Cement and Concrete Research 42 (2012) 1157–1165

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Uptake of chloride and carbonate ions by calcium monosulfoaluminate hydrate

Adel Mesbah a,b, Céline Cau-dit-Coumes a,⁎, Guillaume Renaudin b,d, Fabien Frizon a, Fabrice Leroux c,d

a Commissariat à l'Energie Atomique et aux Energies Alternatives, CEA DEN/DTCD/SPDE, F-30207 Bagnols sur Cèze, Franceb Clermont Université, ENSCCF, Laboratoire des Matériaux Inorganiques, BP 10448, F-63000 Clermont-Ferrand, Francec Clermont Université, Université Blaise Pascal, Laboratoire des Matériaux Inorganiques, BP 10448, F-63000 Clermont-Ferrand, Franced CNRS, UMR 6002, F-63177 Aubière, France

⁎ Corresponding author. Tel.: +33 4 66 39 74 50; faxE-mail address: celine.cau-dit-coumes@cea.fr (C. Cau

0008-8846/$ – see front matter © 2012 Elsevier Ltd. Alldoi:10.1016/j.cemconres.2012.05.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 February 2012Accepted 16 May 2012

Keywords:Waste management (E)Monosulfate (D)Chloride (D)Carbonation (C)X-ray diffraction (B)

Decommissioning of old nuclear reactors may produce waste streams containing chlorides and carbonates,including radioactive 36Cl− and 14CO3

2−. Their insolubilization by calcium monosulfoaluminate hydrate wasinvestigated. Carbonates were readily depleted from the solution, giving at thermodynamic equilibriummonocarboaluminate, monocarboaluminate+calcite, or calcite only, depending on the initial ratio betweenthe anion and calcium monosulfoaluminate hydrate. Chloride ions reacted more slowly and were precipitat-ed as Kuzel's salt, Kuzel's and Friedel's salts, or Friedel's salt only. Rietveld refinement of X-Ray powder dif-fraction patterns was successfully used to quantify the phase distributions, which were compared tothermodynamic calculations. Moreover, analysing the lattice parameters of Kuzel's salt as a function of itschloride content showed the occurrence of a restricted solid solution towards the sulfate side with generalformula 3CaO·Al2O3·xCaCl2·(1−x)CaSO4·(12−2x)·H2O (0.36≤x≤0.50).

© 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Calcium sulfoaluminate (CSA) cements may have a good potentialto stabilize hazardous wastes such as heavy metals [1–7], ion ex-change resins [8], aluminum-containing wastes [9] or radioactivestreams containing high amounts of borate and sulfate ions [10]. Un-like Portland cement, hydration of CSA cement leads to the formationof two principal hydrates: calcium monosulfoaluminate hydrate [11]and ettringite [12] with general formula 3CaO·Al2O3·CaSO4·12H2Oand 3CaO·Al2O3·3CaSO4·32H2O, respectively. Their contents arevery sensitive to the amount of added sulfate: the former dominatesin a gypsum-deficient environment, whereas the latter tends to in-crease in a gypsum-rich system [13]. Both ettringite and calciummonosulfoaluminate hydrate have a flexible structure and can admitvarious substitutions on their cationic or anionic positions [14–18].

In this study, the focus is placed on chloride and carbonate anions,which may contain long-lived radioactive isotopes 36Cl and 14C, inwaste streams produced during the decommissioning of old nuclearreactors (UNGG-type, graphite moderated, cooled by carbon dioxide,and fuelled with natural uranium metal). In the literature, AFmphases are considered as good candidates to bind chloride anionscontrarily to AFt phases [15,19,20]. Two Cl-containing AFm phasesare involved: Kuzel's salt 3CaO·Al2O3·1/2CaSO4·1/2CaCl2·11H2O,a crystallized compound containing ordered chloride and sulfate

: +33 4 66 33 90 37.-dit-Coumes).

rights reserved.

anions, and Friedel's salt 3CaO·Al2O3·CaCl2·10H2O, a phase containingchloride anions only.Moreover, it iswell known that cement-basedma-terials can be easily carbonated, leading to the formation of calciumhemicarboaluminate hydrate 3.5CaO·Al2O3·1/2CaCO3·12H2O, calciummonocarboaluminatehydrate 3CaO·Al2O3·CaCO3·11H2O and/or calcite[21–26]. The AFm phases belong to the lamellar double hydroxide(LDH) large family. Their structure is composed of positively chargedmain layers [Ca2Al(OH)6]+ and negatively charged interlayers[X·nH2O] −, where X is one monovalent anion or half a divalentanion. The following general formulae 3CaO∙Al2O3∙CaX2∙nH2O for amonovalent anion, or 3CaO∙Al2O3∙CaX∙nH2O for a divalent anion,are generally used in cement chemistry. Several structural studieshave been performed on AFm compounds incorporating one typeof anion only in the interlayer: SO4

2− [11], Cl− [27–29], CO32−

[30,31], NO3− [32,33], I− [34,35] and Br− [34,35]. Others studies

were devoted to bi anionic-AFm compounds formed by Cl−\Br−

[35], CO32−\OH− [36], CO3

2−\Cl− [37–39] or SO42−\Cl− permuta-

tion [40].The present study was undertaken within a project aiming at

assessing the binding capacities of chloride and carbonate anions byhydrated CSA cements. The first step was to investigate the uptakeof Cl− and CO3

2− by calcium monosulfoaluminate hydrate.

