# Prediction of Long-Term Chemical Evolution of a Low-pH Cement Designed for Underground Radioactive Waste Repositories

Thi Thuy Ha Bach, Isabelle Pochard, Céline Cau-Dit-Coumes, Cyrille Mercier, André Nonat

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## ► To cite this version:

Thi Thuy Ha Bach, Isabelle Pochard, Céline Cau-Dit-Coumes, Cyrille Mercier, André Nonat. Prediction of Long-Term Chemical Evolution of a Low-pH Cement Designed for Underground Radioactive Waste Repositories. Cement-Based Materials for Nuclear Waste Storage, Springer, pp.239-249, 2013, 9781489989680. 10.1007/978-1-4614-3445-0_21. hal-03065972

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Submitted on 14 Jan 2026

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# Prediction of Long-Term Chemical Evolution of a Low-pH Cement Designed for Underground Radioactive Waste Repositories

T. Bach, I. Pochard, C. Cau-dit-Coumes, C. Mercier, and A. Nonat

## 1 Introduction

A solution to manage long-lived radioactive wastes over the very long time is their disposal in deep geological structures. The disposal concepts are based on a multibarrier design approach including the waste package (the waste and the material used to stabilize it in a suitable overpack), the engineered barrier inserted between the waste package and the rock and the geological barrier, i.e. the actual rock. In most designs, bentonite is a key component of the engineered barrier system. It is however unstable at high pHs (≈12–13) [1], such as those encountered in ordinary concrete leachates. Since it seems more and more difficult to exclude concrete from any sensitive areas containing bentonite in a repository, some work has been undertaken in order to develop low-pH cements which would show an improved compatibility with clay. Low-pH cements, also referred as low-alkalinity cements, are binders with a pore solution pH ≤ 11. They can be designed by replacing significant amounts of Portland cement (OPC) (≥40 %) by silica fume, which can be associated in some cases to low-CaO fly ash and/or ground granulated blast furnace slag [2–4]. Such a blend has several positive effects: (1) consumption

T. Bach

Commissariat à l'Energie Atomique et aux Energies Alternatives, DEN/MAR/DTCD/SPDE, BP17171, 30207 Bagnols sur Cèze, France

ICB, UMR 5209 CNRS Université de Bourgogne, 21078 Dijon, France

I. Pochard (✉) A. Nonat

ICB, UMR 5209 CNRS Université de Bourgogne, 21078 Dijon, France
e mail: isabelle.pochard@u.bourgogne.fr

C. Cau dit Coumes

Commissariat à l'Energie Atomique et aux Energies Alternatives, DEN/MAR/DTCD/SPDE, BP17171, 30207 Bagnols sur Cèze, France

C. Mercier

LMCPA, Université de Valenciennes et du Hainaut Cambrésis, 59313 Valenciennes cedex, France
of portlandite, which is formed by the hydration of OPC and which buffers the pore solution pH up to 12.5, to produce calcium hydrosilicates (C-S-H) by the pozzolanic reaction, (2) decrease in the CaO/SiO₂ ratio of the C-S-H and partial substitution of Al³⁺ for Si⁴⁺ in the silicate chains of the C-S-H, which could enhance their sorption capacity of alkalis [5], and (3) dilution of OPC, which decreases the heat output during hydration. The pore solution pH of the cement paste is then controlled by the solubility of C-S-H: the smaller the CaO/SiO₂ ratio of C-S-H, the lower the pH [6, 7]. With the prospect of using these materials in a geological repository, it is of main importance to estimate their long-term properties and the influence of external and internal factors (chemical composition of the binder, storage temperature) on their characteristics at equilibrium. This chapter describes the mineralogical evolution of a low-pH binder comprising OPC, silica fume, and fly ash over one year. Since hydration was still incomplete after this period, a three-way original approach was adopted to assess the behaviour of this hydrated low-pH cement at thermodynamic equilibrium, i.e. at the end of its evolution under fixed conditions in the temperature range 20-80°C. Firstly, thermodynamic modelling was carried out to predict the mineral assemblage and composition of the solution at equilibrium, starting from the composition of the initial low-pH cement studied. Secondly, hydration of blended cements was accelerated by milling cement slurries (water to solid ratio of 9) with zirconia beads. Thirdly, the low-pH cement pastes were mimicked with mixtures of appropriate highly reactive oxides (lime, silica, calcium aluminate, and calcium sulphate) with a water to solid ratio of 50.

