Investigation into the hydro-mechanical behaviour and microstructural evolution of MX80 bentonite pellet upon wetting/drying

Abstract

Bentonite pellet/powder mixtures have been proposed as the candidate sealing plugs of boreholes or galleries in deep geological disposal of high-level radioactive waste. During the long-term operation of repository, the filled bentonite pellets will be subjected to a coupled wetting/drying process, and their hydro-mechanical behaviors are accordingly expected to undergo complex changes. To clarify this point, the water retention and volume change behaviors of bentonite pellets were investigated by conducting hydration/dehydration tests under free-swell condition. Afterwards, the microstructure of bentonite pellets was studied by mercury intrusion porosimetry (MIP) tests. Results revealed that the water retention curves of pellets exhibited an obvious hysteresis in the high-suction range. The volume change of pellet was attributed to the combined actions from the expansion of micro-pores and the propagation of cracks. Meanwhile, the swelling of bentonite pellets exhibited a significant anisotropy during hydration, with a greater swelling strain in the axial direction. But this phenomenon became insignificant during the subsequent drying/rewetting processes. After a wetting/drying cycle, the bentonite pellets yielded an obvious plastic deformation, which was primarily related to the irrecoverable wetting-induced cracks.