Fly ash-based geopolymers for immobilization of nuclear waste containing cesium

Geopolymers (GP) are a class of relatively new sustainable inorganic materials considered as an alternative to ordinary Portland cement (OPC). GP technology provides an economical solution to utilizing fly ash with a positive environmental impact. The performance advantages of GP relate to their resistance to acid/sulfate attack, thermal stability, and durability. The growing nuclear safety concerns call for better solutions than OPC-based materials to retain radionuclides. Fly ash-based geopolymers (FA-GP) are targeted in this work as a potential sub-class of GP with an ability to immobilize radionuclides, especially cesium (Cs). The work includes a series of fundamental studies and engineering process development of low-cost NaOH-activated FA-GP, exploring the effects of process parameters on non-radioactive equivalent Cs immobilization (quantified as Cs leachability index LX and effective diffusion coefficient, D), in-situ Cs-containing zeolite crystallization, and microstructural development. The applied aspect of the work was to maximize Cs immobilization in FA-GP systems. Phase analysis (XRD), microstructural (SEM), pore structure (BET), and mechanical (compressive strength) studies led to a deeper understanding of the fundamental transformations that occur during the geopolymerization and zeolite crystallization processes. The factorial design of experiments and analysis of variance enabled us to establish quantitative relationships between the degree of Cs immobilization (LX) and the processing parameters in the FA-GP systems. Enhanced immobilization of Cs (measured through LX = 14.6, De = 2.5×10⁻¹⁵ cm²/s), not reported previously, was achieved by in-situ pollucite crystallization via a one-step synthesis route at a modest temperature of 90 °C. The degree of Cs immobilization and other properties were quantified as a function of in-situ pollucite content and shown to have a linear correlation. Johnson-Mehl-Avrami-Kolmogorov kinetics model described well in-situ pollucite crystallization below 90 °C. The activation energy of pollucite formation was found to be 25 kJ/mol. In addition to the advancement of understanding of the fundamentals of Cs-FA-GP system, this work also demonstrated the significant potential of FA-GP processed at relatively low temperatures as a conditioning matrix for long-term immobilization of cesium-containing nuclear waste.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat