Advances in Cement Research

http://dx.doi.org/10.1680/adcr.12.00044

Paper 1200044

Received 06/08/2012; revised 15/10/2012; accepted 28/11/2012

ICE Publishing: All rights reserved

Advances in Cement Research

Alite sulfoaluminate clinker: Rietveldmineralogical and SEM-EDX analysisPerez-Bravo, Alvarez-Pinazo, Compana et al.

Alite sulfoaluminate clinker:Rietveld mineralogical andSEM-EDX analysisRaquel Perez-BravoUndergraduate student, Departamento de Quımica Inorganica,Cristalografıa y Mineralogıa, Universidad de Malaga, Malaga, Spain

Gema Alvarez-PinazoPhD student, Departamento de Quımica Inorganica, Cristalografıa yMineralogıa, Universidad de Malaga, Malaga, Spain

Jose M CompanaPhD student, Departamento de Quımica Inorganica, Cristalografıa yMineralogıa, Universidad de Malaga, Malaga, Spain

Isabel SantacruzResearch associate, Departamento de Quımica Inorganica, Cristalografıa yMineralogıa, Universidad de Malaga, Malaga, Spain

Enrique R. LosillaLecturer, Departamento de Quımica Inorganica, Cristalografıa yMineralogıa, Universidad de Malaga, Malaga, Spain

Sebastian BruqueProfessor, Departamento de Quımica Inorganica, Cristalografıa yMineralogıa, Universidad de Malaga, Malaga, Spain

Angeles G. De la TorreLecturer, Departamento de Quımica Inorganica, Cristalografıa yMineralogıa, Universidad de Malaga, Malaga, Spain

The effect of zinc oxide and calcium fluoride on the clinkering of alite-sulfoaluminate (ACSA) materials has been

studied by laboratory X-ray powder diffraction (EDX) and scanning electron microscopy (SEM). ACSA clinkers without

the addition of calcium fluoride did not contained C3S, and showed CaOfree (5.0(1) wt%). However, by adding

0.25 wt% of calcium fluoride, the obtained clinker did not contain free lime and showed C3S (20.0(2) wt%). The

suitable clinkering temperature has been proved to be 13008C, in order to avoid C4A3S decomposition. The optimum

amount of zinc oxide was found to be 1.0 wt% as the maximum percentage of C3S was formed. An ACSA clinker

prepared at 13008C with 1.0 wt% of zinc oxide and 1.0 wt% of calcium fluoride contained 34.2(2) wt% of C3S

coexisting with 15.2(2) wt% of C4A3S, determined by Rietveld methodology, including amorphous content. SEM-EDX

results have been statistically analysed and particles have been univocally identified. It has been proved that zinc

oxide is preferentially placed in C3S.

NotationA aluminium oxide (Al2O3)

C calcium oxide (CaO)

F iron oxide (Fe2O3)

H water (H2O)

M magnesium oxide (MgO)

S silica (SiO2)

S sulfate (SO3)

T titanium dioxide (TiO2)

IntroductionCalcium sulfoaluminate cements are considered to be environ-

mentally friendly materials because their manufacturing process

releases less carbon dioxide into the atmosphere than that of

ordinary Portland cements (OPC) (Gartner, 2004). These binders

may have quite variable compositions, but all of them contain

high amounts of yeelimite, also called Klein’s salt, calcium

sulfoaluminate or tetracalcium trialuminate sulfate (C4A3S)