# 2 Materials and Methods

## 2.1 Raw Materials and Cement Pastes

The low-pH binder investigated in this work comprised 37.5 % Ordinary Portland Cement, 32.5 % Silica Fume, and 30 % Fly Ash. Table 21.1 gives the chemical composition and properties of the raw materials used in this formulation. Note that the quite important proportion of “minors” in the fly ash is due to a significant loss on ignition (near 5 %). The binder constituents were ground together in a planetary ball mill (Retsch PM400 mixer-mill: 250 rpm, 150 g of material, and 50 g of 10 mm diameter stainless steel balls in a 250 mL grinding jar). Reference cement pastes were prepared at water to solid ratio of 0.55, stored in hermetically sealed 50 mL polypropylene containers, and submitted to three types of curing (1) in a controlled-environment chamber at 20°C and 95 % relative humidity, in an oven at (2) 50°C, or (3) 80°C. To prevent drying, the samples were placed in water-filled boxes. Curing was initiated 30 min after mixing. All the binders were characterized after 3, 6, and 12 months of hydration. A detailed procedure for the solid and pore solution characterization is presented elsewhere [8].
<table><caption>**Table 21.1** Characteristics of binder constituents</caption><thead><tr><th></th><th></th><th>CEM I</th><th>Silica fume</th><th>Fly ash</th></tr></thead><tbody><tr><td rowspan="7">Chemical composition (wt%)</td><td>CaO</td><td>66.90</td><td>0.50</td><td>5.10</td></tr><tr><td>SiO<sub>2</sub></td><td>22.00</td><td>96.30</td><td>51.50</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>3.30</td><td>&lt;0.20</td><td>25.20</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>2.74</td><td>0.10</td><td>5.80</td></tr><tr><td>MgO</td><td>0.60</td><td>&lt;0.20</td><td>1.80</td></tr><tr><td>Sulphates</td><td>2.40</td><td>0.24</td><td>0.66</td></tr><tr><td>Na<sub>2</sub>O + K<sub>2</sub>O</td><td>&lt;0.25</td><td>&lt;0.54</td><td>1.73</td></tr><tr><td>Minor</td><td>1.81</td><td>1.92</td><td>8.21</td></tr><tr><td rowspan="7">Phases observed by X ray diffraction<sup>a</sup></td><td></td><td>C<sub>3</sub>S<sup>+++</sup></td><td>C<sub>3</sub>S<sup>+++</sup></td><td>C<sub>3</sub>S<sup>+++</sup></td></tr><tr><td></td><td>Brownmillerite<sup>+</sup></td><td>Quartz<sup>++</sup></td><td>CaO<sup>+</sup></td></tr><tr><td></td><td>C<sub>3</sub>A<sup>+</sup></td><td>Maghemite<sup>++</sup></td><td>Maghemite<sup>+</sup></td></tr><tr><td></td><td>Gypsum<sup>+</sup></td><td>Silicon<sup>+</sup></td><td></td></tr><tr><td></td><td>Anhydrite<sup>+</sup></td><td>Silicon oxide<sup>+</sup></td><td></td></tr><tr><td></td><td>Calcite<sup>+</sup></td><td></td><td></td></tr></tbody></table>

