The Effect of Strontium and Caesium Salts on the Kinetics and Mechanisms of Geopolymer Cement Formation

Strontium-90 and caesium-137 are two problematic fission products arising from nuclear power. These are categorised as intermediate level waste (ILW); the current technology for the safe containment and disposal of ILW is through encapsulation in Portland cement (PC) and ground granulated blast furnace slag (GGBFS) grouts. However, interactions between PC and the Sr and Cs means that alternative grouts may be needed for their safe encapsulation. Geopolymers (GP) have received significant interest due to favourable properties and potential to chemically immobilise cations. This thesis investigates the interactions of metakaolin-based, potassium-activated geopolymers with various salts of Sr and Cs. Metakaolin and potassium silicate reacted to form a geopolymer cement with a nominal formulation of Si/Al = 1.5, K/Al = 1, and H2O/K2O = 11. Incorporation of both Sr and Cs salts caused distinct changes to the kinetics of the geopolymer polycondensation reaction, depending on the solubility of the salt. The mechanisms for the change in kinetics fall into three categories: interactions with the metakaolin surface, reactions with the potassium silicate, or the filler effect caused by solid particles of insoluble Sr salts. Despite the change to the kinetics, the incorporation of the salts does not cause a significant change in geopolymer nanostructure, and does not prevent formation of a hardened monolith after 24 hours. The main binding phase is a K-A-S-H gel, although some of the potassium is displaced by the Sr2+ and Cs+, and in one case the presence of a crystalline Cs-A-S-H phase is detected. Sr is present in multiple forms within the geopolymer sample, including within a pseudo-zeolitic framework consistent with its incorporation as the charge balancing cation in the K-A-S-H gel. This thesis concludes that geopolymer cements show promise for incorporating Sr and Cs salts, and furthers their use case for long-term disposal of radioactive waste.