2. Experimental section

2.1. Samples preparation

Calcium monosulfoaluminate hydrate with general formula3CaO∙Al2O3∙CaSO4∙12H2O was synthesized in aqueous solution. The

1158 A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165

starting reagents, tricalcium aluminate (Ca3Al2O6) and gypsum(CaSO4∙2H2O), were mixed in decarbonated water (boiled during1 h and cooled under nitrogen atmosphere) to reach a final water/solid mass ratio of 50. The suspension was introduced in a Teflon re-actor and stirred continuously at 80 °C during three weeks. The tem-perature was chosen to avoid the formation of ettringite, which isunstable over ~70 °C under these conditions [41]. Then, the suspen-sion was filtered twice using pure water and finally rinsed with iso-propanol. The precipitates were subsequently dried in a desiccatorover potassium acetate (≈20% r.h.) at room temperature.

Sorption tests were carried out by adding 0.5 g (8.03×10−4mole)of calcium monosulfoaluminate hydrate to 10 mL of sodium chloride(NaCl) or sodium carbonate (Na2CO3) solutions. The suspensionswere kept under stirring at 20 °C. Two series of sorption tests wereperformed, firstly to study the exchange rate between sulfate andchloride or carbonate, and secondly to investigate the influence ofthe initial concentration of chloride or carbonate on this exchange.

In the first series of experiments, the Cl−/monosulfoaluminateand CO3

2−/monosulfoaluminate molar ratios were fixed to 2 and 1 re-spectively, and the stirring time varied from 1 to 28 days. The secondseries of experiments lasted for 28 days. The Cl-/monosulfoaluminatemolar ratios were fixed to n=0.2, 0.5, 1, 1.5, 2, 5 and the CO3

2−/monosulfoaluminate ratios to 0.25, 0.75, 1, 2, 3.5 and 5. At the endof an experiment, the suspensionwas filtered at 0.45 μm. The solid frac-tion was analyzed by X-ray powder diffraction and thermogravimetry.The anion concentrations in the solution were determined by ion chro-matography and total inorganic carbon analysis.

b

E

E

Ms

K

EE

E

Ms

K

E

E

Ms

F

K

E

E

Ms

F

K

a

Fig. 1. XRPD patterns recorded after sorption tests with chloride anions (zoom on (00l)reflexions). Influence of the reaction time (a) and of the initial chloride concentration(b) on the phase assemblage. Hydrates are labelled: Ms (calcium monosulfoaluminatehydrate), K (Kuzel's salt), E (ettringite) and F (Friedel's salt). n indicates the initialchloride/monosulfoaluminate molar ratio. The time series was performed at n=2.

2.2. Solid characterization

Thermogravimetric analyses were carried out under nitrogen on50±2 mg of sample using a TGA/DSC Netzsch STA409 PC instrumentat 10 °C/min up to 1000 °C.

X-ray powder diffraction (XRPD) patterns were recorded on aBruker D8 diffractometer, equipped with a “Lynx eye” detector,using Cu Kα radiation (λ=1.54184 Å). XRPD patterns wererecorded at room temperature within the interval 4°b2θb120°,with a step size Δ2θ=0.011° and a total counting time of 8 h foreach sample. The measurements were carried out with the DebyeScherrer geometry (capillary) to reduce the preferred orientationeffect. Quantitative phase analysis was performed using Rietveldmethod with the FullProf_Suite program [42]. All refinementsused the Thomson–Cox–Hastings Pseudo Voigt convoluted withan axial divergence asymmetry function [43]. The instrumentalfunction was extracted from pure silicon measured under thesame conditions.