## 2.2 Acceleration of the Hydration by Milling

To accelerate the reaction, slurries of low-pH cement were submitted to milling instead of simple curing. By means of the stirring, the zirconia beads ground away the hydrates formed on the grain surfaces and thus re-exposed the anhydrous phases to water. The mechanical energy input to the system is able to change the nature of the hydrosilicate phase. For instance, the same procedure for the hydration of C₃S with aluminium balls leads to the formation of αfwillite [9]. In the present study, this phase was never observed. The tests were conducted at controlled temperature (20, 50, or 80°C) in the absence of carbon dioxide. The water/binder ratio was set at 9 mL/g by the addition of 5 g of binder to 45 mL of deaerated deionized water. For curing at 20°C, 51 g of zirconia beads (diameter: 0.8 to 2.5 mm) were added to the slurry in hermetically sealed polypropylene containers continuously stirred in a turbula for 3 months. The samples at 50 and 80°C were milled with 30 g of zirconia beads (diameter: 2.5 mm) in hermetically sealed Teflon containers under continuous magnetic stirring in an oven for 2 months at the selected temperature. To prevent drying, the suspensions were allowed to stand without stirring at the studied temperature for 1 month. The pH was determined at the sample curing temperature (20, 50, or 80°C). The high alkalinity measurement electrode was first calibrated at the same temperature with buffer solutions: borate (pH 9.23 at 20°C, 9.01 at 50°C, and 8.88 at 80°C), carbonate (pH 10.06 at 20°C, 9.83 at 50°C, and 9.73 at 80°C), and calcium hydroxide (pH 12.63 at 20°C, 11.71 at 50°C, and 10.99 at 80°C). The suspension was then filtered at the test temperature and the filtrate was acidified. Its composition was analysed by ICP-AES (Vista Pro Varian, Ca, Si, Al, S, Na, K standardization with matrix reconstitution). The solids were recovered by using successive water/ethanol and pure ethanol

<footnote><sup>a</sup>Intensity of main diffraction peak: <sup>+++</sup> strong, <sup>++</sup> moderate, <sup>+</sup> weak</footnote>
washing/drying. The mineralogy of the cement pastes was characterized by X-ray diffraction (Siemens D8, copper anode, λ<sub>Kα1</sub> = 1.54056 Å, 40 mA and 40 kV, scanning from 2θ = 5° to 60° in 0.017° steps, 50 s measurement time per step).

## 2.3 Reactive Oxides in Suspension

The tests were conducted at controlled temperature in the absence of carbon dioxide on dilute suspensions (water/solid ratio = 250 mL/5 g). The hydrated binders were precipitated from mixtures of 1.05 g of CaO (CaCO₃ Prolabo decarbonated at 1,000°C for 3 h), 2.75 g of SiO₂ (Aerosil 200), 0.69 g of calcium aluminate (CaAl₂O₄) and 0.13 g of calcium sulphate (CaSO₄, 2H₂O), and 250 g of 7 mmol/L alkaline aqueous solution. These quantities are so that the Ca, Si, Al, SO₄, and alkali proportions are the same as in the low-pH binder (Ca/Si = 0.52; Al/Si = 0.19). These reagents were chosen for their high reactivity in water. Compared to the actual composition of the ternary binder, the composition of the model systems was simplified. Indeed, the minor elements (Fe, Mg, Mn, Ti, P) were not taken into account. The suspensions were stored in hermetically sealed 250 mL polypropylene containers under continuous stirring for 2 months at 20°C, 3 weeks at 50°C, and 1 week at 80°C. The solid and aqueous fractions were characterized in the same way as the accelerated hydration samples.

## 2.4 Thermodynamic Calculations

Thermodynamic modelling was carried out to predict the mineral assemblage and composition of the solution at equilibrium, starting from the composition of the initial low-pH cement studied. Simulations were performed with PHREEQC software provided by US Geological Survey [http://www.USGS.gov] and using the database developed by Damidot et al. for cement-based systems [10 14]. Simulations were ruled by the solubility products which governed the actual system. The goal of these calculations was twofold. It could point out the possible presence of amorphous phases not observable by XRD. Besides, comparing the calculated results with phase assemblages obtained by the two experimental methods could help predicting the final state of equilibrium of the fully hydrated low-pH cement paste. The minor elements were not taken into account, and the crystalline phases from fly ash were treated as the amorphous reactive part. This means that it is the total amounts of Ca (8.69 mol/kg), Si (16.67 mol/kg), and Al (3.16 mol/kg) present in the low-pH cement formulation, including all phases from fly ash that were the input data in mol per kg of water. Taking into account the solubility of such phases was not necessary in a first approach because their amount was negligible compared to the large proportion of amorphous silica and alumina present in silica fume and fly ash. The supersaturated phases were let to precipitate
and the ionic solution was charge balanced with pH. Sulphates and alkalis adsorptions on C-S-H were taken into account to calculate their concentrations at equilibrium [15, 16]. The substitution of Al³⁺ for Si⁴⁺ in the C-S-H was not taken into account. For a more accurate description, it should not be neglected. This will be the subject of future work.

