Incorporation of strontium and calcium in geopolymer gels

Radioactive waste streams containing ⁹⁰Sr, 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 °C and 80 °C is a fully polymerised Al-rich (N,K)-A-S-H gel comprising Al and Si in tetrahedral coordination, with Si in Q⁴(4Al) and Q⁴(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 °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 ⁹⁰Sr.