Incorporation of strontium and calcium in geopolymer gels

Radioactive waste streams containing Sr-90, from nuclear power generation and environmental cleanup operations, are often immobilised in cements 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 show that the disordered geopolymers ((N,K)-A-S-H gels) formed by alkali-activation of metakaolin can readily accommodate the alkaline earth cations Sr2+ and Ca2+ into their aluminosilicate framework structure. The main reaction product identified in gels cured at both 20 degrees C and 80 degrees C is a fully polymerised Al-rich (N,K)-A-S-H gel comprising Al and Si in tetrahedral coordination, with Si in Q(4)(4Al) and Q(4)(3Al) sites, and Na+ and K+ balancing the negative charge resulting from Al3+ in tetrahedral coordination. Faujasite-Na and partially Sr-substituted zeolite Na-A form within the gels cured at 80 degrees 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. Ca2+ and Sr2+ induce essentially the same structural changes in the gels. This is important for understanding the mechanism of incorporation of Sr2+ and Ca2+ in geopolymer cements, and suggests that geopolymer gels are excellent candidates for immobilisation of radioactive waste containing Sr-90.