(Odler, 2000). Recently, belite calcium sulfoaluminate (BCSA) or

sulfobelite cements have become the subject of intense research,

as these binders have the potential to substitute OPC at a large

scale (Cuberos et al., 2010; Gartner and Li, 2006; Li et al.,

2007a; Morin et al., 2011). On the one hand, these BCSA

cements contain C2S (.50 wt%) and yeelimite as main and

secondary phases (,30 wt%), respectively. However, the low

mechanical strengths at intermediate ages developed by BCSA

cements are a technological disadvantage that has to be over-

come. The activation of BCSA clinkers by obtaining modified �-

C2S or stabilising Æ9-C2S may be the solution to reach the

objective of substituting OPC (Cuberos et al., 2010; Gartner and

Li, 2006; Morin et al., 2011). On the other hand, alite sulfoalumi-

nate (ACSA) cements have also been investigated with the same

aim of enhancing final performances of clinkers with 20–30 wt%

of C4A3S (Li et al., 2007b; Lili et al., 2009; Lingchao et al.,

2005; Liu and Li, 2005; Liu et al., 2009; Ma et al., 2006; Yanjun

et al., 2007). These ACSA binders should contain more than

30 wt% of C3S in order to develop high strengths at medium

ages. However, there are some difficulties concerning the clinker-

ing of these ACSA binders because the optimum temperatures for

the synthesis of alite and yeelimite differ considerably. C3S

formation is promoted by the appearance of melting phases (De

La Torre et al., 2007) and requires a temperature of at least

13508C. Furthermore, yeelimite phase decomposition/dissolution

takes place above 13508C and is enhanced by melting phases (De

1

La Torre et al., 2011a, 2011b). Nevertheless, the addition of a

small amount of calcium fluoride (CaF2) (Li et al., 2007b; Yanjun

et al., 2007) and other minor elements, such as magnesium (Liu

and Li, 2005), titanium (Liu et al., 2009), manganese (Lili et al.,

2009), barium (Lingchao et al., 2005) or copper (Ma et al., 2006)

to the raw mixture allows the coexistence of both phases at

temperatures ranging between 1250 and 13008C. The influence of

zinc oxide (ZnO) on OPC clinkering has been studied (Kakali

and Parissakis, 1995; Kolovos et al., 2005a) as this element can

be found in a variety of sources, mainly from waste-derived fuels

such as car tyres. Minor elements act as fluxes to reduce C3S

temperature formation. Moreover, calcium fluoride also increases

the crystallisation volume of C3S, acting as a mineraliser

(Blanco-Varela et al., 1997). The main aim of this work is to

study the suitability of zinc oxide to obtain ACSA clinkers at low

temperatures, when compared to OPC, with an estimated compo-

sition of ,40 wt% of alite, ,25 wt% of C2S and ,20 wt% of

C4A3S. The joint role of calcium fluoride addition will also be

discussed.

Materials and methods

Clinker preparation

Raw materials (kaolin (Aldrich), calcium carbonate (99.95–

100.05% AlfaAesar), iron oxide (Fe2O3) (99.945% AlfaAesar),

pure natural gypsum (SiO2) (99.31% ABCR) and zinc sulfate

monohydrate (ZnSO4.H2O) (min. 97%, Probus)) were mixed to

prepare ACSA clinkers with an expected nominal mineralogical

composition of .40 wt% of C3S, ,25 wt% of C2S and ,20 wt%

of C4A3S, as main phases. ACSA clinkers were prepared by

adding zinc oxide as zinc sulfate monohydrate and calcium

fluoride. Hereafter ACSA clinkers will be termed ZnX_CaF2Y,

where X stands for zinc oxide weight percentage (X ¼ 0.0, 1.0,

2.0 and 3.0) and Y stands for calcium fluoride weight percentage

(Y ¼ 0.0, 0.25, 0.5 and 1.0). Table 1 shows the elemental

composition of the raw mixtures, expressed as parent oxide

content.

Raw materials were mixed by hand in an agate mortar with

absolute ethanol and dried in a stove at 608C. This treatment was

performed three times. The clinkering was carried out by pressing

the raw mixtures into ,3 g, 20 mm dia. pellets. The pellets were

placed in platinum/rhodium alloy (Pt/Rh) crucibles and heated at

9008C for 30 min at a heating rate of 58C/min under ambient

furnace atmosphere. Then, the temperature was raised at the same

rate to the final temperature, ranging from 1300 to 13508C and

held for 15 min. Finally, the clinkers were quenched from high

temperature by applying air flow.

Laboratory X-ray powder diffraction patterns

All clinker samples were ground into fine powder to perform

laboratory X-ray powder diffraction (LXRPD) studies. Patterns

were recorded on an X’Pert MPD Pro diffractometer (PANalytical)

using strictly monochromatic CuKÆ1 radiation (º ¼ 1.54059 A)

[Ge(111) primary monochromator] and working in reflection

geometry (Ł/2Ł). The X-ray tube worked at 45 kV and 40 mA.

The optics configuration was a fixed divergence slit (1/28), a fixed

incident anti-scatter slit (18), a fixed diffracted anti-scatter slit

(1/28) and X’Celerator real-time multiple strip detector, working

in scanning mode with maximum active length. Data were

collected from 58 to 708 (2Ł) during ,2 h. The samples were

rotated during data collection at 16 r/min in order to enhance

particle statistics.

