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

The evolution characteristics of thermal conductivity of the bentonite buffer layer material under alkaline-thermal conditions is a key factor in the conceptual design of a deep geological repository for nuclear waste. In this paper，the evolution law of thermal conductivity of the bentonite buffer material under alkaline-thermal conditions was investigated by using the thermal probe method for MX80 bentonite powder pretreated under different alkaline-thermal conditions. On this basis, XRD, SEM, MIP and TGA tests were carried out on 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 specimens, and to further elucidate the microscopic mechanisms of the evolution of thermal conductivity of bentonite buffer materials under alkali-thermal conditions. Test results show that under alkali-thermal conditions, the thermal conductivity (λ) of the bentonite specimen decreases with increasing pH value of the alkaline solution and increases with 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 that the dissolution of original mineral components in the specimen under the action of the 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 specimen, increase in porosity, reduction in dry density and a weakening of the water absorption properties, while the ambient temperature plays a good role in facilitating the process.