# 3 Results and Discussion

## 3.1 Low-pH Cement Pastes over 6 months

The mineralogy of the reference cement pastes was investigated at 20, 50, and 80°C. Figure 21.1 illustrates the XRD results recorded on a 6-month-old sample. The pastes mainly consisted of calcium silicate hydrate, probably containing aluminium (C-A-S-H). No portlandite was detected, as expected for low-pH cements. The disappearance of ettringite was observed above 50°C. It is well known that the stability range of ettringite decreases in favour of calcium monosulfoaluminate hydrate when temperature increases [17]. The absence of ettringite for samples cured above 50°C ensures that the procedure used to recover the solids after curing is suitable and does not lead to mineralogy change during filtration and drying. Obviously, the crystalline phases of the fly ash (mullite, quartz, hematite), which are unreactive, were still detected. Although C₃S was not observed, the presence of C₂S showed that hydration was not complete after

<figure><img src="image_3.png" /><figcaption>**Fig. 21.1** XRD patterns of the low pH cement pastes cured 6 months at 20, 50, and 80 °C. E ettringite, M mullite, Q quartz, H hematite, asterisk C₂S</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 21.2** Elemental analysis mapping of the low pH cement paste after 6 months of curing at 20 °C. SF residual silica fume, FA residual fly ash</figcaption></figure>

<table><caption>**Table 21.2** Composition (in mmol/L) and pH of the liquid fraction of the cement pastes after 6 months curing</caption><thead><tr><th>T(°C)</th><th>Ca</th><th>Si</th><th>S</th><th>Al</th><th>Alkali</th><th>pH</th></tr></thead><tbody><tr><td>20</td><td>3.2</td><td>2.1</td><td>3.2</td><td>&lt;d.l.</td><td>3.0</td><td>11.0</td></tr><tr><td>50</td><td>6.0</td><td>2.0</td><td>6.6</td><td>0.01</td><td>3.6</td><td>10.0</td></tr><tr><td>80</td><td>6.5</td><td>3.0</td><td>7.4</td><td>&lt;d.l.</td><td>4.2</td><td>9.1</td></tr></tbody></table>

<d.l. means below the detection limit

this time. Indeed, hydration was very slow after 3 months and remained incomplete even after 1 year.

The EDS(SEM) mapping was used in addition to XRD to determine the mineralogy of the pastes as this technique enables the detection of amorphous compounds. Figure 21.2 shows that there were still amorphous silica fume and fly ash in the low-pH cement paste at 20°C after 6 months of hydration. This was not surprising as these two constituents were present in large proportions in the formulation.

Finally, the description of the pore solution after 6 months and for the three temperatures is given in Table 21.2.

The pH reduction mainly resulted from the temperature-dependent variation of the activity coefficients [18]. Raising the curing temperature resulted in an increase in the sulphate concentration in solution. This was due to the disappearance of ettringite from the samples stored at 50°C and 80°C, without any significant
<table><caption>**Table 21.3** Thermodynamic simulation results of the low pH cement paste fully hydrated at 20 °C</caption><thead><tr><th></th><th>Initial quantity<sup>a</sup><br>(mol/kg water)</th><th>Calculated<br>concentration<sup>b</sup> (mM)</th><th>pH</th><th>Solid phases<sup>c</sup></th></tr></thead><tbody><tr><td>Ca</td><td>8.69</td><td>1.79</td><td rowspan="5">10.20</td><td rowspan="5">C S H, silica, Al(OH)<sub>3</sub></td></tr><tr><td>Si</td><td>16.67</td><td>5.06</td></tr><tr><td>Al</td><td>3.16</td><td>0.05</td></tr><tr><td>SO<sub>4</sub></td><td>0.22</td><td>1.00</td></tr><tr><td>Alkalis</td><td>0.36</td><td>2.31</td></tr></tbody></table>

precipitation of other sulphate phases. The concentrations of the other species tended to increase more slowly with the temperature, which could mainly result from the variation of the activity coefficients [18].

