Sulphate attack on concrete: limits of the AFt stability domain

CEMENT and CONCRETE RESEARCH. Vol. 22, pp. 229-234, 1992. Printed in the USA. 0008-8846/92 $5.00 + .00. Copyright © 1992 Pergamon Press Ltd.

Sulphate attack on concrete: limits of the AFt stability domain D. Damidot, M. Atkins, A. Kindness and F.P. Glasser University of Aberdeen, Meston Walk, Aberdeen AB9 2UE, Scotland

Abstract The compositions of the solutions at the four invariant points that define the AFt equilibrium surface, have been calculated at 25°C with and without sodium ions. The results indicate that AFt is stable over a large range of sulphate concentration depending on both the calcium and aluminium concentrations. The presence of sodium modifies the AFt stability domain but it remains stable even at high sodium concentrations. 1. I N T R O D U C T I O N Cement used in low and medium radwaste repositories can be attacked by groundwaters particularly if these contain sulphate. Sulphate attack on concrete is often reported as destructive due to the formation of ettringite (3CaO.A1203.3CaSO4.32H20: also called AFt). In order to define regimes of sulphate attack, and to delineate cement durability, the formation limits of AFt regarding solution composition, temperature and other accessible physico-chemical variables have to be defined. The reported work, which is a part of a wider program to model cement durability, deals with the stability of AFt as regards solution composition and in particular, sulphate concentration. From analyses of the composition of pore fluids from cement pastes, it is known that AFt is destabilised when the sulphate concentration becomes very low. However AFt can form if the sulphate content of the pore fluid increases, by reaction with sulphate containing waters. In this case, the sulphate concentration could be high and the stability of AFt not certain, i.e. an upper limit of AFt stability with respect to the sulphate concentration may exist. So definition of the AFt stability domain is essential for predicting the conditions of formation of AFt. This stability domain in the CaO-AI203-CaSO4-H20 system is represented by the AFt equilibrium surface which is defined by four boundary curves: AH3-AFt, C3AH6-AFt, CH-AFt and gypsum-AFt. These curves cross at four invariant points defined previously by Jones (1): H2 (C3AH6-AFt-AH3), E2 (AH3-AFt-gypsum), F (CH-AFt-gypsum) and G (C3AHr-AFt-CH) (Fig 1). In this first approach, we represent the curves joining the invariant points by straight lines instead of by the real boundary curves. The boundary curves are the intersection of two equilibrium surfaces, so the AFt stability domain changes when the equilibrium surfaces are modified by a change in temperature or by the introduction of additional ions such as alkalies. This work concentrates on the influence of sodium. The composition of the solution at the invariant points was calculated with the speciation program MINEQL (2). This program gives mathematical results and is not able to predict if AFt is stable at high sodium concentrations. Thus complementary experiments where AFt is equilibrated with sodium hydroxide or sodium sulphate solutions (giving low and high sulphate concentrations respectively) were undertaken. 229

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[S04] E2

F

tAq [Ca] Figure 1. CaO-A1203-CaSO4-H20 phase diagram drawn from the data of Jones (1) 2. E X P E R I M E N T A L AFt was synthesized using the saccharose method described by Carlson and Berman (3). The resultant solid was washed several times with water, and aliquots transferred to polyethylene bottles filled with sodium hydroxide or sodium sulphate solutions of various concentrations (Table 1). The bottles were periodically shaken and maintained at 25°C. After 8 weeks, the solution was filtered and the solid dried over silica gel in a desiccator. The solution was analysed for calcium, aluminium, sulphate and sodium contents (Table 1) and the solid analysed by XRD and AEM. Experimental conditions were 'CO 2 free' throughout. The calculation of the solution composition at the invariant points was performed using a modified version of the MINEQL program with an upgraded thermodynamic database specifically for cement applications (4). The species used in the calculation are: Ca2+, CaOH+, CaSO4 °, SO42-, NaSO4, AI(OH)3°, AI(OH)4, Na+, OH- and H3O+. The calculations use the Davies expression to calculate the activity coefficient and assume a temperature of 25°C. The results, which are identical for NaOH or Na2SO4 (with the same sodium concentration), are stated as total calcium, aluminium, sulphate and sodium concentrations (Table 2), to facilitate comparison with the experimental results obtained by Jones (1,5) and D'Ans and Eick (6). 3. R E S U L T S AND DISCUSSION 3.1. AFt stability domain in water The calculated compositions of the solution at the different invariant points are in good accordance with the data of Jones (25°C) and D'Ans and Eick (20°C) (Table 2), especially when the concentrations are higher than lmM/l. For lower concentrations the differences are greater, particularly for aluminium concentrations. These differences can be induced by variations of the

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Table 1 Composition Experiment 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