Rietveld refinement were performed considering the structuralmodels described by Allmann for monosulfoaluminate [11], Rapin et al.for Friedel's salt [28], Mesbah et al. for Kuzel's salt [40], Goetz-Neunhoeffer and Neubauer for ettringite [44], François et al. for mon-ocarboaluminate [30] and for calcite [45]. Rietveld treatments wereused to characterize the mineralogical composition of the samples aftercontact with chloride and carbonate solutions (quantitative analyses),and to characterize the restricted Kuzel's salt solid solution (lattice pa-rameters and anionic contents in the interlayer regions). Concerningthe refinements involving the restricted Kuzel's salt solid solution, thefollowing three constraintswere applied: 1) full occupancy of the anionic3a and 3b sites by considering the presence ofwatermoleculewith XCl+XOw=1 in chloride interlayer andXS+XOw=1 in sulfate interlayer (X isan occupancy parameter, the corresponding atom being given in sub-script), 2) XCl+2 XS=1 to respect electroneutrality of the crystal,and 3) XOs1=XS and XOs2=3 XS to respect the sulfate geometry (Os1and Os2 are the apical and basal oxygen atoms of the sulfate anion, re-spectively). For more details, see the complete structural description ofKuzel's salt recently reported [40].

2.3. Solution analysis

Chloride and sulfate anions were analyzed using ionic chromatog-raphy (Dionex DX 500 equipped with AS9 HC analytical column andAG9 HC guard column, injection volume=100 μL, eluent=Na2CO3

9 mmol/L, flow rate=1 mL/min, detection = suppressed conductivi-ty (ASRS 300 suppressor in the autosuppression recycle mode)). Car-bonates were analyzed using a total organic carbon analyzer fromShimadzu (TOC V-VPN). The quantification limits were 0.5 mg/L forchloride, 1 mg/L for sulfate, and 0.1 mg/L for carbonates. Concentra-tions were measured with ±5% accuracy.

2.4. Thermodynamic modelling

Thermodynamic calculations were carried out using the CHESSsoftware [46]. The solubility constants of the selected cement phaseswere taken from the CEMDATA 07 database [47], and from [20] forKuzel's salt.

3. The CaO\Al2O3\CaSO4\CaCl2\H2O system

Details of the X-ray powder patterns corresponding to the chlo-ride sorption tests are presented in Fig. 1.

KMs

DT

G (

%/°

C)

Temperature(°C)

KMs

KMs

Fig. 2. DTG curves of the phase assemblage obtained after sorption tests (calciummonosulfoaluminate hydrates+NaCl): influence of contact time (Ms: calciummonosulfoaluminate hydrate, E: ettringite, K: Kuzel's salt) (n=initial chloride/monosulfoaluminate ratio=2).

Fig. 3. Refined molar amounts of hydrates at the end of the sorption tests (calciummonosulfoaluminate hydrate+NaCl): influence of contact time (a) and initial chlorideconcentration (b). n indicates the initial chloride/monosulfoaluminate ratio (n=2 forthe time series).

1159A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165

3.1. Investigating the exchange rate between sulfate and chloride anions

Five sorption tests were performed with a Cl−/monosulfoaluminatemolar ratio of 2, and increasing contact times of 1, 4, 7, 14 and 28 days.The corresponding X-ray patterns showed the presence of three hy-drates. Precipitation of Kuzel's salt and ettringite was already observedafter one day. From 4 days, the peak intensity of calcium mono-sulfoaluminate hydrate decreased significantly, while the relative in-tensities of the two other hydrates increased. After 28 days, calciummonosulfoaluminate hydrate was fully depleted. The diffraction peakclose to 2θ=10° with weak intensity corresponded to ettringite(Fig. 1). Thermogravimetry analysis (Fig. 2) confirmed the precipitationof ettringite and Kuzel's salt. The weight amounts of each crystallizedphase were determined by Rietveld refinement. Their equivalentmolar amounts are reported in Table 1 and presented in Fig. 3-a. The re-sults confirmed the previous qualitative observations. At 28 days, calci-um monosulfoaluminate hydrate was converted to Kuzel's salt andettringite in 77:23 molar ratio, not far from balance equation (Eq. (1))

9 ½3CaO ˙Al2O3 ˙CaSO4 ˙12H2O� þ 6 Cl− þ 28 H2O→

6 ½3CaO ˙Al2O3 ˙1=2CaSO4 ˙1=2CaCl2 ˙11H2O� þ 2 ½3CaO ˙Al2O3 ˙3CaSO4 ˙32H2O� þ 2

Al2O3 ˙3H2Oþ 6 OH−

ð1Þ

Table 1Influence of reaction time on the phase assemblage formed by mixing calcium monosulforatio=2). Indicated errors correspond to standard deviation extracted from Rietveld refine

Time(days)

Refined molar amounts (%) Calculated amounts (μm

Monosulfoaluminate(x)

Kuzel's salt(y)

Ettringite(z)

Monosulfoaluminate

0 100 0 0 8031 43.4±0.8 43.4±0.8 13.2±0.5 3274 20.5±0.8 62.9±1.0 16.7±1.0 1527 11.0±0.6 64.5±0.8 24.5±0.6 7914 7.4±0.6 71.7±0.8 20.8±0.6 5428 0 77.1±0.6 23.0±0.5 0

a Calculation performed by assuming the balance equation:[3CaO·Al2O3·CaSO4·12H2O]+α Cl−+β H2O→γ [3CaO·Al2O3·1/2CaSO4·1/2CaCl2·11H2O]calculated from the Rietveld refinement results and conservation of matter:

γ ¼ μ ¼ α ¼ 11þ3

2zy, δ ¼

zy

1þ32zy, ε ¼

12zy

1þ32zy, β ¼

312zy−1

2

1þ32zywhere y and z are the refined molar amounts of K

(where a0 stands for the initial mol number of monosulfoaluminate), and the precipitatedmol nb Amount derived from the measured concentration, by taking into account the volume decre

(Eq. (1)) assumed the precipitation of aluminum hydroxide. Thisphase,which can be poorly crystallized,was not detected byXRD. How-ever, even on the TGA diagrams (Fig. 2), we did not observe any charac-teristic weight loss near 250 °C, due to the dehydration of AH3. Thisresult could be explained by the small amount of AH3 formed, belowthe detection limit of the method. For instance, after 28 days, the AH3

amount should represent 0.9 wt.%, versus 59.0% for Kuzel's salt, and40.1% for ettringite (Table 1) according to the stoichiometry of (Eq. (1)).

aluminate hydrate with NaCl solution (n=initial chloride/monosulfoaluminate molarments.

ol)a Calculated residualchloride amount(mmol)

Measured residualchloride amount(mmol)b

Kuzel'ssalt

Ettringite Aluminumhydroxide

0 0 0 1.61 –

326 100 50 1.28 1.18466 124 62 1.14 1.22461 175 88 1.15 1.22522 152 76 1.09 1.20515 165 83 1.05 1.09

+δ [3CaO·Al2O3·3CaSO4·32H2O]+ε Al2O3·3H2O+μ OH−, stoichiometric coefficients

uzel's salt and ettringite.The depleted mol number of monosulfate is given by a ¼ a0yþ3

2z1þ1

2z

umbers of Kuzel's salt, ettringite and aluminum hydroxide are γ·a, δ·a and ε·a respectively.ase of the aqueous phase due to consumption of water.

1.0E+00

1.1E+00

1.2E+00

1.3E+00

1.4E+00

1.5E+00

1.6E+00

1.7E+00

1.8E+00

Time (days)

Res

idua

l chl

orid

e (m

mol

)

4.0E-034.5E-035.0E-035.5E-036.0E-036.5E-037.0E-037.5E-038.0E-038.5E-03

Rel

ease

d S

ulfa

te (

mm

ol)

ChlorideSulfate

b

0

0.5

1

1.5

2

2.5

3

3.5

4

0 5 10 15 20 25 30

0 1 2 3 4 5 6n

Res

idua

l chl

orid

e (m

mol

)

0

0.005

0.01

0.015

0.02

0.025

0.03

Res

idua

l sul

fate

(m

mol

)

Chloride

Sulfate

a

Fig. 4. Investigation of the chloride uptake by calcium monosulfoaluminate hydrate—evolution of the chloride and sulfate amounts in solution versus time (n=2) (a) orversus initial Cl−/monosulfoaluminate molar ratio (n=0.1 to 5) after 28 days of reac-tion (b).

Table 2Reaction of calcium monosulfoaluminate hydrate with chloride anions. Predictedphase assemblage composition at thermodynamic equilibrium.

Depletedchlorideamount(mmol)

Actualna

Correspondingtime(d)

Predicted amounts (μmol)

Monosulfoaluminate Kuzel'ssalt

Ettringite Gibbsite

0.433 0.539 1 485 190 68 660.388 0.484 4 522 165 60 610.394 0.490 7 519 167 61 610.426 0.530 14 491 186 67 650.536 0.667 28 401 248 86 78

a Molar ratio between depleted chloride and initial calcium sulfoaluminate hydrateamounts.

1160 A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165

Analysing the solutions for chloride and sulfate anions showed arapid decrease, by a factor 1.4, of the residual chloride concentrationduring the first day (Fig. 4-a). Then, depletion of chloride stronglyslowed down, and, after 28 days, the residual fraction still reached66%. On the contrary, only very small amounts of sulfate weredetected, irrespectively of the contact time, due to ettringite precipi-tation. The consistency between the refined data on the solid phaseassemblages and the solution analyses was checked by calculatingthe mol number of Kuzel's salt formed versus time, and the resultingresidual chloride concentration was compared with that determinedby ion chromatography. The deviation between the two results wasless than 10% (Table 1).

