Assessment of Swelling Behavior in Thermally Treated Soil Mixtures for Use as Barrier Fillers in High-Level Radioactive Waste Repositories

Bentonite-based mixtures are promising buffer materials for high-level radioactive waste repositories, but their performance can be affected by heat from decaying nuclear waste. This study investigated the swelling behavior of bentonite–sand–graphite (BSG) mixtures under temperatures ranging from 25°C to 80°C, with varying sand content and dry density. Test results indicated that higher temperatures promote bentonite hydration and swelling, increasing final swelling pressure and strain. While swelling evolves smoothly at room temperature, elevated temperatures cause pronounced fluctuations. This thermal effect is modulated by material composition: higher sand content weakens swelling by disrupting the bentonite structure and inducing microcracks; whereas higher dry density enhances swelling by improving bentonite compactness. Thus, temperature sensitivity of BSG mixtures is jointly governed by sand content and dry density. Interestingly, the mineral composition of bentonite remained unchanged when the temperature changed from 25°C to 105°C. However, changes were observed in certain functional groups. The interlayer spacing of montmorillonite d (001) decreased, and the formation of silicon oxides was detected. These microstructural changes contributed to the cementation of mineral particles, leading to particle aggregation and a “condensation” behavior in bentonite.