Long-term effect of water chemistry on the swelling pressure of a bentonite-based material

Compacted bentonite-based materials have been proposed as possible sealing and backfill materials in geological repositories for the high-level radioactive waste disposal in several countries. During the long time lifespan of a repository, as the chemical composition of pore water can change, the swelling and sealing capacity of the material may also change. From a point of view of storage safety assessment, it is important to evaluate this possible change of swelling capacity. In this study, a mixture of bentonite and crushed Callovo-Oxfordian claystone was investigated. The long-term effect of pore water chemistry on the swelling pressure was studied at constant-volume conditions for 700days. Distilled water and synthetic water having the chemical composition similar to that of in-situ pore water were used for hydration. The results obtained in an initial period of 100h revealed no significant influence of the water composition on the swelling pressure evolution, and the maximum swelling pressures observed were close to 4.30MPa for a dry density of 1.70Mg/m³. Over a longer time period, on the contrary, the swelling pressure eventually decreased for all samples, especially for the sample saturated with synthetic water. In addition, comparison of a one-step soaking test with a multi-step soaking test showed no wetting procedure effect on the long term swelling behaviour. All the results are analysed by considering the physico-chemical interaction between the minerals of claystone, minerals of bentonite and different fluids involved.