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

• There is negligible effect of water chemistry on the swelling pressure in short term.
• The swelling pressure decreases after a long period of 700 days.
• There is no effect of hydration procedure on the long term swelling behaviour.
• The chemical effect is related to changes in microstructure.

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 700 days. 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 100 h revealed no significant influence of the water composition on the swelling pressure evolution, and the maximum swelling pressures observed were close to 4.30 MPa for a dry density of 1.70 Mg/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.