The G-factor approach (which is explained below) requires the

recording of a standard pattern collected in identical diffractometer

configuration/conditions and as close in time as possible to the

clinker measurements. The diffractometer experimental set-up for

standards was the same as detailed above except for the spinning

of the sample. The methodology detailed in Jansen et al. (2011a)

was performed by using a polished polycrystalline quartz rock as

secondary standard, placed on the diffractometer in the very same

orientation for each measurement. The suitability of this quartz

rock was tested against NIST standard reference material SRM-

676a (Æ-aluminium oxide (Æ-Al2O3)) (Cline et al., 2011).

CaO SiO2 Al2O3 Fe2O3 K2O* Na2O* SO3 MgO ZnO P2O5a CaF2

Zn0.0_CaF20.0 60.77 20.40 10.98 1.63 0.46 0.04 5.53 0.10 0.00 0.06 0.00

Zn1.0_CaF20.0 60.17 20.20 10.88 1.61 0.46 0.04 5.47 0.10 0.99 0.06 0.00

Zn2.0_CaF20.0 59.53 19.99 10.76 1.60 0.45 0.04 5.41 0.09 2.04 0.06 0.00

Zn0.0_CaF20.25 60.62 20.35 10.96 1.63 0.46 0.04 5.51 0.10 0.00 0.06 0.25

Zn1.0_CaF20.25 60.04 20.16 10.85 1.61 0.46 0.04 5.43 0.10 0.99 0.06 0.25

Zn2.0_CaF20.25 59.38 19.94 10.73 1.59 0.45 0.04 5.40 0.09 2.04 0.06 0.25

Zn3.0_CaF20.25 58.77 19.73 10.62 1.58 0.45 0.04 5.34 0.09 3.06 0.06 0.25

Zn1.0_CaF20.5 59.87 20.10 10.82 1.61 0.46 0.04 5.44 0.10 0.99 0.06 0.49

Zn1.0_CaF21.0 59.58 20.00 10.77 1.60 0.45 0.04 5.42 0.09 0.98 0.06 0.98

a Coming from kaolin (Aldrich) (Morsli et al., 2007).

Table 1. Nominal elemental composition expressed in weigh

percentage of oxides of the proposed ACSA clinkers

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Advances in Cement Research Alite sulfoaluminate clinker: Rietveldmineralogical and SEM-EDX analysisPerez-Bravo, Alvarez-Pinazo, Compana et al.

LXRPD data analysis

Crystalline fraction

The LXRPD patterns of ACSA clinkers and the external standard

were analysed by the Rietveld method as implemented in the

general structure analysis system software package (Larson and

Von Dreele, 2000) to extract the quantitative phase analyses. The

refined overall parameters were cell parameters, zero-shift error,

peak shape parameters and phase fractions. Peak shapes were

fitted by using the pseudo-Voigt function (Thompson et al., 1987)

corrected for axial divergence (Finger et al., 1994). Table 2 gives

the bibliographic references and Inorganic Crystal Structure

Database (ICSD) (Belsky et al., 2002) collection codes for the

structural descriptions of all crystalline phases used in this work.

Amorphous and crystalline non-quantified fraction

The simplest approach to derive the phase content from the

Rietveld refined scale factor is by the approximation that the

sample is only composed of crystalline phases with known

structures (normalisation to full crystalline content method). This

methodology has been used to evaluate the clinkering procedure.

However, in order to obtain a full mineralogical phase assem-

blage, which includes amorphous contents, the G-factor approach

may be used. This approach quantifies not only amorphous/sub-

cooled phases but also misfitting problems of the analysed

crystalline phases and the non-included crystalline phases. Here-

after, this derived value will be called ‘amorphous and crystalline

non-quantified’, ACn (Aranda et al., 2012). In the G-factor

approach a suitable external standard is employed that allows the

determination of the absolute weight fractions by previously

obtaining the diffractometer constant, knowing the mass attenua-

tion coefficients of the samples (Jansen et al., 2011b; O’Connor

and Raven, 1988).

Microstructural characterisation

Microstructure characterisation was performed in a Jeol JSM-

6490LV scanning electron microscope. X-ray powder diffraction

(EDX) measurements were carried out with the Oxford Inca

Energy 350 attachment. This unit has a Si(Li) detector with a

super-atmospheric thin window. Two different sample prepara-

tions were performed: (a) polished cross-sections up to 6 �m of

diamond powder and covered with gold (Au) in order to obtain

backscattered electron (BSE) images and (b) dry pressed pellets

coated with graphite.