Kuzel's salt was the only chloride-containing AFm phase. Friedel'ssalt was not observed under our conditions, unlike Hirao et al. [19]who detected it directly after 2 days of experiment. Thermodynamiccalculations predicted the precipitation of Kuzel's salt, Friedel's salt,gibbsite and ettringite (Fig. 5). Even when the reacting chloride

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6n

Rel

ativ

e am

ount

of s

olid

(%

mol

)

MonosulfoKuzelFriedelEttringiteGibbsite

Fig. 5. Thermodynamic modelling of the phase assemblage formed in the calci-um sulfoaluminate hydrate–NaCl system as a function of n (initial chloride/monosulfoaluminate molar ratio).

concentration in the thermodynamic model was corrected by sub-stracting the residual concentration measured experimentally in solu-tion from the initial concentration, the calculated phase assemblagewas significantly different from that observed, with lower contents ofKuzel's salt and ettringite (Tables 1 and 2). The system was far fromthermodynamic equilibrium, apparently because of a delay in the pre-cipitation of Friedel's salt which should involve the solubilisation ofKuzel's salt.

3.2. Investigating the influence of the chloride concentration

Calciummonosulfoaluminate hydrate was mixed with solutions ofincreasing chloride concentration, corresponding to n ratios of 0.2,0.5, 1, 1.5, 2 and 5. After 28 days, the crystallized phases identifiedby X-ray diffraction were calcium monosulfoaluminate hydrate,Kuzel's salt, ettringite, and Friedel's salt. Their respective proportionswere quantified as a function of n by Rietveld refinement (Fig. 3-b,Table 3).

At low chloride concentrations (0bn≤1), residual calcium mono-sulfoaluminate hydrate dominated over Kuzel's salt and ettringite.The proportions of these two phases increased however with n. At in-termediate chloride concentrations (1bn≤2), calcium mono-sulfoaluminate hydrate was fully converted into Kuzel's salt andettringite. At higher chloride concentrations (n=5), Friedel's salt ad-ditionally precipitated. The chloride and sulfate concentrations in so-lution after 28 days increased almost linearly versus n (Fig. 4-b).Chlorides always remained in great excess over sulfates. The differentsystems were far from equilibrium after 28 days of reaction. Thermo-dynamic modelling predicted indeed the depletion of calcium mono-sulfoaluminate hydrate from n=1.3, the precipitation of Friedel's saltfrom n=1.65, and the depletion of Kuzel's salt from n=2.35 (Fig. 5).This showed again that the restricting parameter was the transforma-tion of Kuzel's salt into Friedel's salt.

Qualitative analysis of the X-ray patterns presented in Fig. 1showed a shift of the (003) reflexion of the Kuzel's salt (towards

Table 3Influence of the initial chloride concentration on the phase assemblage formed bymixing calcium monosulfoaluminate hydrate with NaCl solution (t=28 days). Indicat-ed errors correspond to standard deviation extracted from Rietveld refinements.

na Refined molar fraction (%)

Monosulfoaluminate Kuzel's salt Ettringite Friedel's salt

0.2 87.5±0.8 6.7±0.2 5.8±0.2 00.5 74.0±0.8 17.2±0.4 8.8±0.4 01 50.4±0.8 35.6±0.6 14.1±0.4 01.5 0 77.1±0.8 22.9±0.6 02 0 77.1±0.6 23.0±0.5 05 0 65.3±1.0 25.6±0.7 9.1±0.8

a n=initial chloride/monosulfoaluminate molar ratio.

Table 4Refined lattice parameters of Kuzel's salt obtained after reaction of calcium mono-sulfoaluminate hydrate with chloride anions with general formula of 3CaO·Al2O3·xCaCl2·(1−x)CaSO4·(12−2x)H2O. Standard deviations are indicated in parentheses.

na Time (days) a (Å) c (Å) Unit cell volume Refined x Cl2

2 1 5.7599(1) 50.6259(11) 1454.59(4) 0.40(1)2 4 5.7568(1) 50.5862(10) 1451.89(2) 0.43(1)2 7 5.7559(1) 50.5119(9) 1449.31(3) 0.44(1)2 14 5.7562(1) 50.5009(8) 1449.12(3) 0.44(1)2 28 5.7538(1) 50.4588(9) 1446.69(3) 0.46(2)0.2 28 5.7667(1) 50.7721(52) 1462.23(15) 0.36(1)0.5 28 5.7659(1) 50.6873(27) 1459.37(8) 0.38(1)1 28 5.7600(1) 50.6276(17) 1454.67(6) 0.41(1)2 28 5.7563(1) 50.5185(10) 1449.68(2) 0.43(1)1.5 28 5.7538(1) 50.4588(9) 1446.69(3) 0.46(1)5 28 5.7504(1) 50.3278(11) 1441.23(3) 0.49(1)

a n=initial chloride/monosulfoaluminate molar ratio.

