Effect of aging on thermal conductivity of compacted bentonites

The thermal conductivity of compacted bentonite is one of the most crucial parameters for the safe operation of high-level radioactive waste (HLW) repositories where bentonite can act as a feasible buffer. In this study, the potential effects of aging on the thermal conductivity of Gaomiaozi (GMZ07) and Wyoming (MX80) bentonites were investigated experimentally. The bentonite specimens were prepared under different conditions, such as varying water content and dry density, through the static compaction method in a special mould. The water content of the compacted bentonite specimens was kept constant during curing periods of 0, 5, 15, 30, 60, and 100 d under constant volume conditions, and the thermal conductivity of the two bentonites was then measured using the thermal probe method. The test results showed that the thermal conductivity decreased with increasing aging time for both of the compacted bentonites, with a trend that decreased significantly at the early aging periods and then tended to be constant when the aging time exceeded 60 d. The effects of aging are more pronounced for specimens with higher dry densities and water contents. Mercury intrusion porosimetry (MIP) test results confirmed that the reduction in thermal conductivity with aging time could be attributed to smectite hydration within the bentonites during the aging process. With this hydration, part of the soil water moves into the interlayer spaces of smectite crystals, resulting in a decrease in the pore water outside the bentonite particles, which decreases the thermal conductivity of the bentonites.