In this study, Na2CO3-activated slag cements were produced from four different blast furnace slags, each blended with a calcined layered double hydroxide (CLDH) derived from thermally treated hydrotalcite. The aim was to expedite the reaction kinetics of these cements, which would otherwise react and harden very slowly. The inclusion of CLDH in these Na2CO3-activated cements accelerates the reaction, and promotes hardening within 24h. The MgO content of the slag also defines the reaction kinetics, associated with the formation of hydrotalcite-type LDH as a reaction product. The effectiveness of the CLDH is associated with removal of dissolved CO32− from the fresh cement, yielding a significant rise in the pH, and also potential seeding effects. The key factor controlling the reaction kinetics of Na2CO3-activated slag cements is the activator functional group, and therefore these cements can be designed to react more rapidly by controlling the slag chemistry and/or including reactive additives

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.