Investigation of ASR resistance of nuclear cemented waste forms containing high-nitrate sludge

This study investigates the alkali-silica reaction (ASR) resistance and performance of blended cementitious systems intended for the immobilization of high-nitrate liquid radioactive waste. The binder system, comprising blast furnace slag (BFS), silica fume and ordinary Portland cement, was evaluated under varying water/binder ratios of 0.35, 0.41, and 0.48. Accelerated and long-term ASR testing was conducted at 60 °C and 38 °C to assess their ASR resistance. The results indicate that ASR expansion was suppressed in all formulations, with specimens containing recycled concrete aggregate, registering maximum expansions of 0.13% at 60 °C and 0.05% at 38 °C. XRD and SEM analyses highlighted the crucial role of nitrated-AFm phases (e.g., Ca₂Al(OH)₆(NO₃)·H₂O) in reducing hydroxyl ion availability, while aluminium contributions from BFS and the waste enhanced aluminosilicate formation, further mitigating ASR. Thermogravimetric analysis revealed distinct decomposition stages between 50 °C and 800 °C, correlating with hydration product degradation and leaching. Compressive strengths consistently exceeded 8 MPa, as required by the Belgian Waste Acceptance Criteria, with peak values reaching up to 38 MPa at early ages. However, long-term exposure to high temperatures and relative humidity (∼98–100%) caused slight strength reductions due to leaching. These findings indicate that BFS- and silica fume based systems have the potential to serve as a promising and durable conditioning matrix for nuclear waste immobilization and emphasize the need for optimized curing conditions to mitigate leaching over time.