Cement and Concrete Research 42 (2012) 1157–1165
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Cement and Concrete Research
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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, France
Clermont Université, ENSCCF, Laboratoire des Matériaux Inorganiques, BP 10448, F-63000 Clermont-Ferrand, France
c
Clermont Université, Université Blaise Pascal, Laboratoire des Matériaux Inorganiques, BP 10448, F-63000 Clermont-Ferrand, France
d
CNRS, UMR 6002, F-63177 Aubière, France
b
a r t i c l e
i n f o
Article history:
Received 14 February 2012
Accepted 16 May 2012
Keywords:
Waste management (E)
Monosulfate (D)
Chloride (D)
Carbonation (C)
X-ray diffraction (B)
a b s t r a c t
Decommissioning of old nuclear reactors may produce waste streams containing chlorides and carbonates,
including radioactive 36Cl − and 14CO32−. Their insolubilization by calcium monosulfoaluminate hydrate was
investigated. Carbonates were readily depleted from the solution, giving at thermodynamic equilibrium
monocarboaluminate, monocarboaluminate + calcite, or calcite only, depending on the initial ratio between
the anion and calcium monosulfoaluminate hydrate. Chloride ions reacted more slowly and were precipitated as Kuzel's salt, Kuzel's and Friedel's salts, or Friedel's salt only. Rietveld refinement of X-Ray powder diffraction patterns was successfully used to quantify the phase distributions, which were compared to
thermodynamic calculations. Moreover, analysing the lattice parameters of Kuzel's salt as a function of its
chloride content showed the occurrence of a restricted solid solution towards the sulfate side with general
formula 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 potential
to stabilize hazardous wastes such as heavy metals [1–7], ion exchange resins [8], aluminum-containing wastes [9] or radioactive
streams containing high amounts of borate and sulfate ions [10]. Unlike Portland cement, hydration of CSA cement leads to the formation
of two principal hydrates: calcium monosulfoaluminate hydrate [11]
and ettringite [12] with general formula 3CaO·Al2O3·CaSO4·12H2O
and 3CaO·Al2O3·3CaSO4·32H2O, respectively. Their contents are
very sensitive to the amount of added sulfate: the former dominates
in a gypsum-deficient environment, whereas the latter tends to increase in a gypsum-rich system [13]. Both ettringite and calcium
monosulfoaluminate hydrate have a flexible structure and can admit
various 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, in
waste streams produced during the decommissioning of old nuclear
reactors (UNGG-type, graphite moderated, cooled by carbon dioxide,
and fuelled with natural uranium metal). In the literature, AFm
phases are considered as good candidates to bind chloride anions
contrarily to AFt phases [15,19,20]. Two Cl-containing AFm phases
are involved: Kuzel's salt 3CaO·Al2O3·1/2CaSO4·1/2CaCl2·11H2O,
a crystallized compound containing ordered chloride and sulfate
anions, and Friedel's salt 3CaO·Al2O3·CaCl2·10H2O, a phase containing
chloride anions only. Moreover, it is well known that cement-based materials can be easily carbonated, leading to the formation of calcium
hemicarboaluminate hydrate 3.5CaO·Al2O3·1/2CaCO3·12H2O, calcium
monocarboaluminate hydrate 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 charged
main layers [Ca2Al(OH)6] + and negatively charged interlayers
[X·nH2O] −, where X is one monovalent anion or half a divalent
anion. The following general formulae 3CaO∙Al2O3∙CaX2∙nH2O for a
monovalent anion, or 3CaO∙Al2O3∙CaX∙nH2O for a divalent anion,
are generally used in cement chemistry. Several structural studies
have been performed on AFm compounds incorporating one type
of anion only in the interlayer: SO42− [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], CO32−\Cl − [37–39] or SO42−\Cl − permutation [40].
The present study was undertaken within a project aiming at
assessing the binding capacities of chloride and carbonate anions by
hydrated CSA cements. The first step was to investigate the uptake
of Cl − and CO32− by calcium monosulfoaluminate hydrate.
2. Experimental section
2.1. Samples preparation
⁎ Corresponding author. Tel.: + 33 4 66 39 74 50; fax: + 33 4 66 33 90 37.
E-mail address: [email protected] (C. Cau-dit-Coumes).
