Thermal conductivity evaluation of Ca-type bentonite-sand mixtures as buffer materials for a high-level radioactive waste repository

The thermal diffusion capacity of buffer materials in engineered barrier systems is critical for controlling peak temperatures below 100 °C in high-level radioactive waste disposal repositories. As an enhanced buffer strategy, sand has been added to bentonite to improve thermal conductivity, but its effectiveness in Ca-type bentonite mixtures is less well understood than in Na-type bentonite. This study investigates the effects of sand content, dry density, and saturation on thermal conductivity and microstructure of Ca-type bentonite-sand mixtures, and develops a thermal conductivity prediction model using categorical multiple regression with dummy variables. Thermal conductivity increased by up to 37% with sand content up to 30%, increasing with dry density and degree of saturation. Moreover, SEM and MIP analyses revealed interfacial void formation and pore structure changes responsible for the nonlinear thermal response. Sand addition also shifted the compaction curve, enabling target dry densities to be achieved at lower initial water contents. Overall, sand addition provides dual benefits by enhancing thermal conductivity through solid-phase heat transfer pathways and facilitating favorable density conditions at low moisture levels, which together may contribute to reducing the required disposal site area from a thermal design perspective.