Role of microstructure evolution in temperature effects on swelling behaviour of compacted bentonite at different vertical pressures

Abstract

Compacted bentonite is often proposed to construct the buffer barrier in deep geological disposal of high-level radioactive waste. Temperature effects on swelling behaviour are significant for the performance assessment of the bentonite buffer. However, the contribution of microstructure evolution to temperature effects on swelling is poorly understood. This study focused on the role of microstructure change in the swelling of compacted GMZ bentonite under various temperatures and vertical pressures. Swell strain tests were conducted using a temperature-controlled oedometer, and mercury intrusion porosimetry was selected to investigate microstructure changes. Intra-aggregate and inter-aggregate pore size distributions were separated based on fitting the cumulative mercury intrusion curve using a bimodal van Genuchten-type equation. The pores larger than 0.1 μm were found to belong not only to the inter-aggregate pore family but also to the intra-aggregate pore family. The significance of the collapse of inter-aggregate porosity under high vertical pressure was highlighted, which resulted in the deceleration in swelling strain growth in the intermediate primary stage and the compression of inter-aggregate pores upon wetting. The temperature effects on the collapse are critical to the temperature dependence of swelling development under high vertical pressure. More collapse at elevated temperature gave rise to more pronounced deceleration in the intermediate stage and a larger amount of the inter-aggregate compression, which in turn led to a greater decrease in maximum swelling strain with temperature under higher vertical pressure. The intra-aggregate pore size distribution can be affected by temperature and vertical pressure, although the total void ratio of intra-aggregate pores was insensitive to temperature and vertical pressure. The volume enlargement of the intra-aggregate pores smaller than 6.5 nm was significant and decreased with increasing temperature and vertical pressure. In contrast, the volume enlargement of the intra-aggregate pores larger than 6.5 nm was relatively small due to the filling effects and exhibited increases with increasing temperature and vertical pressure.