6(a)–(c). TG and DTG curves of HR-SSC hydrated for 1, 7

and 28 days before (HR-SSC) and after (HR-SSC_SE) the selective

extraction with 5% sodium carbonate solution

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1500

2000

2500

HR-SSC_28d

HR-SSC_28d_SE

C

C

DC C C C C

A

E

GHt E

EE

EE

GDAE E

A

A

Figure 7(a)–(c). XRD analyses of HR-SSC hydrated for 1, 7 and

28 days before (HR-SSC) and after (HR-SSC_SE) the selective

extraction with 5% sodium carbonate solution. E, ettringite;

G, gypsum; Ht, hydrotalcite; A, anhydrite; C, calcite; D, dolomite

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Quantification of hydration phases insupersulfated cements: review and newapproachesGruskovnjak, Lothenbach, Winnefeld et al.

glassy phase of the slag dissolves homogeneously, (b) the determi-

nation of aqueous sulfide is associated with a measurement

uncertainty of 20%, (c) the dissolved sulfide may oxidise depend-

ing on the quantity of oxygen present and on the kinetic of the

oxidation reaction. As stated by different authors (Chen and

Morris, 1972; Fischer et al., 1997; O’Brien and Birkner, 1977) the

oxidation of the intermediate products thiosulfate (S2O32�) and

sulfite (SO32�) are very slow under alkaline conditions. In addition,

solid (AFm) phases may act as a sink for sulfides (Vernet, 1982).

Selective dissolution by EDTA (EDTA method)

The EDTA method aims to selectively dissolve all hydrates and

activators without dissolving the unhydrated slag. TGA/DTG

analyses of the residues after EDTA extraction (Figure 9) indicate

a complete dissolution of the hydration products ettringite,

gypsum and C–S–H. Hydrotalcite, dolomite and calcite, how-

ever, are not dissolved or only to a small extent. In addition,

small amounts of a non-identified hydrate can be observed in the

TGA between 30 and 2308C. While the literature generally just

mentions the dissolution of hydration phases, this study demon-

strates that a large amount of the unhydrated HR-slag dissolves in

the EDTA-solution, which leads to an erroneous quantification of

the amount of slag that has hydrated; 13 wt% (0.063 g of 0.5 g

sample) of the unhydrated slag (without activator) is dissolved

(Table 7). Omitting leads to an overestimation of the amount of

slag reacted at early ages but may be more accurate at later ages

as the fraction of the slag that dissolves during the selective

dissolution may have reacted during hydration. In addition,

corrections for the presence of hydrotalcite (which does not

dissolve in the EDTA extract), lead to another set of values, still

significantly lower than the values estimated by SEM or by SE.

Although the reproducibility of the EDTA method was very good,

the largely different results obtained based on different assump-

tions and the presence of hydrates after the selective dissolution

indicate that this is not an adequate method to determine the

amount of slag reacted.

Differential thermal analysis (DTA method)

The exothermic reaction during devitrification can theoretically

be used to quantify the amount of unreacted slag (Figure 10).

Phases HR-SSC_1d: wt% HR-SSC_7d: wt% HR-SSC_28d: wt%

Amorphous 70.5 � 7 68.6 � 7 68.0 � 7

Ettringite 14.8 � 2 18.0 � 2 19.6 � 2

Anhydrite 5.5 � 1 5.4 � 1 6.5 � 1

Gypsum 7.1 � 1 4.5 � 1 3.0 � 0.5

Calcite 0.0 � 0.5 0.8 � 0.5 0.5 � 0.5

Dolomite 2.0 � 0.5 2.5 � 0.5 2.4 � 0.5

Sum 99.9 99.8 100.0

Slag reacted

(from

ettringite)

24 � 3 30 � 3 33 � 3

Table 5. The results from XRD-Rietveld calculations for each phase

are shown. The amorphous phase is mainly composed of the

glassy phase of the slag, C–S–H and probably some hydrotalcite.

