Microscopic analysis of thermal conductivity of bentonite as buffer materials under alkaline-thermal conditions

The evolution characteristics of thermal conductivity of the bentonite as buffer layer materials under alkaline-thermal conditions are a key factor in the conceptual design of a deep geological repository for nuclear waste. The evolution laws of thermal conductivity of the bentonite buffer materials under alkaline-thermal conditions are investigated by using the thermal probe method for MX80 bentonite powder pretreated under different alkaline-thermal conditions. On this basis, the XRD, SEM, MIP and TGA tests are carried out on the representative specimens to reveal the mechanisms of alkaline-thermal effects on the evolution of mineral composition, microscopic morphology, pore structure and water-binding morphology of bentonite samples, and to further elucidate the microscopic mechanisms of the evolution of thermal conductivity of bentonite buffer materials under alkali-thermal conditions. The test results show that under alkali-thermal conditions, the thermal conductivity ( \lambda ) of the bentonite samples decreases with the increasing pH value of the alkaline solution and increases with the increasing ambient temperature (T). This characteristic is particularly significant in highly alkaline solutions (pH=13.0~14.0) and high temperature environments (T=60℃~90℃). The underlying cause is the dissolution of the original mineral components in the samples under the action of alkaline solution to varying degrees, as evidenced by the reduction in the contents of montmorillonite and quartz and the increase in the zeolite content, resulting in reduction in the solid content of the samples, increase in the porosity, and reduction in the dry density and water absorption properties, while the ambient temperature plays a good role in facilitating the process.