Quantitative analysis of mineralized white Portland clinkers:The structure of Fluorellestadite

Isabel PajaresInstituto de Ciencias de la Construccio´n ‘‘Eduardo Torroja,’’ CSIC, C/Serrano Galvache s/n,28033-Madrid, Spain

Angeles G. De la TorreDepartamento de Quı´mica Inorganica, Cristalografı´a y Mineralogı´a, Universidad de Ma´laga,29071-Malaga, Spain

Sagrario Martınez-Ramırez, Francisca Puertas, and Marıa-Teresa Blanco-VarelaInstituto de Ciencias de la Construccio´n ‘‘Eduardo Torroja,’’ CSIC, C/Serrano Galvache s/n,28033-Madrid, Spain

Miguel A. G. Arandaa)

Departamento de Quı´mica Inorganica, Cristalografı´a y Mineralogı´a, Universidad de Ma´laga,29071-Malaga, Spain

~Received 1 April 2002; accepted 15 July 2002!

Fluorellestadite, Ca10(SiO4)3(SO4)3F2 , has been synthesized as single phase. This compoundcrystallizes in the apatite type structure, s.g. P63 /m, with parametersa59.4417(1) Å, c56.9396(1) Å andV5535.76(1) Å3. The refinement of its crystal structure converged toRWP

512.33% andRF54.58%. The atomic parameters have been used to analyze the phase content ofmineralized white Portland clinkers. These clinkers contain Ca3SiO5 , Ca2SiO4 , Ca12Al14O32F2 andCa10(SiO4)3(SO4)3F2 . The agreement between the elemental composition inferred from theRietveld phase analysis and that measured by XRF is noteworthy. This comparison does not takeinto account the presence of amorphous phases and unmodeled elemental substitutions in crystallinephases. Similar Rietveld studies on commercial white Portland clinkers are also shown to befeasible. © 2002 International Centre for Diffraction Data.@DOI: 10.1154/1.1505045#

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INTRODUCTION

For the production of Portland clinkers, mineralizeand/or fluxes are often added to the raw mixes to accelereactions and enhance burnability. Traditional fluxes suchFe2O3 and Al2O3 have been partially substituted by the mieralizing pair CaF2 /CaSO4 to produce clinkers with low alu-minate contents at temperatures between 1300 and 140~Gimenez et al., 1991; Blanco-Varelaet al., 1985, 1990;Moir, 1982; Blanco-Varelaet al., 1997; Wenxiet al., 1992!.The use of this mineralizing pair to partially replace alumnates is particularly useful in manufacturing white clinkebecause of the potential for energy conservation and seater resistance~Pajareset al., 2001!.

The phases that may be present in this type of mineized clinkers~Gimenez and Blanco-Varela, 1995! are: alite,@C3S, Ca3SiO5]; belite, @C2S, Ca2SiO4]; calcium aluminate@C12A7 , Ca12Al14O33] or a related compound@C11A7CaF2 ,Ca12Al14O32F2]; and fluorellestadite @6CaO•3SiO2•3CaSO4•CaF2 or Ca10(SiO4)3(SO4)3F2]. The alite rate for-mation, in the presence of the mineralizing pa(CaF2 /CaSO4) is higher than that found for mixes mineraized only with CaF2 ~Christensen and Johansen, 1980!. Thisfaster rate for C3S is mainly due to the thermodynamic effeof widening its stability range and the primary phase fieldcrystallization. Additionally, ion mobility in the melt is enhanced by CaSO4 and CaF2 as a new liquid phase appea

a! Author to whom correspondence should be addressed: DepartamenQuımica Inorganica, Cristalografı´a y Mineralogı´a, Universidad de Ma´laga,29071-Malaga, Spain, Phone: Int134 952131874, Fax: Int134 952132000, electronic mail: g–aranda@uma.es

281 Powder Diffraction 17 (4), December 2002 0885-715

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below 1200 °C~Gimenez and Blanco-Varela, 1995; BlancoVarelaet al., 1986!.

It is of great importance to know the mineralogical compositions of these clinkers in order to understand and prethe mechanical strength of mortars or concrete elaborawith these cements. So far, calculations with the methodBogue~1929! are used to deduce the mineralogical compsition by using the elemental content usually measuredX-Ray Fluorescence~XRF!. This method has well knownproblems~Taylor, 1997! mainly due to the lack of thermodynamical equilibrium in the kiln. Hence, alternative analyticmethods are being studied to measure the mineralogcompositions of clinkers and cements. Laboratory X-RPowder Diffraction, LXRPD, is now widely used to carry oquantitative phase analyses of crystalline mixtures~Hill andHoward, 1987; Bish and Howard, 1988; Madsenet al., 2001!by using the Rietveld method~Rietveld, 1969; McCuskeret al., 1999!. The method compares the measured patternthat calculated with the crystal structures. This methodolohas recently been applied to Portland clinkers~De la Torreet al., 2001!, aluminous cements~Guiradoet al., 2000! andcalcium sulpho-aluminate cements~Schmidt and Po¨llmann,2000!. The interest on the application of XRD to Portlancements is recently increasing~Goswami and Panda, 1999Taylor et al., 2000!.