The amount of ettringite was corrected and hence the percentage

of slag reacted was determined

Sulfide concentrations: mmol/l Slag reacted: %

8 h 87 � 17 26 � 3

1 day 175 � 35 35 � 3

7 days 349 � 70 41 � 3

28 days 349 � 70 41 � 3

Table 6. The sulfide concentration after 8 h, 1, 7 and 28 days

were taken as indicators and combined with thermodynamic

modelling to get the percentage of reacted slag

5040302010

Ca

0·00001

0·0001

0·001

0·01

0·1

1

10

0% of slag hydrated

[M]

8 h 1 d 7 d28 d

Si

K

S(-II)

OH�

Na

Stot

S(VI)

Figure 8. The concentration of the dissolved species as a function

of the amount of slag dissolved. The sulfide concentration of the

pore solution was measured after 8 h, 1, 7 and 28 days and used

as an indicator for the dissolution of the slag

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Quantification of hydration phases insupersulfated cements: review and newapproachesGruskovnjak, Lothenbach, Winnefeld et al.

However, the unbiased definition of a clear baseline in the DTA

curve was not possible. A number of baselines are possible to be

drawn as can be imagined from Figure 10 resulting in a wide

range of results. Thus the method does not fulfil the requirements

of a reliable quantification method and is not further discussed.

Comparison of the different methodsFigure 11 compares the results from the different methods that

have been used for the quantification of the amount of reacted

slag.

The TGA, SE, XRD, and SP methods determine the amount of

slag indirectly using the quantity of hydrates formed or sulfide

released combined with mass balance considerations. The inher-

ent assumptions of the mass balance can lead to additional

errors.

1130103093083073063053043033023013090

92

94

96

98

100

30

Temperature: °C

TG: w

eigh

t %

�0·02

�0·01

0

0·01

0·02

DTG

: der

. of

wei

ght

%

Residues afterEDTA extraction

Calcite

DolomiteHydrotalcite

HR-slag

HR-SSC_1dHR-SSC_7d

HR-SSC_28d

Figure 9. Residues of unhydrated HR-slag and HR-SSC hydrated

for 1, 7 and 28 days after EDTA extraction

Residue: g Slag reacted*: % Slag reacted: % Slag reacted†: %

Slag 0.44 � 0.01 13 � 3 0 0

HR-SSC_1d 0.30 � 0.01 27 � 3 17 � 3 20 � 3

HR-SSC_7d 0.27 � 0.01 33 � 3 22 � 3 27 � 3

HR-SSC_28d 0.25 � 0.01 36 � 3 25 � 3 31 � 3

Table 7. The undissolved residue after extraction by EDTA, the

calculated percentage of reacted slag not corrected for the initial

dissolution of anhydrous slag (*), corrected for the initial

dissolution of slag, and corrected for the presence of hydrotalcite

(†) are shown

120011001000900800700600�2·3

�2·1

�1·9

�1·7

�1·5

�1·3

�1·1

�0·9

�0·7

�0·5

500Temperature: °C

DTA

:V

/mg

µ

HR-SSC_unhydrated

HR-SSC_1d

HR-SSC_7d

HR-SSC_28d

51·35 Vs/mgµ

33·57 Vs/mgµ

28·34 Vs/mgµ

20·99 Vs/mgµ

Figure 10. DTA curves for unhydrated and hydrated (1, 7 and 28

days) HR-SSC with manually fitted baselines and the calculated

area under each curve

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Quantification of hydration phases insupersulfated cements: review and newapproachesGruskovnjak, Lothenbach, Winnefeld et al.

This is especially visible in the results of the TGA and XRD

methods. Both XRD and TGA methods give comparable amounts

of ettringite and thus a comparable amount of slag reacted results

from both methods. However, the amount of slag reacted based

on the amount of C–S–H determined by TGA and SE was

significantly higher, as shown in Figure 11.

The ettringite-based TGA and XRD methods rely on a mass

balance of Al (and Mg), assuming a homogeneous dissolution of

the slag and that all Mg dissolved precipitates as hydrotalcite

(Mg4Al2(CO3)(OH)12:4H2O). These methods are highly depen-

dent on the Al and Mg content of the slag, of ettringite, C–S–H

and hydrotalcite present and thus introduce sources of large errors

that propagate into the calculations for the reacted slag. Uptake

of Al in C–S–H or a higher Al/Mg in hydrotalcite would result

in a higher amount of reacted slag.

The C–S–H based TGA and SE methods use the simple

assumption that all silicate from the dissolving slag precipitates as

C–S–H. In addition, SiO2 is one of the main constituents of the

slag studied (Table 1). The values obtained by the two C–S–H

methods (TGA and SE) agree relatively well and are similar to

the amount of slag reacted as obtained by the SEM method (Fig.

