Reactions between bentonite/montmorillonite and portlandite have been studied in the context of the engineered barriers of a purpose built repository for the deep geological disposal of spent fuel and high-level radioactive wastes. Portlandite was selected as the representative material of cement leaching in the early alkaline stage expected in a repository when conventional Ordinary Portland Cement (OPC) is used. Eight different batch experiments were performed for a reaction time near to two months, including bentonite or montmorillonite at montmorillonite/portlandite molar ratios of 2？:？1 and 3？:？1 under hydrothermal conditions. Temperatures of reactions were maintained constant at either 60 or 120°C. Calcium silicates hydrates with limited substitution of Al for Si (C-(A)-S-H phases with Al/Si <0.3) were formed with different structures and compositions as a function of the reaction conditions. Orthorhombic 11？？-tobermorite-type phase was detected in experiments at 120°C while a more disordered monoclinic tobermorite formed at 60°C. These results are useful for the interpretation of experimental data in more complex experiments using concrete or cement pastes and bentonite, where C-(A)-S-H phases of variable compositions can precipitate, in addition to the characteristic cement hydrates and other secondary minerals carbonates. 1. Introduction Concrete and compacted bentonite are considered by many international organizations as engineered barriers for the disposal of spent fuel and high-level radioactive waste in deep geological repositories. Reactions between the concrete and bentonite will likely produce an impact on the mineralogical and physicochemical properties at the interface of both materials that could compromise their safety functions in the long term. Concrete is made of cement, aggregates (inert granular materials such as sand, gravel, or crushed stone), and water. Additionally, supplementary cementing materials (e.g., fly ash, silica fume) and chemical admixtures can be incorporated. For laboratory experiments, usually cement mortar is used instead of concrete. The cement/bentonite interactions, and more generically, cement/clay interactions, have been studied by laboratory experiments, either at low water/solid ratios in advective transport cells, using compacted bentonite and cement mortar probes [1, 2], or at high water/solid ratios, studying mineral reactions in batch reactors [3, 4]. In addition, geochemical models have been used to explain the results or pursuing the extrapolation of mineral reactivity in the long term [5–7]. OPC
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