0008-8846/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.cemconres.2012.05.012
Calcium monosulfoaluminate hydrate with general formula
3CaO∙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 during
1 h and cooled under nitrogen atmosphere) to reach a final water/
solid mass ratio of 50. The suspension was introduced in a Teflon reactor and stirred continuously at 80 °C during three weeks. The temperature was chosen to avoid the formation of ettringite, which is
unstable over ~70 °C under these conditions [41]. Then, the suspension was filtered twice using pure water and finally rinsed with isopropanol. The precipitates were subsequently dried in a desiccator
over potassium acetate (≈20% r.h.) at room temperature.
Sorption tests were carried out by adding 0.5 g (8.03 × 10 − 4 mole)
of calcium monosulfoaluminate hydrate to 10 mL of sodium chloride
(NaCl) or sodium carbonate (Na2CO3) solutions. The suspensions
were kept under stirring at 20 °C. Two series of sorption tests were
performed, firstly to study the exchange rate between sulfate and
chloride or carbonate, and secondly to investigate the influence of
the initial concentration of chloride or carbonate on this exchange.
In the first series of experiments, the Cl −/monosulfoaluminate
and CO32−/monosulfoaluminate molar ratios were fixed to 2 and 1 respectively, and the stirring time varied from 1 to 28 days. The second
series of experiments lasted for 28 days. The Cl -/monosulfoaluminate
molar ratios were fixed to n = 0.2, 0.5, 1, 1.5, 2, 5 and the CO32−/
monosulfoaluminate ratios to 0.25, 0.75, 1, 2, 3.5 and 5. At the end
of an experiment, the suspension was filtered at 0.45 μm. The solid fraction was analyzed by X-ray powder diffraction and thermogravimetry.
The anion concentrations in the solution were determined by ion chromatography and total inorganic carbon analysis.
2.3. Solution analysis
Chloride and sulfate anions were analyzed using ionic chromatography (Dionex DX 500 equipped with AS9 HC analytical column and
AG9 HC guard column, injection volume = 100 μL, eluent = Na2CO3
9 mmol/L, flow rate = 1 mL/min, detection = suppressed conductivity (ASRS 300 suppressor in the autosuppression recycle mode)). Carbonates were analyzed using a total organic carbon analyzer from
Shimadzu (TOC V-VPN). The quantification limits were 0.5 mg/L for
chloride, 1 mg/L for sulfate, and 0.1 mg/L for carbonates. Concentrations were measured with ±5% accuracy.
2.4. Thermodynamic modelling
Thermodynamic calculations were carried out using the CHESS
software [46]. The solubility constants of the selected cement phases
were taken from the CEMDATA 07 database [47], and from [20] for
Kuzel's salt.
3. The CaO\Al2O3\CaSO4\CaCl2\H2O system
Details of the X-ray powder patterns corresponding to the chloride sorption tests are presented in Fig. 1.
a
K
E
2.2. Solid characterization
Thermogravimetric analyses were carried out under nitrogen on
50 ± 2 mg of sample using a TGA/DSC Netzsch STA409 PC instrument
at 10 °C/min up to 1000 °C.
X-ray powder diffraction (XRPD) patterns were recorded on a
Bruker D8 diffractometer, equipped with a “Lynx eye” detector,
using Cu Kα radiation (λ = 1.54184 Å). XRPD patterns were
recorded at room temperature within the interval 4° b 2θ b 120°,
with a step size Δ2θ = 0.011° and a total counting time of 8 h for
each sample. The measurements were carried out with the Debye
Scherrer geometry (capillary) to reduce the preferred orientation
effect. Quantitative phase analysis was performed using Rietveld
method with the FullProf_Suite program [42]. All refinements
used the Thomson–Cox–Hastings Pseudo Voigt convoluted with
an axial divergence asymmetry function [43]. The instrumental
function was extracted from pure silicon measured under the
same conditions.
Rietveld refinement were performed considering the structural
models described by Allmann for monosulfoaluminate [11], Rapin et al.
for Friedel's salt [28], Mesbah et al. for Kuzel's salt [40], GoetzNeunhoeffer and Neubauer for ettringite [44], François et al. for monocarboaluminate [30] and for calcite [45]. Rietveld treatments were
used to characterize the mineralogical composition of the samples after
contact with chloride and carbonate solutions (quantitative analyses),
and to characterize the restricted Kuzel's salt solid solution (lattice parameters and anionic contents in the interlayer regions). Concerning
the refinements involving the restricted Kuzel's salt solid solution, the
following three constraints were applied: 1) full occupancy of the anionic
3a and 3b sites by considering the presence of water molecule with XCl +
XOw =1 in chloride interlayer and XS + XOw =1 in sulfate interlayer (X is
an occupancy parameter, the corresponding atom being given in subscript), 2) XCl + 2 XS = 1 to respect electroneutrality of the crystal,
and 3) XOs1 = XS and XOs2 =3 XS to respect the sulfate geometry (Os1
and Os2 are the apical and basal oxygen atoms of the sulfate anion, respectively). For more details, see the complete structural description of
Kuzel's salt recently reported [40].
