Thermodynamic Properties of (N,K)-a-S-H Gels for Radioactive Waste Solidification

Radioactive waste streams are often immobilised in Portland cement grouts to limit radionuclide leaching. Due to poor compatibility of certain wastes with Portland cement, alternatives such as alkali aluminosilicate ‘geopolymers’ are being investigated. Here, we describe the solubility and thermodynamic properties of synthetic sodium aluminosilicate hydrate gels (N-A-S-H) gels as the main reaction producing forming in metakaolin-based geopolymers. The solubility of solid N-A-S-H gels was determined by dissolution. The activities of the dissolved ionic species measured by ICP-OES were calculated using the Gibbs energy minimisation software GEMS v3.3 and the extended Debye-Hückel model. The aqueous and solid phases were characterised using X-ray diffraction, solid state ²⁷Al and ²⁹Si MAS and ¹H-²⁹Si CP MAS NMR and SEM-EDX. The (N,K)-A-S-H gels formed in the presence of trace amounts of Sr or Ca were a fully polymerised Al-rich (N,K)-A-S-H gels. Faujasite-Na and partially Sr-substituted zeolite Na-A form within the gels cured at 80 °C. Incorporation of Sr²⁺ or Ca²⁺ displaces some Na⁺ and K⁺ from the charge-balancing sites, with a slight decrease in the Si/Al ratio of the (N,K)-A-S-H gel. These results suggest that geopolymer gels are excellent candidates for immobilisation of radioactive waste containing alkaline earth radionuclides.