On the other hand, in order to apply the Rietveld methto mineralized clinkers it is necessary to know the structof fluorellestadite. Unfortunately, the crystal structure of thphase was not known although it can be prepared as a ctalline single phase. Its powder pattern is present in the PDatabase~JCPDS-ICDD! with a hexagonal cell, s.g. P63 /m,of a59.441 Å and c56.939 Å ~Pollman and Neubauer

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TABLE I. Chemical analysis for the raw materials and mineralized clinker~% w/w!.

Sample Sand Limestone Kaolin Fluorite Gypsum Clinker-ECa Clinker-EAb Clinker-PAc

SiO2 89.43 3.79 57.72 - 1.68 23.04 22.25 26.2Al2O3 5.11 0.75 27.94 - 0.00 2.61 2.70 1.05Fe2O3 0.22 0.32 0.60 - 0.10 0.45 0.34 -CaO 1.46 53.76 1.12 - 37.86 67.73 69.06 69.3MgO 0.42 0.35 0.98 - 0.07 0.53 0.88 -SO3 0.00 0.00 0.00 - 40.05 2.75 2.98 2.50K2O 0.93 0.11 0.23 - 0.05 0.32 nd -Na2O 0.11 0.22 0.11 - 0.08 0.29 nd -CaF2 - - - 99.9 - 2.28 nd 0.95Lossd 2.30 40.70 11.29 - 20.11 - 0.48 -Ratioe 12.45 79.72 1.76 1.51 4.56 - - -

aExpected composition for the mineralized clinker taking into account the raw materials ratio.bElemental analysis of the mineralized clinker by XRF and expressed as oxide content.cPhase analysis of the mineralized clinker by the Rietveld method and expressed as oxide content.dWeight loss by heating at 1000 °C.eRaw materials ratio for the preparation of the mineralized clinker.

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1993!. The structure must be similar to that of apatite whethe phosphate groups are randomly replaced by silicatesulphate tetrahedra and the hydroxyl groups are replacethe fluoride anions. In fact, a structure for ellestadite@ap-proximately Ca10(SiO4)3(SO4)3(OH)2] has been reportedfor a mineral from Cuba~Organovaet al., 1994!. This crystalwas monoclinic but with the apatite framework and it cotained chloride anions and even a small amount of carbon

The aim of this work is to determine the crystal structuof fluorellestadite and use it for the quantitative analysisthe mineralized white Portland clinkers.

EXPERIMENTFluorellestadite synthesis

The raw materials used for fluorellestadite, FLEL(6CaO•3SiO2•3CaSO4•CaF2) synthesis were CaCO3,CaSO4•2H2O, SiO2 and CaF2 with high purity grade in thestoichiometric ratio. The ground mixture was heated1000 °C for 2 h. Then, the sample was ground again inagate mortar and heated at 1000 °C for 32 h. To followsynthetic procedure, free lime concentration was determiby the ethylene-glycol method~UNE Standard 80-243-861986! giving 0.42% w/w of CaO. Routine LXRPD data indicated that the sample was a highly crystalline single ph

Mineralized white Portland clinker preparation

The chemical analysis of the raw materials used forpreparation of the mineralized clinker is presented in TablIn order to get the required mineralizers content, the dosgiven at the bottom of Table I was selected. The resultsilica modulus was 7.50~% w/w!. The raw materials wereground to particle size smaller than 45mm except limestonewhich was ground to particle size smaller than 125mm. Themixes were hand homogenized, using ethanol as dispermedium, taking great care to avoid any modifications of pticle size and then dried. Cylindrical pellets of 2.5 cm diaeter and 1.5–2.0 cm length were prepared. The clinketemperature was 1350 °C for 1 h in alaboratory furnace. Thechemical composition of the clinker was determined accoing to UNE Standard 80-225-93~1993!.

282 Powder Diffr., Vol. 17, No. 4, December 2002

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White Portland clinker

A typical commercial white Portland clinker wasampled from a factory and characterized by powder diffrtion.

LXRPD studies

The LXRPD pattern for FLELL was recorded onSiemens D5000u/2u diffractometer~flat reflection mode! byusing CuKa1,2 radiation~1.542 Å! with a secondary curvedgraphite monochromator at 25 °C. The samples were loain an aluminum holder by sample-front pressing. The expmental details are given in Table II. The LXRPD data forcommercial white Portland clinker were collected as for tmineralized clinker but spinning the sample at 15 rpmimprove the particles statistics.

RESULTS AND DISCUSSION

The final goal of this work is the determination of thmineralogical composition of mineralized white Portlanclinker by powder diffraction using the Rietveld methodoogy. However, to do so, the structural information of all crytalline phases is needed. As the crystal structure of FLEwas not known, this structure was determined prior tomineralogical quantification.

Crystal structure of Fluorellestadite

FLELL crystallizes in an hexagonal cell, s.g. P63/m ,PDF no. 45-0009, with edges very similar to those of tapatite type structure. Hence, the fluorapatite structurebeen used as starting model~Mackie and Young, 1973!. Thepowder pattern was analyzed by the Rietveld method wthe GSAS suite of programs~Larson and Von Dreele, 1994!by using the pseudo-Voigt peak shape function~Thompsonet al., 1987! corrected for axial divergency~Finger et al.,1994!. The background was fitted by linear interpolatiofunction.

The P position in fluorapatite was substituted by Si anwith 1

2 occupation factor for each type of atoms. Cell paraeters and the spatial atomic positions were refined. Fina

282Pajares et al.

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the isotropic temperature factors were optimized. Preferorientation was corrected by the March-Dollase algorit~Dollase, 1986! along@0 0 1# direction and the refined parameter was 0.978~2!. The pseudo-Voigt parameters were G~Gaussian part!, LX ( d* -independent Lorentzian part! andLY ( d* -dependent Lorentzian part!. Some details of theFLELL refinement are given in Table II.

The last refinement converged toRWP512.3%, seeTable II. The refined atomic positions and isotropic vibratitemperature factors are given in Table III. The Rietveld pis displayed in Fig. 1. The inset shows an enlarged viewthe fit in the high angle part of the pattern. Inter-atomic bodistances range between 1.54 and 1.56 Å for Si/S–O bo2.39 and 2.85 for Ca–O bonds, and the Ca–F bond distais 2.30 Å. The angles for the tetrahedra (SiO4 or SO4) rangesbetween 107.4 and 110.8° The structure of FLELL belonto the well known apatite type structure, so no discusswill be reported.

Phase analysis of the mineralized white Portlandclinker

The sample was rotated during LXRPD data collectas the particles statistic increases notably. The intensities

TABLE II. Data collection details and refined parameters for FLELL.

Soller slits 2 setsDivergence slit/mm 2Antidivergence slit/mm 2Receiving slit/mm 0.2X-ray tube, V/kV and I/mA 40–30Step-size/°2u 0.03Angular range/°2u 10–120Sample spinner noneCounting time/s/step 14

Chemical formula Ca10(SiO4)3(SO4)3F2

a/Å 9.441 74~9!

c/Å 6.939 64~8!

V/Å 3 535.76~1!

M /g/mol 1003.25Z 1No. reflections (Ka1) 292GW/0.01°2 5.8~2!

LX/0.01° 0.5~1!

LY/0.01° 8.5~3!

S/L 0.031H/L 0.016RWP /% 12.33RP /% 8.85RF /% 4.58

283 Powder Diffr., Vol. 17, No. 4, December 2002

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better and so, the phase analysis gives more accurate reFirst, the phases present in the synthetic clinker were idefied: C3S ~De la Torre et al., 2002!, C2S ~Mumme et al.,1995!, C11A7CaF2 ~Williams, 1973! and fluorellestadite. TheLXRPD multi-phase pattern was analyzed with the GSsuite of programs as above. Scale factors and unit cellrameters were refined for each phase and the~positional andthermal! atomic parameters were NOT refined for any phaIt is worthy to point out that the peak shape parametersphases in low concentrations are unstable and usually harefine. In this case, a number of variables as low as possshould be used. Hence, only two parameters to describepseudo-Voigt were used and only one parameter~LY ! wasrefined for the phases in low concentrations. Alite presepreferred orientation due to its large particle size. In this cthe effect was corrected using the spherical-harmonic cortion ~Von Dreele, 1997!. This method gives much better fitfor alite than the March–Dollase algorithm~Dollase, 1986!.It used the cylindrical symmetry and the order of the sphecal harmonics was 10. The final texture index was 4.79~1texture index represents an ideal ‘‘random powder’’ and`stands for a single crystal!.