Ms
E
b
K
E
Ms
E
F
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 monosulfoaluminate
hydrate), K (Kuzel's salt), E (ettringite) and F (Friedel's salt). n indicates the initial
chloride/monosulfoaluminate molar ratio. The time series was performed at n = 2.
A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165
1159
DTG (%/°C)
Ms
K
Temperature(°C)
Fig. 2. DTG curves of the phase assemblage obtained after sorption tests (calcium
monosulfoaluminate hydrates + NaCl): influence of contact time (Ms: calcium
monosulfoaluminate hydrate, E: ettringite, K: Kuzel's salt) (n = initial chloride/
monosulfoaluminate ratio = 2).
3.1. Investigating the exchange rate between sulfate and chloride anions
Five sorption tests were performed with a Cl−/monosulfoaluminate
molar 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 hydrates. Precipitation of Kuzel's salt and ettringite was already observed
after one day. From 4 days, the peak intensity of calcium monosulfoaluminate hydrate decreased significantly, while the relative intensities of the two other hydrates increased. After 28 days, calcium
monosulfoaluminate hydrate was fully depleted. The diffraction peak
close to 2θ = 10° with weak intensity corresponded to ettringite
(Fig. 1). Thermogravimetry analysis (Fig. 2) confirmed the precipitation
of ettringite and Kuzel's salt. The weight amounts of each crystallized
phase were determined by Rietveld refinement. Their equivalent
molar amounts are reported in Table 1 and presented in Fig. 3-a. The results confirmed the previous qualitative observations. At 28 days, calcium monosulfoaluminate hydrate was converted to Kuzel's salt and
ettringite in 77:23 molar ratio, not far from balance equation (Eq. (1))
−
ð1Þ
9 ½3CaO ˙ Al2 O3 ˙ CaSO4 ˙ 12H2 O þ 6 Cl þ 28 H2 O→
6 ½3CaO ˙ Al2 O3 ˙ 1=2CaSO4 ˙ 1=2CaCl2 ˙ 11H2 O þ 2 ½3CaO ˙ Al2 O3 ˙ 3CaSO4 ˙ 32H2 O þ 2
−
Al2 O3 ˙ 3H2 O þ 6 OH
Fig. 3. Refined molar amounts of hydrates at the end of the sorption tests (calcium
monosulfoaluminate hydrate + NaCl): influence of contact time (a) and initial chloride
concentration (b). n indicates the initial chloride/monosulfoaluminate ratio (n = 2 for
the time series).
(Eq. (1)) assumed the precipitation of aluminum hydroxide. This
phase, which can be poorly crystallized, was not detected by XRD. However, even on the TGA diagrams (Fig. 2), we did not observe any characteristic weight loss near 250 °C, due to the dehydration of AH3. This
result could be explained by the small amount of AH3 formed, below
the 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, and
40.1% for ettringite (Table 1) according to the stoichiometry of (Eq. (1)).
Table 1
Influence of reaction time on the phase assemblage formed by mixing calcium monosulfoaluminate hydrate with NaCl solution (n = initial chloride/monosulfoaluminate molar
ratio = 2). Indicated errors correspond to standard deviation extracted from Rietveld refinements.
