On the impact of long-term thermal treatment on rehydration and swelling behaviours of bentonite

Bentonite-based buffer materials in radioactive waste repositories are exposed to prolonged elevated temperatures due to decay heat from the waste and long-term variations in subsurface thermal conditions associated with climate evolution. This study investigated the impact of long-term thermal treatment on the rehydration and swelling behaviours of calcium-type bentonite (Kunibond). Samples were thermally treated at 130°C and 300°C for times up to 800 days, followed by rehydration tests under varying relative humidity conditions and free swelling ratio measurements. The results indicated that higher treatment temperatures and longer durations progressively reduced the rehydration capacity and swelling ratio of the bentonite. X-ray diffraction and thermogravimetric analyses revealed that thermal treatment led to the collapse of the montmorillonite interlayer space and desorption of bound water. Furthermore, a machine learning framework combined with partial dependence plot analysis was developed to investigate the mechanisms controlling the swelling behaviour of thermally treated bentonite.