SULPHATE ATTACK. AFt, STABILITY, SODIUM EFFECT

of the solution when AFt is equilibrated Solution [Ca] mM/l [A1]mM/l NaOH 0.351 2.035 NaOH 0.55 0.451 NaOH 0.5425 0.37 NaOH 0.602 0.326 NaOH 0.6175 0.311 NaOH 0.515 0.443 NaOH 0.8075 0.323 NaOH 0.935 0.342 NaOH 1.175 0.457 NaOH 1.395 0.431 NaOH 1.68 0.513 NaOH 1.65 0.649 Na2SO4 0.825 0.777 Na2SO4 1.15 0.629 Na2SO4 1,50 0.592 water 1.995 0.552

231

with NaOH or Na2SO4 solutions. [SO4]mM/l [Na] mM/l losKsp 4.084 997.5 -43.80 2.20 503.8 -44.12 1.124 248.3 -44.62 1.1 95.65 -44.30 0.91 53.48 -44.54 1.196 27.82 -44.67 0.806 16.52 -44.57 0.676 9.78 -44.79 0.819 5.65 -44.35 0.728 3 -44.63 0.81 1.87 -44.24 0.884 1.26 -44.54 122.5 249.2 -44.60 49.2 99.3 -44.22 26.0 50.8 -44.48 0.81 0 -44.55

Table 2 Calculated composition of the solution at the different invariant points and experimental composition from D'Ans and Eick at 20°C (E) and from Jones at 25°C (J). Invariant point [Ca] mM/l [AI] mM/l [SO4] mM/l [Na] mM/l G 20.74 0.074 0.034 0 O(J) 19.17 0.01 0.180 0 G(E) 21.46 0.025 0.107 0 H2 7.6 1.174 0.074 0 H2(J) 3.495 0.09 0.375 0 H2(E) 3.103 0.158 0.263 0 F 32.13 0.0003 12.24 0 F(J) 31.56 0.06 12.33 0 F(E) 32.8 0.027 12.2 0 E2 15.34 0.0178 15.20 0 E2(J) 15.645 0.025 14.97 0 E2(E) 15.136 0.09 14.82 0 G 8.747 0.131 0.09 50 H2 1.059 4.23 0.522 50 F 22.18 0.0004 21.26 50 E2 11.21 0.027 36.04 50 G 4.532 0.2181 0.175 100 H2 0.4509 8.205 1.764 100 F 17.82 0.0005 35.4 100 E2 10.13 0.034 59.91 100 G 1.636 0.5134 0.675 250 H2 0.153 20.18 7.80 250 F 13.23 0.0008 90.98 250 E2 8.76 0.0479 133.34 250 G(J) 1.62 0.085 0.93 250 H2(J) =0 16.8 60.6 250 F(J) 15.39 0.025 93.9 250 E2(J) 10.17 0.14 134.7 250

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solubility of AH3, depending on the different forms: amorphous, ct-AH3 (bayerite) or ~,-AH3 (gibbsite) and difficulties of measurement at low concentrations. Jones used a mixture of ct and ?-AH3 in his experiments. In our calculations, pure ct-AH3 is assumed. However, the sequence of the invariant points relative to sulphate concentration is the same: G < H2 < F < E2. The range of the sulphate concentration where AFt is in equilibrium with the solution as a function of its calcium and aluminium concentrations, is 0.034 to 15.2mM/l. The projection of the AF t stable domain on the calcium-sulphate axis (Fig 2.), defines the calcium and sulphate concentrations where AFt is stable (surface delimited by the projection of the boundary curves). The projection of the AF t stable domain on the aluminate-sulphate axis is drawn on Fig. 3. For sulphate concentrations less than 0.034mM/l or greater than 15.2raM/l, AFt is not stable at any calcium or aluminate concentrations. In the CaO-A1203-CaSO4-H20 system, the maximum sulphate concentration corresponds to the solubility of gypsum in water: 15.22mM/1. These results confirm the existence of a lower limit of sulphate concentration for AF t stability (0.034mM/1) but also indicate the existence of an upper limit (15.2mM/l). Thus, there appears to be two domains where AF t is unstable(<0.034 and [15.2 to 15.22] mM/l). These domains of AF t instability are very small: their exact magnitudes are subject to uncertainty from possible errors in calculation. In order to investigate the existence of these domains of AF t, the system has been studied in the presence of sodium ions.

3.2. AFt stability domain in the presence of sodium ions 3.2.1. Investigation of the stability of AFt in sodium hydroxide and sodium sulphate solutions Two solutions containing sodium, sodium hydroxide and sodium sulphate, have been used as these solutions represent two distinct cases. When AFt is in equilibrium with a sodium hydroxide solution, the sulphate concentration should be low (close to invariant points G and H2: lower limit of the sulphate range). When AFt is in equilibrium with a sodium sulphate solution, the sulphate concentration will be high (close to invariant points F and E2: higher limit of the sulphate range). For all the experiments, investigation by XRD and AEM showed that no other hydrates were present with AFt, indicating that AFt is stable in contact with such solutions. If correct, the solubility product of AFt calculated from the composition of the solution should not vary between the different experiments and an apparent solubility product can be calculated by assuming that Ca2+, AI(OH)4 and SO42- represent approximatively total calcium, aluminium and sulphate concentrations in solution: the hydroxyl concentration is calculated from the charge balance. The apparent solubility product can be written as: Ksp = (Ca2+) 6 (AI(OH)4-) 2 (SO42-) 3 (OH-)I2 The Log Ksp values (Table 1) are, as predicted, essentially invariant. A slight deviation is observed for the higher sodium content, probably due to the high ionic strength reducing the accuracy of calculation of the activity coefficient. These results appear to confirm the stability of AF t in sodium solutions up to 0.25M/1; further investigations are needed at higher concentrations.