Calculated amounts (μmol)a
Time
(days)
Refined molar amounts (%)
Monosulfoaluminate
(x)
Kuzel's salt
(y)
Ettringite
(z)
Monosulfoaluminate
Kuzel's
salt
Ettringite
Aluminum
hydroxide
0
1
4
7
14
28
100
43.4 ± 0.8
20.5 ± 0.8
11.0 ± 0.6
7.4 ± 0.6
0
0
43.4 ± 0.8
62.9 ± 1.0
64.5 ± 0.8
71.7 ± 0.8
77.1 ± 0.6
0
13.2 ± 0.5
16.7 ± 1.0
24.5 ± 0.6
20.8 ± 0.6
23.0 ± 0.5
803
327
152
79
54
0
0
326
466
461
522
515
0
100
124
175
152
165
0
50
62
88
76
83
Calculated residual
chloride amount
(mmol)
Measured residual
chloride amount
(mmol)b
1.61
1.28
1.14
1.15
1.09
1.05
–
1.18
1.22
1.22
1.20
1.09
a
Calculation performed by assuming the balance equation:
[3CaO·Al2O3·CaSO4·12H2O]+ α Cl− + β H2O → γ [3CaO·Al2O3·1/2CaSO4·1/2CaCl2·11H2O]+ δ [3CaO·Al2O3·3CaSO4·32H2O]+ ε Al2O3·3H2O + μ OH−, stoichiometric coefficients
calculated from the Rietveld refinement results and conservation of matter:
z
1z
γ ¼ μ ¼ α ¼ 1þ13z, δ ¼ 1þy 3z, ε ¼ 1þ2y3z, β ¼
2y
2y
2y
31z 1
2 y−2
1þ32yz
yþ3z
where y and z are the refined molar amounts of Kuzel's salt and ettringite.The depleted mol number of monosulfate is given by a ¼ a0 1þ21z
2
(where a0 stands for the initial mol number of monosulfoaluminate), and the precipitated mol numbers of Kuzel's salt, ettringite and aluminum hydroxide are γ·a, δ·a and ε·a respectively.
b
Amount derived from the measured concentration, by taking into account the volume decrease of the aqueous phase due to consumption of water.
1160
A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165
1.8E+00
1.7E+00
1.6E+00
1.5E+00
1.4E+00
1.3E+00
1.2E+00
1.1E+00
1.0E+00
8.5E-03
8.0E-03
7.5E-03
7.0E-03
6.5E-03
6.0E-03
5.5E-03
5.0E-03
4.5E-03
4.0E-03
Chloride
Sulfate
0
5
10
15
20
25
Released Sulfate (mmol)
Residual chloride (mmol)
a
30
Time (days)
Chloride
0.025
Sulfate
0.02
2.5
0.015
2
1.5
0.01
1
0.005
0.5
0
0
0
1
2
3
4
5
Residual sulfate (mmol)
Residual chloride (mmol)
0.03
4
3
Depleted Actual Corresponding Predicted amounts (μmol)
chloride na
time
Monosulfoaluminate Kuzel's Ettringite Gibbsite
amount
(d)
salt
(mmol)
0.433
0.388
0.394
0.426
0.536
0.539
0.484
0.490
0.530
0.667
1
4
7
14
28
485
522
519
491
401
190
165
167
186
248
68
60
61
67
86
66
61
61
65
78
a
Molar ratio between depleted chloride and initial calcium sulfoaluminate hydrate
amounts.
b
3.5
Table 2
Reaction of calcium monosulfoaluminate hydrate with chloride anions. Predicted
phase assemblage composition at thermodynamic equilibrium.
6
concentration in the thermodynamic model was corrected by substracting the residual concentration measured experimentally in solution from the initial concentration, the calculated phase assemblage
was significantly different from that observed, with lower contents of
Kuzel's salt and ettringite (Tables 1 and 2). The system was far from
thermodynamic equilibrium, apparently because of a delay in the precipitation of Friedel's salt which should involve the solubilisation of
Kuzel's salt.
n
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) or
versus initial Cl−/monosulfoaluminate molar ratio (n = 0.1 to 5) after 28 days of reaction (b).
Relative amount of solid (% mol)
Analysing the solutions for chloride and sulfate anions showed a
rapid decrease, by a factor 1.4, of the residual chloride concentration
during the first day (Fig. 4-a). Then, depletion of chloride strongly
slowed down, and, after 28 days, the residual fraction still reached
66%. On the contrary, only very small amounts of sulfate were
detected, irrespectively of the contact time, due to ettringite precipitation. The consistency between the refined data on the solid phase
assemblages and the solution analyses was checked by calculating
the mol number of Kuzel's salt formed versus time, and the resulting
residual chloride concentration was compared with that determined
by ion chromatography. The deviation between the two results was
less than 10% (Table 1).
Kuzel's salt was the only chloride-containing AFm phase. Friedel's
salt was not observed under our conditions, unlike Hirao et al. [19]
who detected it directly after 2 days of experiment. Thermodynamic
calculations predicted the precipitation of Kuzel's salt, Friedel's salt,
gibbsite and ettringite (Fig. 5). Even when the reacting chloride
100
Monosulfo
Kuzel
Friedel
Ettringite
Gibbsite
90
80
70
3.2. Investigating the influence of the chloride concentration
Calcium monosulfoaluminate hydrate was mixed with solutions of
increasing chloride concentration, corresponding to n ratios of 0.2,
0.5, 1, 1.5, 2 and 5. After 28 days, the crystallized phases identified
by X-ray diffraction were calcium monosulfoaluminate hydrate,
Kuzel's salt, ettringite, and Friedel's salt. Their respective proportions
were quantified as a function of n by Rietveld refinement (Fig. 3-b,
Table 3).