Statistical analysis of EDX data

A cluster analysis using Statgraphics Centurion XVI V. 16.1.15

(32 bits) was performed using EDX data. The dissimilarity

between each pair of points was measured using Euclidean

distances, which are easily interpretable in a two-dimensional

space.

Results and discussion

LXRPD studies

Clinkers with zinc oxide

The ZnX_CaF20.0 clinkers were prepared at two different tem-

peratures, 13008C and 13508C. LXRPD was performed for all the

samples in order to determine the burnability of the raw materials.

Figure 1 shows raw LXRPD patterns of ZnX_CaF20.0 prepared at

(a) 13008C and (b) 13508C, where main peaks due to a phase are

labelled. Table 3 gives Rietveld quantitative phase analysis

normalised to 100 wt% of crystalline phases for all the samples.

The first result extracted by inspecting Figure 1 and Table 3 is that

zinc oxide promoted burnability at 13008C (Kakali and Parissakis,

1995), as free lime decreased from 5.0(1) wt% to 0.4(1) wt%.

Preparing the clinkers at 13508C is, at first sight, the best option

as free lime is always present at very low amounts. However, the

amount of C4A3S is systematically lower and the amount of C3A

increases, see open square symbols in Figure 1 and Table 3. These

results show that C4A3S may be decomposing to give C3A (Odler,

2000). Moreover, C3S contents were much lower for every

composition than expected. According to these results it was not

worth performing the ACn determination for these samples. As a

conclusion of these first results, calcium fluoride has to be added

to the samples to promote C3S formation and samples were

clinkered at 13008C to avoid C4A3S decomposition.

Clinkers with zinc oxide and calcium fluoride

The ZnX_CaF20.25 (X ¼ 0.0, 1.0, 2.0 and 3.0) series were

prepared at 13008C. The addition of 0.25 wt% of calcium fluoride

increased the burnability of the raw mixtures, as calcium oxide

was not detected by LXRPD. These clinkers showed higher C3S

content than the corresponding clinker without calcium fluoride

addition. Table 4 gives Rietveld quantitative phase analysis

RQPA, including ACn contents determined by the G-factor

approach, for ZnX_CaF20.25 clinkers and Figure 2 shows a

Rietveld plot of Zn1.0_CaF20.25 as an example. It should be

highlighted that the maximum C3S content, 27.8(2) wt%, was

obtained when 1.0 wt% of zinc oxide was added to the raw

mixture. Higher additions of zinc oxide did not yield higher

percentages of C3S and even zinc-bearing phases were formed, as

Ca3ZnAl4O10 in Zn3.0_CaF20.25 (Bolio-Arceo and Glasser,

Reference ICSD code

C3S (De La Torre et al., 2002) 94742

�-C2S (Mumme et al., 1995) 81096

C4A3S (Calos et al., 1995) 80361

C4AF (Colville and Geller, 1971) 9197

C3A (Mondal and Jeffery, 1975) 1841

C (Natta and Passerini, 1929) 61550

CS (Kirfel and Will, 1980) 16382

Ca3ZnAl4O10 (Barbanyagre et al., 1997) 50293

ZnFe2O4 (Shinoda et al., 1995) 81205

F-ellesteadite (Pajares et al., 2002) 97203

Table 2. ICSD collection codes and bibliographic references for all

phases used for Rietveld refinements

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Advances in Cement Research Alite sulfoaluminate clinker: Rietveldmineralogical and SEM-EDX analysisPerez-Bravo, Alvarez-Pinazo, Compana et al.

1998). According to these results, higher amounts of zinc oxide

did not favour C3S formation. It is well known that Zn2þ is a

low-volatility cation (Barros et al., 2004; Kolovos et al., 2002),

but its distribution among phases has not been properly clarified

(Garcia-Diaz et al., 2009; Kolovos et al., 2005a). Table 4 also

gives refined unit cell volume (V/Z) of main phases. Refined C3S

volume decreases, at a higher pace than other phases, as zinc

oxide percentage increases. Thus it can be stated that Zn2þ is

preferably incorporated in this phase (Urabe et al., 2002). More-

over, this conclusion has been reinforced by scanning electron

microscopy (SEM) studies, see below.

