Physicochemical, hydromechanical, and Cs retention properties of Ca-bentonite treated with Ca(OH)₂ saturated solution at 150°C for up to 2 years

Understanding the performance of bentonite buffer materials in the presence of cement leachates at > 100°C is required for optimizing the design of deep geological repositories for high-level radioactive wastes. This study investigated the effects of exposure to a Ca(OH)₂ saturated solution at 150°C on Ca-bentonite, focusing on how mineralogical changes in the bentonite affect its physicochemical, swelling and hydraulic, and cesium (Cs) retention properties, which are key attributes for buffer materials. Analytical techniques, including X-ray diffraction, nuclear magnetic resonance, thermal gravimetry and differential thermal analyses, indicated calcium silicate hydrate (CSH) formation. Despite this alteration, cation exchange and Cs retention capacities were largely unaffected. Swelling pressure tests on compacted bentonite specimens (dry density: 1.6 Mg/m³) prepared from hydrothermally treated bentonite showed a reduction in swelling pressure. However, no significant difference in the hydraulic conductivity occurred. X-ray photoelectron spectroscopy analysis of the compacted bentonite specimen indicated consistent Cs retention across the specimen with no effluent detected over a one-year permeation period, suggesting that the specimens were still absorbing fluid. Overall, Ca-bentonite treated with a Ca(OH)₂ saturated solution at 150°C for up to two years showed minimal CSH formation and minor changes in physicochemical, hydraulic, and Cs retention properties, with a slight reduction in swelling pressure.