Influence of thermal ageing time on barrier properties of bentonite as a buffer material at high temperature conditions and its micro-mechanism

The effect of thermal aging time on the engineering barrier performance of bentonite buffer material at high temperature is of great significance for evaluating the long-term operation safety of nuclear waste deep geological repositories. MX80 bentonite powder was pretreated at high temperature of 200℃ with different thermal aging time (t=0, 15, 30, 60, 90, 120d), and the evolution of engineering barrier performance parameters such as thermal conductivity λ, constant volume expansion force Ps, unconfined compressive strength qu, and specific surface area SSA with thermal ageing time t was studied. The variation trend of mineral composition, adsorbed water and microstructure in MX80 bentonite after high temperature aging was discussed by the X-ray diffraction (XRD), thermogravimetric analysis (TGA)，and scanning electron microscope (SEM), and the influence mechanism of thermal aging time on engineering barrier properties of bentonite buffer material was revealed from a microscopic perspective. The test results show that: 1) the engineering barrier performance parameters (λ 、Ps 、qu 、SSA) of MX80 bentonite decrease with thermal aging time t increasing, showing a significant time effect which mainly occurred in 0 to 15 days with a decay rate of 56.89 to 68.51%. 2) During high temperature aging, montmorillonite minerals in bentonite are partially transformed into sodium mica, adsorbed water in soil is lost, and soil microstructure evolves. The above microscopic changes are consistent with the time effect of thermal aging in engineering barrier properties. 3) Under high temperature conditions, the engineering barrier performance parameters (λ 、Ps 、qu 、SSA) of MX80 bentonite are linearly positively correlated with the montmorillonite content Cm and adsorbed water content w, indicating that the transformation of bentonite mineral composition, the loss of adsorbed water in soil, and the evolution of soil microstructure are the essential reasons for influence of thermal ageing time on the engineering barrier properties of bentonite.