The Rietveld plot for the mineralized white Portlanclinker is given in Fig. 2. An enlarged view of the moinformative part of the pattern is given in Fig. 3~top! withthe peaks due to a given phase labeled. Details ofRietveld refinement of this pattern, including the phaanalysis, are given in Table IV. The phase content obtaifrom the Rietveld analysis has been transformed to oveelemental content~expressed as oxides! in Table I. However,one should exercise care when comparing these valuesthose expected from the dosage of the raw materialsthose measured by XRF. In order to compare these numbthe atomic substitution in the crystalline phases and the pence of amorphous phases was neglected. This approxtion is coarse. However, under these constraints, the agment in Table I is remarkably good. Some considerationsbe drawn:~1! Overall CaO is always slightly overestimatein the Rietveld analysis as it includes the Ca/Mg substitutwhich cannot be modeled. Consequently, MgO is not emated.~2! Similarly, SiO2 is overestimated in the Rietvelanalysis as the Si/Al substitution cannot be taken intocount. Consequently, Al2O3 is underestimated.~3! Theagreement for SO3 content is good and CaF2 is underesti-mated in the Rietveld analysis probably due to the preseof amorphous fluorides which are the last crystallizing fration.

TABLE III. Refined atom parameters and thermal factors for Ca10(SiO4)3(SO4)3F2 .

Atom Wyck. pos. x y z Uiso /Å 2

O~1! 6h 0.3234~7! 0.4846~6! 1/4 0.014~2!O~2! 6h 0.5865~7! 0.4683~8! 1/4 0.018~2!O~3! 12i 0.3385~5! 0.2568~5! 0.0709~5! 0.022~1!Sia 6h 0.3961~3! 0.3684~3! 1/4 0.012~1!Sa 6h 0.3961~–! 0.3684~–! 1/4 0.012~–!Ca~1! 4f 1/3 2/3 0.0016~5! 0.031~1!Ca~2! 6h 0.2399~3! 0.9920~3! 1/4 0.022~1!F 2a 0.0 0.0 1/4 0.026~3!

aThe occupation factor is 0.5.

283Quantitative analysis of mineralized white Portland clinkers

Figure 1. Rietveld plot (10-120°/2u) for fluorellestadite. The inset shows an enlarged view of the high angle part of the pattern.

Figure 2. Rietveld plot (20-70°/2u)for the mineralized white Portlandclinker.

284 284Powder Diffr., Vol. 17, No. 4, December 2002 Pajares et al.

Figure 3. Selected region (28-35°/2u)of the Rietveld plot for the mineralizedwhite Portland clinker~top!. Same se-lected region for a commercial whitePortland clinker~bottom!.

TABLE IV. Selected data for the LXRPD Rietveld refinement of the mineralized clinker.

Phase s.g. a/Å b/Å c/Å b/° V/Å 3 GWa LYb w/w %

C3S Cm 33.058~4! 7.0594~4! 18.536~1! 94.207~6! 4314.1~7! 12.8~9! 16.7~6! 72.3„3…C2S P21 /n 5.509~2! 6.756~2! 9.313~2! 94.41~2! 345.6~1! 5.0~–! 39~4! 15.1„7…C11A7CaF2 I 4 3 d 11.959~3! 11.959 11.959 90.0 1710~1! 5.0~–! 25~8! 2.1„3…

FLELL P63 /m 9.446~1! 9.446 6.934~1! 90.0 535.8~2! 5.0~–! 50~4! 10.5„4…

aS/L50.015 andH/L50.015 were used to describe the asymmetry due to axial divergence for all the phases.bGW is expressed in (0.01°)2 units.cLY is expressed in 0.01° units.

285 285Powder Diffr., Vol. 17, No. 4, December 2002 Quantitative analysis of mineralized white Portland clinkers

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Phase analysis of commercial white Portland clinker

The Rietveld analysis of a diffraction pattern for a commercial white Portland clinker manufactured without Ca2was carried out for the sake of comparison. This samplesimpler and it contained C3S, C2S and C3A. The phaseanalysis was carried out as described above and theinformative part of the Rietveld plot is given in Fig. 3~bot-tom!. The results for white Portland clinkers, including thquantification of the non-diffracting part, will be reporteelsewhere.

CONCLUSIONS

This paper reports the crystal structure of fluorellestadwhich belongs to the apatite type structure. This structurused to quantify the mineralogical content of mineralizwhite Portland clinkers. These clinkers contain C3S, C2S,C11A7CaF2 and fluorellestadite. Some guidelines for thetype of Rietveld studies are presented. The agreementtween the elemental composition inferred from the phanalysis and that measured by XRF is noteworthy takenaccount the possible presence of amorphous phases. SiRietveld studies on commercial white Portland clinkersalso shown to be feasible.

ACKNOWLEDGMENTS

This work has been supported by the FEDER 1FD0894 and PGC97-1144 research grants. S. Martı´nez-Ramı´rezthanks DGES~MEC! for the award of ‘‘Contrato de Incorporacion de Doctores.’’ I. Pajares and A. G. De la Torthank two studentships by MCYT and Junta de Andaluc´a,respectively.

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