Gas diffusion property of compacted GMZ bentonite tested under different boundary conditions considering saturation and gas pressure

During the long-term operation of a deep geological repository, hydrogen, methane and carbon dioxide, etc. could be generated and accumulated in bentonite around canisters, threatening integrity and safety of the engineering barrier systems. In this work, self-designed test apparatuses were developed. Gas diffusion tests were conducted on GMZ bentonite specimens under both rigid and flexible boundary conditions with consideration of initial saturations and gas pressures. After experienced the gas diffusion tests, the specimens were cut and submitted for the mercury intrusion porosimetry (MIP) tests. Results revealed that under same initial saturation and gas pressure conditions, diffusion coefficient of the specimen tested under rigid boundary conditions was higher than that tested under flexible boundary conditions. This is because that compared to that of rigid boundary conditions, the effective pore volume for gas diffusion was more easily to be compressed under flexible boundary conditions, resulting in a relatively poor pore connectivity and low gas diffusion coefficient. Regardless of constraint conditions, diffusion coefficient of the specimens with same initial saturations decreased with increases of the initial gas pressure, and the decreasing rate slowed down with increasing initial gas pressure. Explanations could be that increases of gas pressure reduced the mean free path of gas molecules, affecting the diffusion efficiency. Meanwhile, the diffusion coefficient decreased with increasing initial saturations and same initial gas pressures for the flexible boundary specimens, while an opposite trend was recorded for the rigid boundary ones. Explanations to this observation could be that under the rigid boundary conditions, specimens with higher initial saturations have larger diameter macro-pores, facilitating the gas diffusion.