3.2.2. Calculation of the solution composition at the invariant points in the presence of sodium ions The composition of the solution at the invariant points has been calculated for three sodium concentrations: 0.05, 0.1 and 0.25M/1 (Table 2).

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SULPHATEATI'ACK.AFt, STABILITY,SODIUMEFFECT

233

804] mM/I 100 10

1

0.1 t

t

0.01

I

~

~ L iLL

~

L

I

I

I

I ill

1

0.1

I

I

I

I

I [q

100

10

[Cal Water

t

..... INa]o0.05

rnM/I

*

INa]-0.1

--~-- [ N a I - 0 . 2 5 M / I

Figure 2. Projection of AFt equilibrium surface on the calcium-sulphate axis for different sodium concentrations at 25°C. 1804] mM/I ........... [2.~ . . . . . . . . . . . . .

100

Eg. - _

10 1

0.1 0.01 _ I 1.000E-04

I

I Illlll

t

1.000E-03

Water

l

I IIIlll

I

0.01

..... INal-0.05

I

r Illlll

I

I

0.1 [AI] mM/I *

[Na]-0.1

l kllFII

I

1

I

[ IIIrll

p

10

--~- INa]-0.25M/I

Figure 3. Projection of AF t equilibrium surface on the aluminate-sulphate axis for different sodium concentrations at 25°C. The projections of the AF t equilibrium surface on the calcium-aluminium (Figure 2) and on the aluminate-sulphate (Figure 3) axis systems show the displacement of the AF t stability domain as sodium content increases: the AF t stability domain is displaced to lower calcium, but to higher aluminium concentrations. Moreover the range of the sulphate concentration is increased. When the results are compared to those obtained by Jones (5) with a 0.125M/1 Na2SO4 solution (Table 2), the extent of agreement is similar to that found for water, i.e. the largest differences occur at lowest concentrations. Moreover the increase of AH3 solubility in the presence of sodium enhances differences in aluminium concentrations. A large difference exists

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for the sulphate concentration at the point H2:7.8mM/1 calculated instead of 60.6mM/1. An explanation might be that Jones analysed the solution before the attainment of the equilibrium. As Jones used a 0.125M/1 sodium sulphate solution, the sulphate concentration has to decrease from 125 to 7.8mM/l to reach equilibrium. This indicates the importance of reaction kinetics: sufficient time must be allowed for attainment of equilibrium solubilities. The sequence of the invariant points relative to the sulphate concentration is the same as that in water, whether the sulphate content is taken as 7.8 or 60.6mNUI at the invariant point H2: G < H2 < F < E2. There again exist two AFt domains of instability relative to the sulphate concentration: the first from 0 to 0.67mM/l and the second from 133.34mM/1 to 135.28mM/1 (sulphate concentration obtained when gypsum is in equilibrium with a 0.125M/1 sodium sulphate solution). The position of the AFt domains of stability and instability increase relative to water (fig 4). This confirms the existence of the AFt domains of instability that were less evident in water. This simple example demonstrates that even a small amount of additional ions in the system, can induce major changes and lead to different conclusions concerning the stability of AFt.

Na+

0.675

133.34

135.28

804 0

0.034

15.2 15.22 AFt unstable

~

mM/I

iii_..~ AFt stable

Figure 4. Evolution of the AFt domains of stability and instability when sodium is added. 4. C O N C L U S I O N It is now possible to calculate equilibria involving ettringite by computer, thus freeing us from the restraints imposed by graphical solutions. This flexibility will enable cementgroundwater interactions to be calculated more accurately than previously. The computer code calculating these interactions will be incorporated in a more general approach to predict the long term performance of cement in radwaste repositories. 5. A C K N O W L E D G E M E N T This work has been supported by the Commission of the European Communities, contract number F12W-CT90-0035.

6. 1. 2. 3. 4. 5. 6.

REFERENCES F.E. Jones, J, Phys. Chem.,48,311-356,(1944). M. Schweingruber, EIR Internal Report TM 45-82-38, Wurenlingen,(1982). E.T. Carlson and H.A. Berman, J. Res. N.B.S..64A(4),331-341,(1960). M. Atkins, F.P. Glasser, A. Kindness, D. Bennett, A. Dawes, D. Read, DoE (1991). F.E. Jones, J. Phys. Chem.,48,356-378,(1944). J. D'Ans and H. Eick, Zem. Kalk Gips,6,302-311,(1953).