## 3.2 Determination of the Final State by Thermodynamic Calculation

Even if the simulated system was simplified compared to the actual low-pH cement paste, several conclusions could be drawn. The results at 20°C are listed in Table 21.3. Compared to the cement paste at 20°C, there was no C₂S remaining, but silica and Al(OH)₃ were still present besides C-S-H at the final stage. In the cement paste, residual silica was also noticed as amorphous silica and quartz from silica fume, and as amorphous and crystalline phases from fly ash. At equilibrium, no free Al was available, which was already the case in the cement paste cured for 6 months. Actually, aluminium was precipitated in Al(OH)₃ and/or unreacted fly ash. The Ca and Si concentrations fitted the C-S-H solubility curve for a Ca/Si ratio equal to 0.66. On the contrary, the Ca concentration in the pore solution of the 6-month-old cement paste was higher (3.2 mM at 20°C), which was characteristic of a higher Ca/Si ratio. The pozzolonic reaction could thus continue after 6 months of curing. This was corroborated by the calculated pH value, almost one unit smaller than that measured on the cement paste.

The final state of the low-pH formulation at 20°C was thus composed of C-S-H of Ca/Si = 0.66 with excess of silica and Al(OH)₃ coming from the excess of silica fume and fly ash. The C-S-H may include aluminium by Si/Al substitution but this feature could not be taken into account by the simulation. However, the Al content in the C-S-H should be low given the very low Al concentration in the pore solution [19]. The pH of the pore solution was thus governed by the C-S-H solubility. Ettringite, which precipitated at early age when hydration occurred at 20°C, was shown to be unstable at thermodynamic equilibrium. Moreover, apart from the destabilization of ettringite and the variation of the activity coefficients [18], this

<footnote><sup>a</sup>Based on the composition of the low pH binder</footnote>

<footnote><sup>b</sup>Concentrations in the solution in equilibrium with</footnote>

<footnote><sup>c</sup>Precipitated solid phases</footnote>
<table><caption>**Table 21.4** Crystalline phases in the solids, composition (in mmol/L) and pH of the liquid fraction of the cement suspensions obtained by milling, and of the mixed oxides suspensions at 20, 50, 80°C</caption><thead><tr><th rowspan="2"></th><th colspan="2">20°C</th><th colspan="2">50°C</th><th colspan="2">80°C</th></tr><tr><th>Hydration by milling</th><th>Model systems</th><th>Hydration by milling</th><th>Model systems</th><th>Hydration by milling</th><th>Model systems</th></tr></thead><tbody><tr><td>Crystalline phases<sup>a</sup></td><td>C S H<sup>+++</sup><br>Mullite<sup>++</sup><br>Quartz<sup>++</sup><br>Calcite<sup>+</sup></td><td>C S H<sup>+++</sup><br>Ettringite<sup>+</sup></td><td>C S H<sup>+++</sup><br>Mullite<sup>++</sup><br>Quartz<sup>++</sup><br>Calcite<sup>+</sup></td><td>C S H</td><td>C S H<sup>+++</sup><br>Mullite<sup>++</sup><br>Quartz<sup>++</sup><br>Calcite<sup>+</sup></td><td>C S H</td></tr><tr><td>Ca</td><td>5.5</td><td>1.2</td><td>10.0</td><td>2.2</td><td>8.8</td><td>2.8</td></tr><tr><td>Si</td><td>1.9</td><td>3.7</td><td>2.2</td><td>3.3</td><td>1.8</td><td>2.6</td></tr><tr><td>Al</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>&lt;d.l.</td><td>&lt;d.l.</td></tr><tr><td>SO<sub>4</sub></td><td>8.2</td><td>1.0</td><td>13.5</td><td>2.4</td><td>12.3</td><td>3.1</td></tr><tr><td>Alkali</td><td>9.4</td><td>2.3</td><td>9.3</td><td>2.4</td><td>8.8</td><td>2.1</td></tr><tr><td>pH</td><td>10.7</td><td>10.4</td><td>9.7</td><td>9.5</td><td>9.0</td><td>9.2</td></tr></tbody></table>