At low chloride concentrations (0 b n ≤ 1), residual calcium monosulfoaluminate hydrate dominated over Kuzel's salt and ettringite.
The proportions of these two phases increased however with n. At intermediate chloride concentrations (1 b n ≤ 2), calcium monosulfoaluminate hydrate was fully converted into Kuzel's salt and
ettringite. At higher chloride concentrations (n = 5), Friedel's salt additionally precipitated. The chloride and sulfate concentrations in solution after 28 days increased almost linearly versus n (Fig. 4-b).
Chlorides always remained in great excess over sulfates. The different
systems were far from equilibrium after 28 days of reaction. Thermodynamic modelling predicted indeed the depletion of calcium monosulfoaluminate hydrate from n = 1.3, the precipitation of Friedel's salt
from 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 transformation of Kuzel's salt into Friedel's salt.
Qualitative analysis of the X-ray patterns presented in Fig. 1
showed a shift of the (003) reflexion of the Kuzel's salt (towards
Table 3
Influence of the initial chloride concentration on the phase assemblage formed by
mixing calcium monosulfoaluminate hydrate with NaCl solution (t = 28 days). Indicated errors correspond to standard deviation extracted from Rietveld refinements.
60
50
40
30
na
20
Refined molar fraction (%)
Monosulfoaluminate
10
0
0
1
2
3
4
5
6
n
Fig. 5. Thermodynamic modelling of the phase assemblage formed in the calcium sulfoaluminate hydrate–NaCl system as a function of n (initial chloride/
monosulfoaluminate molar ratio).
0.2
0.5
1
1.5
2
5
a
87.5 ± 0.8
74.0 ± 0.8
50.4 ± 0.8
0
0
0
Kuzel's salt
6.7 ± 0.2
17.2 ± 0.4
35.6 ± 0.6
77.1 ± 0.8
77.1 ± 0.6
65.3 ± 1.0
Ettringite
5.8 ± 0.2
8.8 ± 0.4
14.1 ± 0.4
22.9 ± 0.6
23.0 ± 0.5
25.6 ± 0.7
n = initial chloride/monosulfoaluminate molar ratio.
Friedel's salt
0
0
0
0
0
9.1 ± 0.8
A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165
Table 4
Refined lattice parameters of Kuzel's salt obtained after reaction of calcium monosulfoaluminate 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
2
2
2
2
0.2
0.5
1
2
1.5
5
1
4
7
14
28
28
28
28
28
28
28
5.7599(1)
5.7568(1)
5.7559(1)
5.7562(1)
5.7538(1)
5.7667(1)
5.7659(1)
5.7600(1)
5.7563(1)
5.7538(1)
5.7504(1)
50.6259(11)
50.5862(10)
50.5119(9)
50.5009(8)
50.4588(9)
50.7721(52)
50.6873(27)
50.6276(17)
50.5185(10)
50.4588(9)
50.3278(11)
1454.59(4)
1451.89(2)
1449.31(3)
1449.12(3)
1446.69(3)
1462.23(15)
1459.37(8)
1454.67(6)
1449.68(2)
1446.69(3)
1441.23(3)
0.40(1)
0.43(1)
0.44(1)
0.44(1)
0.46(2)
0.36(1)
0.38(1)
0.41(1)
0.43(1)
0.46(1)
0.49(1)
a
n = initial chloride/monosulfoaluminate molar ratio.
1161
interlayers: one composed of chloride and the other composed of sulfate anions. The evolution of the solid solution could be explained assuming the substitution of one chloride anion by one water molecule
in the chloride interlayer, and the insertion of ½SO4 in the sulfate
interlayer space with the departure of two water molecules. The sulfate insertion into the sulfate interlayer region was allowed by the
statistic distribution between one sulfate anion and two water molecules in Kuzel's salt structure [40]. The full occupancy of the chloride
site in the Kuzel's salt structure (chloride interlayer region fully occupied with Cl − anions [40]) prevented the extension of the Kuzel's salt
solid solution towards the chloride side (i.e. with x > 0.50). Fig. 7
shows a projection along [100] of Kuzel's salt and of a solid solution
obtained by replacing 2 chloride ions by 2 water molecules in the
chloride interlayer, and 2 water molecules by 1 sulfate ion in the adjacent sulfate interlayer.
