Thermal conductivity and microstructural evolution of bentonite pellet mixtures under combined temperature and pressure

Abstract

Bentonite pellet mixtures (BPM) are the preferred buffer for filling various seams or voids in deep geological disposal repositories for high-level radioactive waste. A series of thermal property tests of MX80 BPM was conducted under thermo-hydro-mechanical coupled conditions using the thermal probe method in this study. The potential impacts of particle size distribution, pellet density, pressure, and temperature on the thermal conductivity (TC) of specimens in different initial states, as well as the underlying heat transfer mechanisms, were explored. The macroscopic test results indicated that for the same conditions, the TC of compacted powder specimens was higher than that of BPM, and the TC of two-component mixtures was greater than that of three-component ones. The TC of BPM improved as the pellet density increased, and the impact of pellet density on the TC became more significant as the dry density increased. When pellet density increased from 1.8 to 2.0 g/cm³, the TC increase for the specimen with a dry density of 1.5 g/cm³ was 3.5 times greater than that for the 1.1 g/cm³ specimen. The temperature effect on specimen TC diminished with increasing applied pressure. The microscopic test results showed that increasing pellet density reduced larger inter-aggregate pores but increased smaller intra-aggregate pores in low-density specimens, while slightly increasing inter-aggregate pores and decreasing intra-aggregate pores in high-density specimens. Under vertical pressure, both pore types decreased, particularly large pores, with higher water content amplifying this compression. Larger and more numerous soil pores enhance vapor-driven latent heat transfer during temperature changes, intensifying its effect on thermal conductivity. Pressure application reduces soil pores, limiting latent heat transfer pathways and thereby diminishing the temperature effect on TC.