11). The more simple assumptions and the large fraction of SiO2

in the slag seem to give more realistic results. However, it has to

be noted that the TGA measurements can be associated with

significant errors as the presence of overlapping hydrates (ettrin-

gite, C–S–H, gypsum and hydrotalcite) makes the deconvolution

difficult. The SE method is expected to provide a more reliable

estimation of the amount of slag reacted.

The SP method shows a very high degree of reaction at the

beginning, which leaves reasonable doubt if the mechanisms of

the dissolution of sulfur are all well enough understood.

In general, a determination of slag reaction by direct methods

(EDTA, DTA or SEM) seems preferable as no mass balance

assumptions are needed. As mentioned above, the results of the

DTA method are excluded in this discussion as they rely on a

baseline, which cannot be defined properly. The results of the

EDTA method depend strongly on the fraction dissolved in the

unhydrated slag and on whether this fraction is considered at

longer hydration times or not. In addition, the presence of

hydrates in the residue leads at later ages to an underestimation

of the amount of slag reacted (see Figure 11). The SEM method

provides a direct measurement of the fraction of slag reacted, is

less prone to suffer from large errors introduced by calculations

and is thus judged to be a reliable method to measure the amount

of slag reacted in super-sulfated slag systems. In agreement with

the findings reported here, SEM based methods have also been

found to be a reliable method to determine the degree of reaction

in OPC–fly ash systems (Ben Haha et al., 2010).

ConclusionsDifferent methods to determine the degree of reaction of slag in

the presence of anhydrite were evaluated and compared. For all

methods a continuing reaction of the slag during the first month

was observed. Image analysis based on SEM provides the most

direct measurement of the fraction of slag reacted. The newly

developed SE method provides a reliable relative estimation of

the C–S–H phases formed in SSC and gives comparable results

to SEM imaging.

XRD and TGA determine the amount and kind of hydration

product. Calculations based on the amount of ettringite, however,

underestimate strongly the degree of slag reaction.

Two commonly used methods, EDTA and DTA, exhibit serious

sources of error and cannot be used for a reliable quantification

of the amount of reacted slag.

Estimation of the slag reaction using sulfide concentrations

overestimates early reaction while underestimating the late reac-

tion.

AcknowledgementsThe authors express their thanks to Holcim Group Support Ltd

for the financial support. The authors would like to thank Walther

Trindler and his team for the help in the lab and especially Luigi

Brunetti for the tremendous preparation of the samples. Thanks

to Oliver Nagel for the pore solution analyses, Annette Johnson

for the hydrotalcite sample, Mohsen Ben Haha for the help with

the image analysis and Paul Hug for the use of the DSC.

REFERENCES

Barneyback RFJ and Diamond S (1981) Expression and analysis

of pore fluids from hardened cement pastes and mortars.

Cement and Concrete Research 11(2): 279–285.

Ben Haha M, De Weerdt K and Lothenbach B (2010)

Quantification of the degree of reaction of fly ash. Cement

and Concrete Research 40(11): 1620–1629.

Chen KY and Morris JC (1972) Kinetics of oxidation of aqueous

sulfide by O2: Environmental Science and Technology 6(6):

529–536.

3528211470

10

20

30

40

50

60

0

Days of hydration

% s

lag

reac

ted

EDTA

TGA_C–S–H

XRD_ettringite

TGA_ettringite

SP

SE_C–S–HSEM

Figure 11. Comparison of the results of all methods discussed as

a function of percentage of slag dissolved

274

Advances in Cement ResearchVolume 23 Issue 6

Quantification of hydration phases insupersulfated cements: review and newapproachesGruskovnjak, Lothenbach, Winnefeld et al.

Erntroy HC (1987) The determination of clinker content of

composite cements. Zement-Kalk-Gips 40(5): 270–272.

Fischer H, Schulz-Ekloff G andWohrle D (1997) Oxidation of

aqueous sulfide solutions by dioxygen Part I: autooxidation

reaction. Chemical Engineering and Technology 20(7): 462–

468.

Gruskovnjak A, Lothenbach B, Holzer L, Figi R and Winnefeld F

(2006) Hydration of alkali-activated slag: comparison with

ordinary Portland cement. Advances in Cement Research

18(3): 119–128.

Gruskovnjak A, Lothenbach B, Winnefeld F, Figi R and Mader U

(2008) Hydration mechanisms of super sulphated slag

cement. Cement and Concrete Research 38(7): 983–992.

ICSD (2006) Inorganic Crystal Structure Database; ICSD. See

http://icsdweb.fiz-karlsruhe.de/index.php (accessed: 20/02/

2006).

Johnson CA and Glasser FP (2003) Hydrotalcite-like minerals

(M2Al(OH)6(CO3)0:5:XH2O, where M ¼Mg, Zn, Co, Ni) in

the environment: synthesis, characterization and

thermodynamic stability. Clays and Clay Minerals 51(1):

1–8.

Kannan S (2004) Influence of synthesis methodology and post

treatments on structural and textural variations in MgAlCO3

hydrotalcite. Journal of Materials Science 39(21): 6591–

6596.

Kuhl H and Schleicher E (1952) Gipsschlackenzement.

Fachbuchverlag GmbH, Leipzig, Germany.

Kulik D (2005) GEMS-PSI 2.1, PSI-Villigen, Switzerland,

available at http://les.web.psi.ch/Software/GEMS-PSI/

(accessed 15/11/2005).

Lommatzsch A (1956) Untersuchung von Hochofenschlacke mit

der Differentialthermoanalyse. Silikattechnik 7(11): 468.

Lothenbach B and Gruskovnjak A (2007) Hydration of alkali-

activated slag: thermodynamic modelling. Advances in

Cement Research 19(2): 81–92.

Lumley JS, Gollop RS, Moir GK and Taylor HFW (1996) Degrees

of reaction of the slag in some blends with Portland cement.

Cement and Concrete Research 26(1): 139–151.

Matschei T, Bellmann F and Stark J (2005) Hydration behaviour

of sulphate activated slag cements. Advances in Cement

Research 17(4): 167–178.

Nelder JA and Mead R (1965) A simplex method for function

minimization. Computer Journal 7(4): 308–313.

O’Brien DJ and Birkner FB (1977) Kinetics of oxygenation of

reduced sulfur species in aqueous solution. Environmental

Science and Technology 11(12): 1114–1120.

Reichle WT (1986) Synthesis of anionic clay minerals (mixed

metal hydroxides, hydrotalcite). Solid State Ionics 22(1):

135–141.

Rietveld HM (1969) A profile refinement method for nuclear and

magnetic structure. Journal of Applied Crystallography 2(2):

65–71.

Rosina A, Smajic N and Dobovisek B (1966) Beitrag zur

Anwendung der differenz-thermischen Analyse in der

Kalorimetrie (Contribution of the application of differential-

thermal analysis in calorimetry). Microchimica Acta 55(4):

626–638.

Schneider C and Meng B (2000) Bedeutung der Glasstruktur von

Huttensanden fur ihre Reaktivitat (Importance of the glass

structure of blast furnace slag for their reactivity).

Proceedings of the 14th ibausil Conference, Weimar, vol. 1,

pp. 1-0455–1-0463.

Schramli W (1963) Zur Charakterisierung von Hochofenschlacken

mittels Differentialthermoanalyse (For the characterization of

blast furnace slag using differential-thermal analysis).

Zement-Kalk-Gips 26(4): 140–147.

Scrivener KL (2004) Backscattered electron imaging of

cementitious microstructures: understanding and

quantification. Cement and Concrete Composites 26(8): 935–

945.

Scrivener KL, Fullmann T, Gallucci E, Walenta G and Bermejo E

(2004) Quantitative study of Portland cement hydration by

X-ray diffraction/Rietveld analysis and independent methods.

Cement and Concrete Research 34(9): 1541–1547.

Stark J, Ludwig HM and Muller A (1990) Zur Bestimmung des

Hydratationsgrades von Schlackenzementen (For the

determination of the degree of hydration of slag cements).

Zement-Kalk-Gips 43(11): 557–560.

Taylor HFW (1997) Cement Chemistry, 2nd edn. Thomas Telford

Publishing, London, UK.

Van Aardt JHP and Visser S (1975) Thaumasite formation:

A cause of deterioration of Portland cement and related

substances in the presence of sulphates. Cement and Concrete

Research 5(3): 225–232.

Vernet CH (1982) Comportement de l’ion S—au cours de

l’hydratation des ciments riches en laitier (CLK) (The

behaviour of the ion S in the course of the hydration of

cements rich in blast furnace slag). Silicates Industriels 47(3):

85–90.

Walenta G and Fullmann T (2004) Advances in quantitative XRD

analysis for clinker, cements, and cementitious additions.

Powder Diffraction 19(1): 40–44.

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