Water-retention properties and microstructure changes of a bentonite pellet upon wetting/drying; application to radioactive waste disposal

Like bricks of compacted bentonite/sand mixtures, mixtures made up of pellets and powder of bentonite are considered as a possible material to make the sealing plugs used to fill up galleries and ensure long-term watertightness in deep radioactive waste disposal. Pellets/bentonite mixtures have a low permeability, high swelling capacity, good radionuclide retention capability and operational advantages in terms of placement in the galleries. Following a previous in-depth characterisation of bentonite pellets/powder mixture conducted by the same group, an investigation of the water-retention properties and microstructure changes of a bentonite pellet subjected to wetting/drying cycles under free swelling conditions was carried out by means of mercury intrusion porosimetry (MIP) and X-ray microtomography. A complete description of the changes in water content, void ratio and degree of saturation of the pellet was provided. Data showed that the free swelling of the pellet is due to the combined effect of both crack propagation at the macro scale, and the swelling of bentonite grains, governed by hydration mechanisms along the smectite faces at the nano scale. Significant development of a crack network is observed between 38 and 9 MPa. For suctions below 9 MPa, there is a significant decrease of the platelet thickness and an increase in the disorder of the platelet assembly, resulting in the average MIP entrance pore radius increasing to 0·4 μm within the expanded bentonite grains.