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RESEARCH PRODUCT

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

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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 to decrease the heat output during hydration by dilution of OPC and improve the mechanical strength of the final material. 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. For this purpose, a three-way original approach was adopted. Firstly, hydration of low-pH cements was accelerated by milling cement slurries with zirconia beads. Secondly, the low-pH cement pastes were mimicked from mixtures of appropriate highly reactive oxides (lime, silica, calcium aluminate and calcium sulphate) in diluted suspensions. Thirdly, 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. Comparing the different results showed that this three-way approach is suitable to understand and predict the long-term chemical evolution of the cements since the final states obtained in all cases were equivalent. This method was then used to investigate the influence of temperature in the range 20–80°C on the chemical evolution of a low-pH cement.