Table 4 includes ACn contents derived by the G-factor approach,

which stand for both amorphous sub-cooled material and any

misfitting problems of analysed crystalline phases (for instance

lack of solid solution crystal structure description, which is the

case of C3S with zinc). Mass balance calculations have been

performed in order to obtain an estimated ACn elemental

composition, see Table 5. These data have been obtained by

comparing derived elemental composition of the crystalline

fraction, considering the stoichiometry of each phase given in

Table 4, with data given in Table 1. Therefore, it can be inferred

that ACn material may be mainly composed by an ill-crystallised

1300°C 1350°C

Zn2·0_CaF 0·02Zn2·0_CaF 0·02

Zn1·0_CaF 0·02Zn1·0_CaF 0·02

Zn0·0_CaF 0·02Zn0·0_CaF 0·02

15 1520 2025 2530 3035 3540 4045 452 : degθ 2 : degθ

C A4 3S C S3 C S2 C AF4 CS C C A3

(a) (b)

Figure 1. Three-dimensional view of a selected range of the

LXRPD patterns for ZnX_CaF20.0 clinkers obtained at (a) 13008C

and (b) 13508C. Main peaks attributable to a given phase have

been labelled

Zn0.0_CaF20.0 Zn1.0_CaF20.0 Zn2.0_CaF20.0

Temperature: 8C 1300 1350 1300 1350 1300 1350

C3S — — 18.5(2) 9.4(2) 19.6(2) 3.8(2)

�-C2S 74.3(1) 77.0(1) 59.5(1) 67.3(1) 60.5(1) 72.0(1)

C4A3S 14.9(1) 7.0(1) 17.6(1) 12.5(1) 16.5(1) 11.3(1)

C4AF — — 2.5(2) 2.2(2) 2.5(2) 3.3(2)

CS — 0.8(1) — 0.5(1) 0.5(1) 0.6(1)

CaO 5.0(1) 0.9(1) 1.5(1) 1.3(1) 0.4(1) 2.0(1)

C3A 5.8(1) 14.4(1) 0.4(1) 6.7(1) — 6.9(1)

Table 3. Rietveld quantitative phase analysed of ZnX_CaF20.0

clinkers prepared at 13008C and 13508C, expressed in wt%

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Advances in Cement Research Alite sulfoaluminate clinker: Rietveldmineralogical and SEM-EDX analysisPerez-Bravo, Alvarez-Pinazo, Compana et al.

or amorphous sub-cooled phase composed mainly of calcium

sulfo-aluminoferrite phases.

Finally, in order to study the calcium fluoride role in the

mineralogical composition, Zn1.0_CaF2Y (Y ¼ 0.25, 0.5 and 1.0)

series were prepared. This amount of zinc oxide was chosen as it

was the composition with the maximum content of C3S as

previously studied. With the addition of fluorite the amount of C3S

increased from 20.0(2) wt% with 0.25 wt% of calcium fluoride to

34.2(2) wt% with 1.0 wt% of calcium fluoride. This latter compo-

sition also contains some F-ellestadite, Ca10(SiO4)3(SO4)3F2,

(Pajares et al., 2002); hence it is not worth increasing the amount

Zn0.0_CaF20.25 Zn1.0_CaF20.25 Zn2.0_CaF20.25 Zn3.0_CaF20.25a Zn1.0_CaF20.5 Zn1.0_CaF21.0

C3S 20.0(2) 27.8(2) 25.8(2) 20.2(2) 31.9(1) 34.2(2)

�-C2S 45.7(2) 37.7(2) 34.4(2) 36.5(2) 35.1(2) 29.5(2)

C4A3S 14.7(1) 15.5(1) 15.2(1) 12.6(1) 15.9(3) 15.2(1)

C4AF 1.0(1) 2.1(2) 1.9(2) 3.7(2) 2.7(1) 2.8(2)

CS 0.8(1) 0.8(1) 0.9(1) 0.9(1) 0.6(1) 0.4(1)

CaO — — — — — —

C3A 1.5(1) — — — — —

Ca3ZnAl4O10 — — — 3.1(1) — —

F-ellestadite — — — — — 1.9(1)

ACn 16.2(4) 16.0(4) 21.9(4) 22.5(4) 13.8(4) 16.1(4)

V(C3S)/Z 121.17(1) 120.73(1) 120.69(2) 120.58(2) 120.621(7) 120.679(8)

V(�-C2S)/Z 87.325(7) 87.201(8) 87.127(8) 87.061(9) 87.118(7) 86.837(8)

V(C4A3S)/Z 389.04(4) 389.04(5) 389.13(5) 388.97(6) 389.19(4) 389.31(4)

