The cement–clay interaction (CI) experiment was carried out at the Mont Terri rock laboratory to complement the current knowledge on the influence that cementitious materials have on Opalinus Clay (OPA) and bentonite (MX). Drill cores including the interface of OPA, concrete (LAC = low-alkali binder, and OPC = ordinary Portland cement), and MX, which interacted for 4.9 and 10 years, were successfully retrieved after drilling, and detailed analyses were performed to evaluate potential mineralogical changes. The saturated compacted bentonites in core samples were divided into ten slices, profiling bentonite in the direction towards the interface, to evaluate the extent and spatial variation of the mineralogical alteration of bentonite. Regarding the mineral compositions of bentonite, cristobalite was dissolved within a range of 10 mm from the interface in both LAC-MX and OPC-MX, while calcite precipitated near the interface for OPC-MX. In LAC-MX and OPC-MX, secondary products containing Mg (e.g., M-S-H) also precipitated within 20 mm of the interface. These alterations of bentonite developed during the first 4.9 years, with very limited progress observed for the subsequent 5 years. Detectable changes in the mineralogical nature of montmorillonite (i.e., the formation of illite or beidellite, increase in layer charge) did not occur during the 10 years of interaction.

Some low-level radioactive wastes categorized as shallow underground pit disposal may contain high concentrations of sulfate. There is concern that sulfate may interact with cementitious materials and affect the physical properties of the waste. We examined the interaction of sulfate-containing cementitious materials by using drums of the same size as actual waste bodies. Two specimens with different sulfate concentrations were prepared and their chemical and physical properties were analyzed. Results showed that different sulfate concentrations affected the temperature history and phase composition within the waste, and the sulfate concentration also changed the composition of the products. It was observed that sulfate accelerated the rate of the hydration reaction of the cement paste, which may have resulted in a greater temperature increase. It was also shown that sulfate affects the amount of calcium silicate hydrate (C-S-H) produced. In specimens with high sulfate concentrations, the amount of C-S-H formed decreased, as did the reaction rate of the slag. This could be a result of sulfate interfering with the hydration reaction of the slag.