4. CaO\Al2O3\CaSO4\CaCO3\H2O system
smaller interlayer distance when increasing time of contact or increasing n), which could indicate the existence of a solid solution between chloride and sulfate ions in the interlayer space of the AFm
phase, as already postulated by Glasser et al. [15]. This was confirmed
by the Rietveld refinement results (Table 4, Fig. 6). The lattice parameters (a, b and unit cell volume) decreased linearly when the chloride
content increased, following a Vegard's law. The decrease could be
explained by the smaller size of chloride (anionic radius = 1.67 Å)
compared to sulfate (around 2.4 Å). The chloride amount in the interlayer space varied between 0.36(1) ≤ x ≤ 0.50(1), giving general formula 3CaO·Al2O3·xCaCl2·(1 − x)CaSO4·(12 − 2x)H2O. The solid
solution between sulfate and chloride had a restricted range, which
could be linked to the structural properties of Kuzel's salt [40]. This
AFm phase is a two-stage layered compound with two distinct
To investigate the possible exchange between sulfates and carbonates, calcium monosulfoaluminate hydrate was mixed with solutions
of sodium carbonate. The X-ray diffraction patterns recorded on the
solid phases after increasing contact times or carbonate concentrations 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 to
7 days, only two crystallized phases were detected: calcium monosulfoaluminate hydrate and calcite. Their molar fractions were estimated
(during the first week) to be 36 ±1 and 64±1% respectively by Rietveld
Fig. 6. Evolution of the lattice parameters of Kuzel's salt versus its chloride content (assessed from Rietveld refinement) with general formula of 3CaO·Al2O3·xCaCl2·(1 − x)
CaSO4·(12 − 2x)H2O. Triangles correspond to the experiments aiming at investigating the contact time between chlorides and sulfates, and squares to the experiments on the influence of n (initial chloride/monosulfoaluminate molar ratio).
1162
A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165
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 group
and two water molecules, and orientation disorder of sulfate groups (up and down) were ordered.
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 minimum monosulfoaluminate fraction of 42.9%) assuming the consumption of all carbonate ions, which is lower than the Rietveld
estimation.
2
2
75:25 ½3CaO ˙ Al2 O3 ˙ CaSO4 ˙ 12H2 O þ 79CO3 þ 254:5H2 O→32
ð3Þ
½3CaO ˙ Al2 O3 ˙ CaCO3 ˙ 11H2 O þ 21 ½3CaO ˙ Al2 O3 ˙ 3CaSO4 ˙ 32H2 O þ 47 CaCO3 þ 22:25
2
−
½Al2 O3 ˙ 3H2 O þ 12:25 SO4 þ 133:5 OH
2
½3CaO ˙ Al2 O3 ˙ CaSO4 ˙ 12H2 O þ 4 CO3 →4 CaCO3 þ SO4
þ ½Al2 O3 ˙ 3H2 O þ 6 OH þ H2 O
the investigated systems. The Rietveld analysis suggested a balance
equation such as (Eq. (3)).
ð2Þ
Additional information was provided by the analysis of the
aqueous fractions (Table 6). Given the residual concentration of
carbonates and the released concentration of sulfates, the molar
ratio between calcium monosulfoaluminate hydrate and calcite
should be equal to 0.726, which corresponds to relative fractions
of 57.9% and 42.1% for the two phases, and agrees well with balance equation (Eq. (2)) given measurement errors. The Rietveld
analysis thus slightly overestimated calcite, and/or underestimated
calcium monosulfoaluminate hydrate. This could result from very
different crystallinities of the two phases.
