Evolution of swelling pressure properties of bentonite-sand-graphite buffer materials under action of temperature and salt/alkali

The buffer materials are the last engineering barrier for the repository of high-level radioactive wastes. It is necessary to have a deep understanding of the swelling characteristics of unsaturated buffer materials under the action of temperature and chemical fields. The bentonite-sand-graphite (BSG) buffer materials are taken as the research object. Based on the self-developed swelling pressure test devices, the influences of temperature and chemical solution on the swelling pressure of the BSG mixture are systematically studied. The results show that the high temperature and chemical solution reduce the swelling pressure. The influences of cation type are explained by the difference of its chemical activity. Ca²⁺ has higher exchange capacity than Na⁺. The influences of temperature on the swelling pressure are more than those of cation type. At room temperature, the swelling pressure of the BSG mixture with CaCl₂ solution is higher than that with NaCl solution, while the rule is opposite at high temperature. The swelling pressure decreases with the increase of pH value of NaOH solution. At higher temperatures, the rate of decline depends on the pH of the NaOH solution. High OH^- concentration is conducive to double-layer swelling and rearrangement of soil structure. The influences of NaOH solution with different concentrations on the swelling pressure are shown by the interaction of Na⁺and OH^-. The increase and decrease of the double-layer swelling pressure caused by the rearrangement of soil structure are controlled by the decrease of the double-layer thickness and the dissolution of montmorillonite and silicate minerals, respectively.