Shear Characteristics and Microstructural Mechanisms of Bentonite Healing Interfaces under Thermal-Alkaline Coupling

In deep geological repositories for high-level radioactive waste, the shear characteristics of healed interfaces between bentonite blocks directly relate to the long-term stability of engineered barriers. Targeting the near-field coupled thermal-alkaline environment, this study conducted direct shear tests on combined GMZ bentonite blocks with technological voids after 150-day constant volume hydration. By integrating tests of water content, dry density, SEM, and XRD, the mechanisms influencing interfacial shear characteristics were investigated. Results show that at 20°C in deionized water under 1200 kPa normal stress, the peak shear strength of the sample with a 2 mm void decreased by 28.7% compared to intact samples. Elevated temperature reduced the heterogeneity of dry density and water content distribution. As temperature increased from 20°C to 80°C, the cohesion of the 2 mm void sample increased from 61.34 kPa to 141.96 kPa. NaOH concentration showed divergent impacts on the shear strength. At 0.1 mol/L, the peak shear strength increased by 81.9% relative to the deionized water condition, whereas at 1.0 mol/L, it decreased but remained higher than the deionized water baseline. The effect of coupled thermal-alkaline conditions on shear strength depends on alkaline concentration. In the 0.1 mol/L solution, the increase in shear strength was attributed to osmotic suction, which was equivalently transformed into an effective normal stress increment, compensating for the reduced dry density. In the 1.0 mol/L solution, elevated temperature accelerated mineral dissolution, with the d001 peak intensity decreasing by 88.2% compared to the baseline. The dry density further decreased, and structural degradation caused a substantial decline in interfacial shear characteristics. These findings provide a scientific basis for the long-term performance assessment of buffer materials in repositories.