After 7 days, the calcite content decreased while calcium monocarboaluminate hydrate and ettringite precipitated. At 28 days, calcium monosulfoaluminate hydrate was almost fully depleted, and the
phase assemblage comprised ettringite, calcium monocarboaluminate
hydrate, and calcite in respective proportions of 32/21/47 according to
Rietveld refinement. Calcium hemicarboaluminate hydrate was never
observed, which could be explained by the absence of portlandite in
Given the residual carbonate concentration measured in the aqueous fraction (Table 6), the sulfate amount theoretically released
according to (Eq. (3)) was calculated to be 0.120 mmol, in rather
good agreement with the experimental determination (0.140 ±
0.010 mol). However, under these conditions, the residual amount
of calcium monosulfoaluminante hydrate should be slightly higher
than that estimated by Rietveld refinement (monosulfoaluminate/
monocarboaluminate/ettringite/calcite in 6/30/20/44 molar proportions, instead of 0.4/31.7/20.7/47.2). Once again, the Rietveld analysis
seemed to overestimate slightly calcite and/or underestimate calcium
monosulfoaluminate hydrate. The broadening of the diffraction peaks
of calcium monosulfoaluminate hydrate probably affected the quality
of the quantification. The phase assemblage was close to thermodynamic equilibrium (Fig. 10). Calculations predicted the complete depletion of calcium monosulfoaluminate hydrate, and the precipitation
of gibbsite, calcium monocarboaluminate hydrate, ettringite and calcite,
with 35/22/43 proportions for monocarboaluminate, ettringite and
calcite, respectively.
A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165
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 concentration (b) on the phase assemblage. Hydrates are labelled: Ms (calcium monosulfoaluminate hydrate), Mc (calcium monocarboaluminate hydrate), E (ettringite)
and C (calcite). n indicates the initial carbonate/monosulfoaluminate molar ratio. The
time series was performed at n = 1.
4.2. Investigating the influence of the carbonate concentration
Calcium monosulfoaluminate hydrate was mixed for 28 days with
solutions containing increasing carbonate concentrations, giving n ratios of 0.25, 0.75, 1, 2, 3.5 and 5. The evolution of the respective
1163
Fig. 9. Refined molar amounts of hydrates at the end of the sorption tests (calcium
monosulfoaluminate hydrate + Na2CO3); influence of contact time (a) and initial carbonate concentration (b). n indicates the initial carbonate/monosulfoaluminate
molar ratio. The time series was performed at n = 1.
proportions of calcium monosulfoaluminate hydrate, ettringite, calcite and monocarboaluminate, estimated from Rietveld refinement,
is shown in Fig. 9-b. At low carbonate concentration (n = 0.25), calcium monosulfoaluminate hydrate was partly converted into ettringite
and calcium monocarboaluminate hydrate, according to mass balance
equation (Eq. (4)).
2
Table 5
Influence of reaction time and initial carbonate concentration on the phase assemblage
formed by mixing calcium monosulfoaluminate hydrate with Na2CO3 solution. Indicated errors correspond to standard deviation extracted from Rietveld refinements.
n
a
1
1
1
1
1
0.25
0.75
2
3.5
5
a
t
(days)
1
4
7
14
28
28
28
28
28
28
Refined molar fraction (%)
Monosulfoaluminate
Ettringite
Monocarboaluminate
Calcite
36.5 ± 0.7
36.5 ± 0.7
35.3 ± 0.8
31.3 ± 0.7
0.4 ± 0.1
64.7 ± 0.8
5.4 ± 0.6
7.9 ± 1.0
1.2 ± 0.6
0
0
0
0
2.4 ± 0.4
20.8 ± 1.2
23.9 ± 0.6
24.2 ± 1.1
0
0
0
0
0
0
0.9 ± 0.4
31.7 ± 0.3
11.4 ± 0.4
32.1 ± 1.6
11.7 ± 1.5
3.5 ± 1
0
63.5 ± 0.4
63.5 ± 0.4
64.7 ± 0.4
65.5 ± 0.4
47.2 ± 0.8
0
38.4 ± 0.4
80.4 ± 0.8
95.3 ± 0.8
100
n = initial carbonate/monosulfoaluminate molar ratio.
7=2 ½3CaO ˙ Al2 O3 ˙ CaSO4 ˙ 12H2 O þ 2 CO3 þ 15 H2 O→2 ½3CaO ˙ Al2 O3 ˙ CaCO3 ˙ 11H2 O þ
2−
−
½3CaO ˙ Al2 O3 ˙ 3CaSO4 ˙ 32H2 O þ 1=2 ½Al2 O3 ˙ 3H2 O þ 1=2 SO4 þ 3 OH
ð4Þ
According to this equation, the expected sulfate and carbonate
amounts in the aqueous phase would be 0.045 mmol and 0.019 mmol,
Table 6
Amounts (mmol) of carbonates and sulfates in the aqueous fractions at the end of the
sorption tests (initial calcium monosulfoaluminate hydrate amount = 0.803 mmol).
na
t (days)
SO42−
CO32−
1
1
1
1
1
0.25
2
5
1
4
7
14
28
28
28
28
0.239
0.244
Not measured
0.220
0.141
0.044
0.547
0.800
0.025
0.032
0.026
0.033
0.028
0.012
0.045
0.724
a
n = initial carbonate/monosulfoaluminate molar ratio.
