An experimental study on thermal relaxation of BCV bentonite

Bentonites are considered as a suitable buffer material for the high-level radioactive waste disposal. The long-term stability of the hydromechanical properties of the bentonite barrier at high temperature is one of the key conditions for the proper functioning of the bentonite barrier. This paper presents an experimental study of the swelling pressure evolution of compacted samples of BCV bentonite exposed to high temperatures. The samples at constant volume were fully saturated at room temperature from the as-compacted state and then rapidly heated to constant temperatures of 50–150 °C. Temperatures were then held constant and the swelling pressure evolution was measured for the next 30 days. In the next stage, temperatures were decreased back to laboratory conditions and the equilibration of the swelling pressures was monitored. Finally, hydraulic conductivity of the selected samples was determined at room temperature. An additional test was performed to investigate the effect of saturation after heating on swelling pressure development. The results showed a significant decrease in the swelling pressure with time at elevated temperatures in all the samples (denoted as thermal relaxation). This decrease was more significant at higher temperatures and at temperatures above 100 °C the swelling pressure did not reach a steady state value until the end of the high temperature stage. The rate of swelling pressure decrease was quantified by the coefficient of relaxation Crel. A permanent decrease in swelling pressure due to thermal relaxation was identified after cooling. The swelling pressures decreased to 90–41% of the original swelling pressures determined before heating. The magnitude of the swelling pressure reduction increased with the applied temperature. Similar behaviour was observed for samples of different dry densities. The same effect of thermal relaxation was observed for samples saturated before and after heating. Thermal relaxation did not affect the hydraulic conductivity of the bentonite after temperature cycle, which was similar to that of thermally untreated BCV samples.