1161A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165

smaller interlayer distance when increasing time of contact or in-creasing n), which could indicate the existence of a solid solution be-tween chloride and sulfate ions in the interlayer space of the AFmphase, as already postulated by Glasser et al. [15]. This was confirmedby the Rietveld refinement results (Table 4, Fig. 6). The lattice param-eters (a, b and unit cell volume) decreased linearly when the chloridecontent increased, following a Vegard's law. The decrease could beexplained by the smaller size of chloride (anionic radius=1.67 Å)compared to sulfate (around 2.4 Å). The chloride amount in the inter-layer space varied between 0.36(1)≤x≤0.50(1), giving general for-mula 3CaO·Al2O3·xCaCl2·(1−x)CaSO4·(12−2x)H2O. The solidsolution between sulfate and chloride had a restricted range, whichcould be linked to the structural properties of Kuzel's salt [40]. ThisAFm phase is a two-stage layered compound with two distinct

Fig. 6. Evolution of the lattice parameters of Kuzel's salt versus its chloride content (assCaSO4·(12−2x)H2O. Triangles correspond to the experiments aiming at investigating the cfluence of n (initial chloride/monosulfoaluminate molar ratio).

interlayers: one composed of chloride and the other composed of sul-fate anions. The evolution of the solid solution could be explained as-suming the substitution of one chloride anion by one water moleculein the chloride interlayer, and the insertion of ½SO4 in the sulfateinterlayer space with the departure of two water molecules. The sul-fate insertion into the sulfate interlayer region was allowed by thestatistic distribution between one sulfate anion and two water mole-cules in Kuzel's salt structure [40]. The full occupancy of the chloridesite in the Kuzel's salt structure (chloride interlayer region fully occu-pied with Cl− anions [40]) prevented the extension of the Kuzel's saltsolid solution towards the chloride side (i.e. with x>0.50). Fig. 7shows a projection along [100] of Kuzel's salt and of a solid solutionobtained by replacing 2 chloride ions by 2 water molecules in thechloride interlayer, and 2 water molecules by 1 sulfate ion in the ad-jacent sulfate interlayer.

4. CaO\Al2O3\CaSO4\CaCO3\H2O system

To investigate the possible exchange between sulfates and carbon-ates, calcium monosulfoaluminate hydrate was mixed with solutionsof sodium carbonate. The X-ray diffraction patterns recorded on thesolid phases after increasing contact times or carbonate concentra-tions are shown in Fig. 8.

4.1. Investigating the exchange rate between sulfate and carbonate ions

Five experiments were performed with a CO32−/monosulfoaluminate

molar ratio of 1, and contact times increasing from 1 to 28 days. Up to7 days, only two crystallized phases were detected: calcium mono-sulfoaluminate hydrate and calcite. Theirmolar fractionswere estimated(during the first week) to be 36±1 and 64±1% respectively by Rietveld

essed from Rietveld refinement) with general formula of 3CaO·Al2O3·xCaCl2·(1−x)ontact time between chlorides and sulfates, and squares to the experiments on the in-

Fig. 7. Structure of Kuzel's salt (a) and of Kuzel's salt enriched in sulphate (solid solution) (b) represented along [100]. For clarity reasons, distribution between one sulfate groupand two water molecules, and orientation disorder of sulfate groups (up and down) were ordered.

1162 A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165

refinement (Table 5, Fig. 9-a). A balance equation such as (Eq. (2))would lead to a maximum calcite fraction of 57.1% (and to a mini-mum monosulfoaluminate fraction of 42.9%) assuming the con-sumption of all carbonate ions, which is lower than the Rietveldestimation.

½3CaO ˙Al2O3 ˙CaSO4 ˙12H2O� þ 4 CO2�3 →4 CaCO3 þ SO

2�4

þ ½Al2O3 ˙3H2O� þ 6 OH� þ H2O ð2Þ

Additional information was provided by the analysis of theaqueous fractions (Table 6). Given the residual concentration ofcarbonates and the released concentration of sulfates, the molarratio between calcium monosulfoaluminate hydrate and calciteshould be equal to 0.726, which corresponds to relative fractionsof 57.9% and 42.1% for the two phases, and agrees well with bal-ance equation (Eq. (2)) given measurement errors. The Rietveldanalysis thus slightly overestimated calcite, and/or underestimatedcalcium monosulfoaluminate hydrate. This could result from verydifferent crystallinities of the two phases.

After 7 days, the calcite content decreased while calcium mon-ocarboaluminate hydrate and ettringite precipitated. At 28 days, calci-um monosulfoaluminate hydrate was almost fully depleted, and thephase assemblage comprised ettringite, calciummonocarboaluminatehydrate, and calcite in respective proportions of 32/21/47 according toRietveld refinement. Calcium hemicarboaluminate hydrate was neverobserved, which could be explained by the absence of portlandite in

the investigated systems. The Rietveld analysis suggested a balanceequation such as (Eq. (3)).

