Utilization of Cs-Contaminated Fukushima Soil as Natural Pozzolan for Alkali-Activated Materials

The sustainable management of radioactive soil waste remains a critical environmental challenge, particularly in regions affected by nuclear accidents. In Fukushima, post-accident decontamination efforts have produced large volumes of cesium (Cs)-contaminated soils. These soils, derived from granitic and basaltic bedrock, exhibit natural pozzolanic properties, making them promising precursors for reuse. This study proposes a circular strategy to repurpose Cs-contaminated soils into potassium-based alkali-activated materials (K-AAMs). Two types of waste were utilized, natural soil and slurry soil. The slurry soil, which exists as a soil-water mixture, was wet-ground to enhance fineness and reactivity before being combined with raw soil. The mixture was then activated with a combination of potassium silicate solution and potassium hydroxide. The optimized formulation, consisting of 60% total soil (including 4.8% of the soil derived from the wet-ground slurry fraction), 21% alkali-activator, and 19% water, produced a durable cementitious matrix with a compressive strength greater than 1.8 N/mm2, surpassing Japan’s regulatory requirement of 1.47 N/mm2 for low-level radioactive waste disposal. The alkali-activation process promoted the formation of amorphous K-AAM phases, improving both mechanical performance and chemical durability. Leaching tests conducted over a period of 90 days revealed Cs release of approximately 10%, comparable to that of metakaolin-based systems. The findings demonstrate that Cs-contaminated soils can serve as sole precursors for alkali-activated matrices while maintaining stable Cs immobilization, thereby eliminating the need for external additives and reducing the overall volume of secondary waste. This provides a practical, scalable, and circular pathway for the long-term containment and safe disposal of radioactive soils.