V(C4AF)/Z 215.8(1) 217.05(7) 215.81(8) 214.77(7) 216.34(8) 215.77(8)

a Also contains 0.5(1) wt% of ZnFe2O4

Table 4. Rietveld quantitative phase analyses of ZnX_CaF2Y at

13008C clinkers, including ACn content obtained by the G-factor

approach (in wt%). Refined volume/Z for main phases (in A3)

0

0·5

1·0

10·0 20·0 30·0 40·0

Cou

nts:

10�

�4

2 : degθ

CA

F4

CA 4

3S

CA 4

3S

CA 4

3S

CA 4

3S

CS C

S 3C

S 2

CS 3

CS 3 CS 3

CS 3

Figure 2. LXRPD Rietveld plot for Zn1.0_CaF20.25 clinker

prepared at 13008C, with main peaks labelled

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Advances in Cement Research Alite sulfoaluminate clinker: Rietveldmineralogical and SEM-EDX analysisPerez-Bravo, Alvarez-Pinazo, Compana et al.

of calcium fluoride. Thus, the combination of 1.0 wt% of zinc

oxide and 1.0 wt% of calcium fluoride promoted the formation of

alite sulfoaluminate clinker in which 34.2(2) wt% of C3S coexists

with 15.2(1) wt% of C4A3S. Moreover, the amount of C3S formed

at this low temperature with this composition is very close to the

expected value (,40 wt%). The difference between the experi-

mental and the expected composition can be mainly attributed to

the fact that thermodynamic equilibrium has not been achieved in

any composition, as holding time at high temperature is very short

and cooling is rapid. Fluorine ion location has not been determined

in this study. However, it is known that the F� guest ion has

preference for interstitial oxygen sites of alite (Tran et al., 2009).

SEM studies

Figure 3 (parts numbered from 1 to 6) shows the microstructure

of ZnX_CaF2Y clinkers prepared at 13008C. The micrographs

were chosen to be representative as far as the distribution, size

and form of alite, belite and yeelimite crystals are concerned. An

evaluation of size and shape of clinker main phases is presented

in Table 6. The identification was based on previous knowledge

of clinker phase microstructure and EDX measurements. Polished

CaO SiO2 Al2O3 Fe2O3 SO3

Zn0.0_CaF20.25 9.0 0.0 2.9 1.3 3.1

Zn1.0_CaF20.25 8.0 0.0 2.7 0.9 2.9

Zn2.0_CaF20.25 11.2 1.1 2.7 1.0 2.9

Zn3.0_CaF20.25 12.3 1.7 2.1 0.4 3.2

Zn1.0_CaF20.5 6.2 0.0 2.3 0.7 3.0

Zn1.0_CaF21.0 7.2 0.4 2.6 0.7 2.7

Table 5. Estimated elemental compositions of amorphous and

crystalline non-quantified (ACn) phases of ZnX_CaF2Y clinkers

(1) (2)

(3) (4)

(5) (6)

a

b

a

a

b

a

a

a

b

aa

b

a

b

a

a

ab

20 kV

20 kV

20 kV

20 kV

20 kV

20 kV

�500

�500

�500

�500

�500

�500

50 mμ

50 mμ

50 mμ

50 mμ

50 mμ

50 mμ

09 40 SEI

10 40 SEI

23/MAR/11

08 40 SEI

14 40 SEI

10 40 SEI

Figure 3. SEM micrographs of: (1) Zn0.0_CaF20.25;

(2) Zn1.0_CaF20.25; (3) Zn2.0_CaF20.25; (4) Zn3.0_CaF20.25;

(5) Zn1.0_CaF20.5; (6) Zn1.0_CaF21.0 clinkered at 13008C

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Advances in Cement Research Alite sulfoaluminate clinker: Rietveldmineralogical and SEM-EDX analysisPerez-Bravo, Alvarez-Pinazo, Compana et al.

cross-sections of these clinkers were also examined using a BSE

detector. BSE microstructures of selected clinkers, namely

Zn0.0_CaF20.25 and Zn3.0_CaF20.25, are shown in Figure 4,

with the aim of clarifying the particle shapes of the main phases.