1164
Relative amout of solid
(mol%)
a
A. Mesbah et al. / Cement and Concrete Research 42 (2012) 1157–1165
100
90
80
70
60
50
40
30
20
10
0
Monosulfo
Calcite
Monocarbo
Ettringite
Gibbsite
0
1
2
3
4
5
6
n
Relative amout of solid
(mol%)
b
100
90
80
70
60
50
40
30
20
10
0
Monosulfo
Calcite
Monocarbo
Ettringite
exp Monosulfo
exp Monocarbo
exp Ettringite
exp Calcite
0
1
2
3
4
5
6
n
Fig. 10. (a) Thermodynamic modelling of the phase assemblage formed in the calcium sulfoaluminate hydrate–Na2CO3 system as a function of n (initial carbonate/
monosulfoaluminate molar ratio). (b) Comparison with experimental results at
28 days (gibbsite excluded).
given the depleted amount of calcium monosulfoaluminate hydrate
assessed from Rietveld analysis (0.317 mmol). This was in rather good
agreement with the analyses of solution (0.044 mmol of sulfates, and
0.012 mmol of carbonates—Table 6).
Increasing n led to the additional precipitation of calcite. Calcium
monosulfoaluminate hydrate was almost fully depleted at n = 1 but,
quite unexpectedly, was detected again at n = 2 and 3.5. From
n = 2, ettringite was not observed any longer. The fraction of calcium
monocarboaluminate hydrate decreased when n increased beyond 1,
while that of calcite went on increasing. At n = 5, calcite was the sole
detected crystallized phase. These results, as well as the sulfate
amount measured in solution (0.800 mmol), were consistent with
mass balance equation (Eq. (5)).
2−
2−
−
½3CaO ˙ Al2 O3 ˙ CaSO4 ˙ 12H2 O þ 4 CO3 →4 CaCO3 þ SO4 þ ½Al2 O3 ˙ 3H2 O þ 6 OH þ 6 H2 O
ð5Þ
Comparing the experimental results with the phase assemblage
evolution predicted by thermodynamics showed two main differences (Fig. 10-b) the persistence of calcium monosulfoaluminate hydrate at n = 2 and 3.5, and the early disappearance of ettringite (from
n = 2 instead of 3.1 for the model). Thermodynamic modelling was
also performed by considering amorphous Al(OH)3 instead of less soluble gibbsite in the database. In that case, ettringite disappeared for
n = 2.9, a value still significantly higher than the experimental one.
Thus, the type of AH3 selected in the database could not explain the
deviation between the modelled and experimental data at high n
(n ≥ 2). It seems likely that the corresponding samples were not at
thermodynamic equilibrium after 28 days.
5. Conclusion
These results show that calcium monosulfoaluminate hydrate has
a better potential for insolubilizing carbonates than chlorides. When
equimolar amounts of calcium monosulfoaluminate hydrate and carbonates or chloride are made to react (0.803 mmol under the investigated conditions), the residual concentrations at thermodynamic
equilibrium are calculated to be 32 times smaller for carbonates
than for chloride (12.59 μmol against 402.6 μmol). Moreover, carbonates are much more readily insolubilized. Under the same experimental conditions, the aqueous concentration of carbonates is reduced by
a factor 32 after one day for carbonates, and that of chlorides by a factor 1.26 only.
Carbonates are rapidly depleted to form calcium monocarboaluminate
hydrate (nb 0.5), 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 later
partly destabilized into calcium monocarboaluminate hydrate.
Chloride ions react more slowly. Under the investigated experimental conditions, thermodynamic equilibrium is not achieved after
28 days at ambient temperature. The final chloride-containing products are Kuzel's salt (0.2 ≤ n b 1.65), Kuzel's and Friedel's salts
(1.65 ≤ n ≤ 2.35), and Friedel's salt only at higher n. The reaction proceeds by the transient precipitation of Kuzel's salt which is later
converted into Friedel's salt.
Rietveld analysis was successfully used for a quantitative analysis of
the solid phase distribution. The lattice parameters of Kuzel's salt were
shown to decrease linearly when the chloride concentration increased,
which clearly demonstrates the occurrence of a solid solution between
sulphate and chloride within a restricted range of compositions
3CaO·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 the
synthesis experiments. Laurent Petit, from Electricité de France, is
deeply acknowledged for his support on the study.
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