Temporal influence of thermal ageing on thermal conductivity of bentonite buffer layer materials under high temperature conditions

To investigate the temporal influence of thermal ageing on the thermal conductivity of bentonite buffer material under high temperature conditions, MX80 bentonite powder was pretreated at 100 ℃ and 200 ℃ for durations of 0, 15, 30, 60, 90, 120 days. The thermal conductivity of the compacted bentonite samples after pretreatment was measured using the thermal probe method, and the temporal influence was analyzed. The microscopic mechanism underlying the temporal influence on thermal conductivity λ of bentonite samples was revealed through particle size analysis, X-ray diffraction and thermogravimetric analysis tests. The experimental results indicate that: 1) After high-temperature aging (100 ℃ and 200 ℃), the thermal conductivity λ of bentonite samples decreased significantly with increasing thermal aging time t, demonstrating a significant temporal effect. A sharp decline was observed from 0 to 15 days, followed by stabilization after 30 days. The effect was more pronounced at 200 ℃ compared to 100 ℃. 2) High temperatures (100 °C and 200°C) result in the gradual desorption of various forms of water, thinning of the bound water film, and a reduction in the particle size of bentonite samples. Additionally, at 200°C, some montmorillonite minerals in the samples transform into sodium mica. These microstructural evolutions are consistent with the temporal influence observed in the thermal conductivity λ of the samples. 3) The fundamental reason for the temporal influence of thermal aging on the thermal conductivity of bentonite materials is as follows: At 100 °C, as thermal aging time t increases, the temperature effects lead to the gradual desorption of various forms of water, thinning of the bound water film, reduction in particle size, decrease in solid volume, and increase in gas volume, while the mineral composition remains unchanged. At 200 °C, as the thermal aging time t increases, the aforementioned temperature become more pronounced, and the high temperature causes some montmorillonite minerals to transform into sodium mica, which exhibits a lower thermal conductivity λ.