The impact of molten salt waste loadings on the properties of geopolymers and Portland cement for radioactive waste immobilization

The transition to advanced nuclear reactors, such as Molten Salt Reactors (MSRs), introduces new challenges for the safe immobilization of radioactive waste. This study investigates the potential of blended Portland cement (PC) and geopolymers (GPs) using blast furnace slag (BFS) and metakaolin (MK) as precursors to immobilize sodium carbonate-based molten salt (MS) waste. While PC is widely used for the immobilization of low- and intermediate-level waste, its high CO₂ footprint has driven increasing interest in alternative binders such as GPs. The effects of 10 and 15 wt% MS waste loadings on the material properties are evaluated and compared to a reference without MS. Preconditioned MS waste was incorporated into three binder systems: PC, GP-BFS and GP-MK. A comprehensive testing program was conducted, including mechanical strength, setting time, viscosity, carbonation resistance, hydration heat, scanning electron microscopy (SEM), nitrogen adsorption and ultrasonic testing. Results indicate that both PC and GP matrices show potential, but performance strongly depends on the combination of MS waste loading and binder system. MS additions improved strength and reduced porosity, but workability declined with higher loadings for both PC and GP-BFS. MK-based GPs did not meet acceptance criteria and are unsuitable for immobilization. These findings highlight the complexity of waste form development and the importance of using binder systems tailored to the specific waste stream to ensure long-term durability and safety.