In Zn0.0_CaF20.25 clinker (in Figure 3, micrograph 1, labelled

‘a’ in the micrographs and in Figure 4(a), labelled C3S), C3S

appeared as big, compact, basal, angular, prismatic crystals of

hexagonal outline (30–70 �m). C2S (labelled ‘b’, in Figure 3)

appears as uniformly distributed, smaller (3–10 �m), roundish

grains. Alite and belite phases have a very similar chemical

composition, thus the BSE analysis is sufficient to distinguish

both phases. However, at high magnification, some round parti-

cles can be observed, see Figure 4(d). Yeelimite grains appeared

as small (,1–3 �m) prisms with hexagonal outline and they are

not marked in Figure 3 because of their small particle sizes.

Figure 4(c) and (d) shows some small, dark, angular particles;

these were proved to be yeelimite grains by EDX analysis. The

C3S size:

�m

Shape C2S size:

�m

Shape C4A3S size:

�m

Shape

Zn0.0_CaF20.25 30–70 Angular, hexagonal 4–12 Round 1–3 Angular, hexagonal

Zn1.0_CaF20.25 30–70 Rounded in the rims, hexagonal 4–12 Round 1–3 Angular, hexagonal

Zn2.0_CaF20.25 30–70 Irregular, rounded in the rims 4–12 Round 1–3 Angular, hexagonal

Zn3.0_CaF20.25a 30–70

10 3 1

Irregular, rounded in the rims,

elongated shape (length 3 width)

8–15 Round 1–3 Angular, hexagonal

Zn1.0_CaF20.5 30–70 Compact, rounded in the rims 1–5 Round 1–3 Angular, hexagonal

Zn1.0_CaF21.0 30–70 Compact, rounded in the rims 1–5 Round 1–3 Angular, hexagonal

a Two types of C3S particle shapes.

Table 6. Shape and size of main phases in ZnX_CaF2Y samples

clinkered at 13008C

C A4 3SC A4 3S

(a) (b)

(c) (d)

20 kV 20 kV�2000 �200010 mμ 10 mμ09 40 BEC 09 40 BEC0607

20 kV �500 50 mμ 08 40 BEC 20 kV �500 50 mμ 09 40 BEC

C S3

C S3

C S3

C S3

Figure 4. Polished cross-section backscattered electron

micrographs of: (a) and (c) Zn0.0_CaF20.25; (b) and (d)

Zn3.0_CaF20.25 clinkered at 13008C

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Advances in Cement Research Alite sulfoaluminate clinker: Rietveldmineralogical and SEM-EDX analysisPerez-Bravo, Alvarez-Pinazo, Compana et al.

addition of zinc oxide has not caused any change in sizes or

shapes of this phase. These types of clinkers did not show a

significant amount of interstitial material as OPC clinkers

(Kolovos et al., 2005b). Particles of C4AF and CS were not

identified in any composition, mainly owing to their low percent-

age in these ACSA clinkers, see Table 4. Moreover, these phases

may have a very small particle size, ,1 �m, which may

contribute to increase the amount of ACn material determined by

LXRPD. At this point it is possible to state that the SEM studies

support the X-ray powder diffraction results.

Zinc oxide addition affects mainly the shape of C3S and size of

C2S particles in clinkers with the same calcium fluoride content

(i.e. 0.25 wt%). By increasing the zinc oxide content, C3S grains

show a more roundish and irregular shape at the rims (Figure 3,

micrographs 1 to 4, particles labelled ‘a’). To reinforce this

finding, Figure 4(a) and (b) shows BSE images of 0.0 and

3.0 wt% of zinc oxide clinkers, respectively. Figure 4(a) shows

big and well-defined, straight grain boundaries, whereas in Figure

4(b) straight grain boundaries are not found. Moreover, when the

zinc oxide amount is high, that is 3.0 wt%, elongated and small

(10 �m 3 1 �m) C3S crystals appeared (white arrows in Figure 3,

micrograph 4). Moreover, zinc-bearing particles appear in BSE

images as the lightest and the brightest ones, see Figure 4(d).

These particles were not detected in other compositions with less

zinc oxide. The identification of these elongated particles as C3S

was performed by EDX and will be detailed below. Concerning

C2S particles, the addition of 3.0 wt% of zinc oxide has slightly

increased the size, see Table 6 and Figure 3, micrographs 1 to 4,

although it had a negligible effect on particle shape.

The increase of calcium fluoride content has mainly affected the

shape of C3S particles (which were very irregular) and C2S

particles (which are rounded and much smaller than in other

compositions with less calcium fluoride) (Figure 3, micrographs 5

and 6, and Table 6). This behaviour is consistent with the role as

a flux of calcium fluoride.