75:25 ½3CaO ˙Al2O3 ˙CaSO4 ˙12H2O� þ 79CO2�3 þ 254:5H2O→32

½3CaO ˙Al2O3 ˙CaCO3 ˙11H2O� þ 21 ½3CaO ˙Al2O3 ˙3CaSO4 ˙32H2O� þ 47 CaCO3 þ 22:25

½Al2O3 ˙3H2O� þ 12:25 SO2�4 þ 133:5 OH

−

ð3Þ

Given the residual carbonate concentration measured in the aque-ous fraction (Table 6), the sulfate amount theoretically releasedaccording to (Eq. (3)) was calculated to be 0.120 mmol, in rathergood agreement with the experimental determination (0.140±0.010 mol). However, under these conditions, the residual amountof calcium monosulfoaluminante hydrate should be slightly higherthan that estimated by Rietveld refinement (monosulfoaluminate/monocarboaluminate/ettringite/calcite in 6/30/20/44 molar propor-tions, instead of 0.4/31.7/20.7/47.2). Once again, the Rietveld analysisseemed to overestimate slightly calcite and/or underestimate calciummonosulfoaluminate hydrate. The broadening of the diffraction peaksof calcium monosulfoaluminate hydrate probably affected the qualityof the quantification. The phase assemblage was close to thermody-namic equilibrium (Fig. 10). Calculations predicted the complete de-pletion of calcium monosulfoaluminate hydrate, and the precipitationof gibbsite, calciummonocarboaluminate hydrate, ettringite and calcite,with 35/22/43 proportions for monocarboaluminate, ettringite andcalcite, respectively.

Fig. 8. XRPD patterns recorded after sorption tests with carbonate anions (zoom on(00l) reflexions). Influence of the reaction time (a) and the initial carbonate concentra-tion (b) on the phase assemblage. Hydrates are labelled: Ms (calcium mono-sulfoaluminate hydrate), Mc (calcium monocarboaluminate hydrate), E (ettringite)and C (calcite). n indicates the initial carbonate/monosulfoaluminate molar ratio. Thetime series was performed at n=1.

Fig. 9. Refined molar amounts of hydrates at the end of the sorption tests (calciummonosulfoaluminate hydrate+Na2CO3); influence of contact time (a) and initial car-bonate concentration (b). n indicates the initial carbonate/monosulfoaluminatemolar ratio. The time series was performed at n=1.

1163A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165

4.2. Investigating the influence of the carbonate concentration

Calcium monosulfoaluminate hydrate was mixed for 28 days withsolutions containing increasing carbonate concentrations, giving n ra-tios of 0.25, 0.75, 1, 2, 3.5 and 5. The evolution of the respective

Table 5Influence of reaction time and initial carbonate concentration on the phase assemblageformed by mixing calcium monosulfoaluminate hydrate with Na2CO3 solution. Indicat-ed errors correspond to standard deviation extracted from Rietveld refinements.

na t(days)

Refined molar fraction (%)

Monosulfo-aluminate

Ettringite Monocarbo-aluminate

Calcite

1 1 36.5±0.7 0 0 63.5±0.41 4 36.5±0.7 0 0 63.5±0.41 7 35.3±0.8 0 0 64.7±0.41 14 31.3±0.7 2.4±0.4 0.9±0.4 65.5±0.41 28 0.4±0.1 20.8±1.2 31.7±0.3 47.2±0.80.25 28 64.7±0.8 23.9±0.6 11.4±0.4 00.75 28 5.4±0.6 24.2±1.1 32.1±1.6 38.4±0.42 28 7.9±1.0 0 11.7±1.5 80.4±0.83.5 28 1.2±0.6 0 3.5±1 95.3±0.85 28 0 0 0 100

a n=initial carbonate/monosulfoaluminate molar ratio.

proportions of calcium monosulfoaluminate hydrate, ettringite, cal-cite and monocarboaluminate, estimated from Rietveld refinement,is shown in Fig. 9-b. At low carbonate concentration (n=0.25), calci-um monosulfoaluminate hydrate was partly converted into ettringiteand calciummonocarboaluminate hydrate, according to mass balanceequation (Eq. (4)).

7=2 ½3CaO ˙Al2O3 ˙CaSO4 ˙12H2O� þ 2 CO2�3 þ 15 H2O→2 ½3CaO ˙Al2O3 ˙CaCO3 ˙11H2O� þ

½3CaO ˙Al2O3 ˙3CaSO4 ˙32H2O� þ 1=2 ½Al2O3 ˙3H2O� þ 1=2 SO2−4 þ 3 OH

− ð4Þ

According to this equation, the expected sulfate and carbonateamounts in the aqueous phase would be 0.045 mmol and 0.019 mmol,

Table 6Amounts (mmol) of carbonates and sulfates in the aqueous fractions at the end of thesorption tests (initial calcium monosulfoaluminate hydrate amount=0.803 mmol).

na t (days) SO42− CO3

2−

1 1 0.239 0.0251 4 0.244 0.0321 7 Not measured 0.0261 14 0.220 0.0331 28 0.141 0.0280.25 28 0.044 0.0122 28 0.547 0.0455 28 0.800 0.724

a n=initial carbonate/monosulfoaluminate molar ratio.

