Deterioration of swelling pressure of compacted Gaomiaozi bentonite induced by heat combined with hyperalkaline conditions

Compacted bentonite has been considered a suitable engineered barrier material for high-level radioactive waste (HLW) repositories for several decades. However, hyperalkaline groundwater produced by cementitious materials, combined with the heat generated by nuclear decay during the long-term storage of waste canisters, may cause the deterioration of the swelling properties of compacted bentonite. In this study, a series of swelling pressure tests and scanning electron microscopy (SEM) tests were performed on compacted Gaomiaozi (GMZ) bentonite (dry density 1.7 Mg/m³) to investigate the deterioration of the swelling pressure. Results indicated that the deterioration of the swelling pressure was facilitated by the temperature when the same concentration of NaOH solution was infiltrated, and a model of swelling pressure deterioration was developed to predict the long-term swelling pressure. Furthermore, the dissolution of montmorillonite and some silicate minerals, as well as the formation of non-expanding secondary minerals, led to transformations of the agglomeration patterns of the soil particles and structural damage to the bentonite, which controlled the long-term deterioration of the swelling pressure. Therefore, for the long-term operation of an HLW repository, the deterioration of the swelling pressure of compacted bentonite should be monitored, and safety assessments should account for the effects of heat and alkalinity.