A semi-quantitative composition of the particles was obtained by

SEM-EDX. Taking into account the intrinsic uncertainty of SEM-

EDX data, a statistical analysis of the results is essential to obtain

some reliable conclusions. The most significant differences

among the analysed phases are their silicon (Si), sulfur (S),

calcium (Ca) and zinc (Zn) contents, so the Si/(Ca+Zn) and

S/(Ca+Zn) content ratios were chosen to carry out a cluster

analysis. The main aim of this analysis was to match each SEM-

EDX measurement with the theoretical compositions of pure

phases, which were included as raw data, solid rhombus for C3S,

upwards-pointing triangle for C2S and downwards-pointing trian-

gle for C4A3S in Figure 5. Ward’s agglomerative method was

Cluster

123C S3

C S2

C A4 3S

Centre

0

0·2

0·4

0·6

0·8

1·0

0 0·1 0·2 0·3 0·4

Si/(C

a +

Zn)

Si/(Ca + Zn)

(1)

(2) (3)

(4)

Figure 5. Cluster analysis of SEM-EDX results, where solid

symbols stand for the theoretical values for pure phases. Insets:

(1) Zn2.0_CaF20.25; (2) Zn1.0_CaF20.25; (3) Zn3.0_CaF20.25;

(4) Zn1.0_CaF21.0

8

Advances in Cement Research Alite sulfoaluminate clinker: Rietveldmineralogical and SEM-EDX analysisPerez-Bravo, Alvarez-Pinazo, Compana et al.

used, which minimises the increase in the total within-cluster sum

of squares (Everitt et al., 2001). The optimal number of clusters

for this dataset was found to be three (Figure 5). Those clusters

are easily interpretable. Cluster 1 (rhombus) includes those

samples similar to C3S composition. Inset 2 in Figure 5 shows

the big, hexagonal-outline particles for Zn1.0_CaF20.25. It is well

known that this is the typical particle shape of C3S and,

moreover, EDX elemental composition has been included in

cluster 1, which corresponds to particles with elemental composi-

tion similar to C3S. Inset 3 in Figure 5 shows elongated, small

particles for Zn3.0_CaF20.25. It is important to highlight that all

EDX semi-quantitative compositions for these types of particles

contain zinc, and they are included in cluster 1, corresponding to

C3S composition. Thus, it can be concluded that the addition of

zinc oxide promoted a modification in particle size and shape of

C3S. Moreover, the study of unit cell volume variations by

LXRPD suggested that zinc oxide was included in the C3S

structure. It has been demonstrated by two independent method-

ologies that zinc oxide is preferentially placed in C3S when

present in clinkers. On the other hand, cluster 2 (upwards-

pointing triangles) includes all EDX compositions similar to C2S.

Inset 4 in Figure 5 shows round particles of Zn1.0_CaF21.0 as

representative of all compositions. It should be mentioned that

none of these EDX results contained zinc oxide. Finally, cluster 3

(downwards-pointing triangles) includes those samples chemi-

cally similar to C4A3S, whose silica contents were low. These

particle compositions are more similar to C4A3S owing to their

low iron and silicon contents. Inset 1 in Figure 5 shows a

micrograph of Zn2.0_CaF20.25 in which some small, hexagonal-

outline particles are shown as representative of all the composi-

tions. EDX measurements of these particles are included in

cluster 3, so it can be concluded that those small particles, which

are present in all compositions, correspond to C4A3S.

ConclusionsIn order to obtain ACSA clinkers with coexistence of C3S and

C4A3S the addition of calcium fluoride is mandatory. Here,

ACSA clinkers have been prepared with a maximum of 1.0 wt%

of calcium fluoride in which F-ellestadite phase is formed. These

studies have stated that 13008C was shown to be a suitable

clinkering temperature. The addition of zinc oxide promoted the

formation of higher amounts of C3S. It has been demonstrated by

LXRPD and SEM-EDX analysis that zinc oxide is preferentially

placed in C3S. The addition of zinc oxide has promoted changes

in C3S particles from big and hexagonal to very irregular and

rounded in the rims, and small and elongated particles. Using

statistical cluster analysis of SEM-EDX results, phases have been

unequivocally identified.

AcknowledgementsThis work has been supported by Mineco through MAT2010–

16213 research grant, which is co-funded by Feder. Author I.

Santacruz thanks Ramon y Cajal Fellowship (RYC-2008–03523)

and J. M. Compana thanks Micinn for his FPU studentship.

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