0102030405060708090

100

n

Rel

ativ

e am

out o

f sol

id(m

ol%

)

Monosulfo

Calcite

Monocarbo

Ettringite

Gibbsite

b

0102030405060708090

100

0 1 2 3 4 5 6

0 1 2 3 4 5 6

n

Rel

ativ

e am

out o

f sol

id(m

ol%

)

Monosulfo

Calcite

Monocarbo

Ettringite

exp Monosulfo

exp Monocarbo

exp Ettringite

exp Calcite

a

Fig. 10. (a) Thermodynamic modelling of the phase assemblage formed in the calci-um sulfoaluminate hydrate–Na2CO3 system as a function of n (initial carbonate/monosulfoaluminate molar ratio). (b) Comparison with experimental results at28 days (gibbsite excluded).

1164 A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165

given the depleted amount of calcium monosulfoaluminate hydrateassessed from Rietveld analysis (0.317 mmol). This was in rather goodagreement with the analyses of solution (0.044 mmol of sulfates, and0.012 mmol of carbonates—Table 6).

Increasing n led to the additional precipitation of calcite. Calciummonosulfoaluminate hydrate was almost fully depleted at n=1 but,quite unexpectedly, was detected again at n=2 and 3.5. Fromn=2, ettringite was not observed any longer. The fraction of calciummonocarboaluminate hydrate decreased when n increased beyond 1,while that of calcite went on increasing. At n=5, calcite was the soledetected crystallized phase. These results, as well as the sulfateamount measured in solution (0.800 mmol), were consistent withmass balance equation (Eq. (5)).

½3CaO ˙Al2O3 ˙CaSO4 ˙12H2O� þ 4 CO2−3 →4 CaCO3 þ SO

2−4 þ ½Al2O3 ˙3H2O� þ 6 OH

− þ 6 H2O

ð5ÞComparing the experimental results with the phase assemblage

evolution predicted by thermodynamics showed two main differ-ences (Fig. 10-b) the persistence of calcium monosulfoaluminate hy-drate at n=2 and 3.5, and the early disappearance of ettringite (fromn=2 instead of 3.1 for the model). Thermodynamic modelling wasalso performed by considering amorphous Al(OH)3 instead of less sol-uble gibbsite in the database. In that case, ettringite disappeared forn=2.9, a value still significantly higher than the experimental one.Thus, the type of AH3 selected in the database could not explain thedeviation between the modelled and experimental data at high n(n≥2). It seems likely that the corresponding samples were not atthermodynamic equilibrium after 28 days.

5. Conclusion

These results show that calcium monosulfoaluminate hydrate hasa better potential for insolubilizing carbonates than chlorides. Whenequimolar amounts of calcium monosulfoaluminate hydrate and car-bonates or chloride are made to react (0.803 mmol under the investi-gated conditions), the residual concentrations at thermodynamic

equilibrium are calculated to be 32 times smaller for carbonatesthan for chloride (12.59 μmol against 402.6 μmol). Moreover, carbon-ates are muchmore readily insolubilized. Under the same experimen-tal conditions, the aqueous concentration of carbonates is reduced bya factor 32 after one day for carbonates, and that of chlorides by a fac-tor 1.26 only.

Carbonates are rapidly depleted to formcalciummonocarboaluminatehydrate (nb0.5), calcium monocarboaluminate hydrate and calcite(0.5≤n≤4.3), or calcite only (n>4.3) at thermodynamic equilibrium.At n=1, the reaction occurs by the precipitation of calcite which is laterpartly destabilized into calciummonocarboaluminate hydrate.

Chloride ions react more slowly. Under the investigated experi-mental conditions, thermodynamic equilibrium is not achieved after28 days at ambient temperature. The final chloride-containing prod-ucts are Kuzel's salt (0.2≤nb1.65), Kuzel's and Friedel's salts(1.65≤n≤2.35), and Friedel's salt only at higher n. The reaction pro-ceeds by the transient precipitation of Kuzel's salt which is laterconverted into Friedel's salt.

Rietveld analysis was successfully used for a quantitative analysis ofthe solid phase distribution. The lattice parameters of Kuzel's salt wereshown to decrease linearly when the chloride concentration increased,which clearly demonstrates the occurrence of a solid solution betweensulphate and chloride within a restricted range of compositions3CaO·Al2O3·xCaCl2·(1−x)CaSO4·(12−2x)H2O with 0.36≤x≤0.50,towards the sulfate side.

Acknowledgments

The authors are grateful to Pascal Antonucci for his help on thesynthesis experiments. Laurent Petit, from Electricité de France, isdeeply acknowledged for his support on the study.

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