<d.l. means below the detection limit

<sup>a</sup>Intensity of main diffraction peak: <sup>+++</sup> strong, <sup>++</sup> moderate, <sup>+</sup> weak

equilibrium state should not strikingly change with temperature. The pozzolonic reaction was indeed already complete at 20°C (there was no more calcium hydroxide reservoir and the C-S-H got its lower Ca/Si ratio). Besides, the well-known increase in silicate chain polymerization of the C-S-H with temperature [20] should not occur as the C-S-H already gets the lowest Ca/Si ratio at 20°C.

## 3.3 Hydration by Milling and Reactive Oxides in Suspension

The goal of these approaches was to estimate if it was possible to reach equilibrium experimentally by accelerating the reactions. The crystalline phases present in the solid fractions and the ionic composition of the aqueous fractions of the samples obtained by milling and by mixing oxides in dilute suspensions are listed in Table 21.4 at the three temperatures.

All the samples contained C-A-S-H, but were free from portlandite and C₂S. Traces of ettringite were sometimes observed in model samples at 20°C. Using the milling technique, the unreactive phases of fly ash (mullite, quartz) remained present. The samples were not systematically analysed by EDS. However, the few analyses done showed the presence of non-reacted amorphous silica and Al(OH)₃ in both systems (milling and oxides in suspension). It corroborates the fact that the silica fume and fly ash were present in excess compared to OPC in the low-pH formulation. The mineral assemblages were thus similar in the three cases: hydration by milling, oxides in dilute suspensions, and thermodynamic simulation. Nevertheless, the phase proportions may vary from one case to another depending on the pozzolanic reaction progress. As a matter of fact, the Ca/Si of the C-A-S-H would decrease as well. In the cement pastes, the only supplementary phase was C₂S.
<figure><img src="image_5.png" /><figcaption>**Fig. 21.3** Comparison of the pH values obtained with the different methods at the three studied temperatures</figcaption></figure>

At 20°C, the pH decreased within the series cement paste/milling/oxides in suspension/thermodynamic simulation (see Fig. 21.3). This pH decrease was correlated with a Ca concentration decrease and Si concentration increase, except for the milling experiment (in this case, the sulphate concentration was surprisingly high, involving a high Ca concentration as well). These variations may thus be related to a greater progress of the pozzolanic reaction. The same pH decrease was observed at 50°C and 80°C between the three experimental methods, but the deviations were less and less significant when the temperature rose. At 80°C, the pH values were similar considering the experimental errors. It tends to show that the increase in temperature accelerated the pozzolanic reaction and at 80°C, the reaction progress was similar in the three experiments.

In conclusion, the two experimental approaches made it possible to consume all the reactive anhydrous phases from the low-pH binder. The small differences observed at 20°C and 50°C compared to thermodynamic simulation could be explained by a slightly less advanced pozzolanic reaction.

## 4 Conclusion

With the prospect of using low-pH binders in a geological repository, the goal of this work was to estimate the long-term chemical evolution of such materials. For this purpose, a three-way original approach was adopted, which consisted in the comparison of the final state achieved by thermodynamic simulations with experimental data. These data were obtained by milling the cement paste or by mixing
reactive oxides in suspension. Comparing the different results showed that this three-way approach is suitable to understand and predict the long-term chemical evolution of cements since the final states obtained in all cases were equivalent.

The mineral assemblages were similar in the three cases. At 20°C and 50°C, the phase proportions could slightly vary from one case to another, depending on the progress of the pozzolanic reaction. Raising the temperature from 20°C to 80°C accelerated the pozzolanic reaction and, at 80°C, similar results were obtained whatever the experimental method. In conclusion, the three-way approach under investigation was suitable to assess the chemical properties of fully hydrated cementitious materials with complex chemistry.

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