Mass transport behaviour of caesium and strontium in geopolymer cements

Abstract

Geopolymers are a promising alternative to conventional Portland cement-based wasteforms for immobilising hazardous radioactive fission products such as caesium-137 and strontium-90, offering superior durability and lower leach rates. However, the specific mass transport mechanisms governing radionuclide release in geopolymers remain poorly understood, limiting implementation. This study reveals the incorporation and mass transport mechanisms of caesium (Cs) and strontium (Sr) in metakaolin-based geopolymers. Solid-state characterisation showed Sr incorporation via direct chemical binding in the alkali aluminosilicate hydrate gel in charge-balancing extra-framework sites, replacing K⁺ ions, and precipitation of SrCO₃ and Sr(OH)₂, while Cs is predominantly bound within the charge-balancing sites in the alkali aluminosilicate gel. Leach testing confirmed low overall release rates, with all measured leachability indices significantly exceeding the industry minimum of 6 (Li >13 for Cs; Li >18 for Sr), outperforming conventional PC systems. Mass transport modelling revealed distinct mechanisms: Cs release is accurately described by a Diffusion/Surface Exchange Kinetics Model (DSEM), yielding high correlation (R² > 0.99), however, Sr exhibited a complex, staggered release profile. Standard mass transport models (diffusion, dissolution, surface exchange) could not satisfactorily capture this complex behaviour. We hypothesise this rate resumption is caused by the structural reordering or crystallisation of the amorphous K–A–S–H gel into a zeolitic phase, potentially excluding incorporated Sr. This finding highlights that simple diffusive models, commonly assumed for geopolymers, are inadequate for predicting the long-term performance of Sr-containing geopolymer wasteforms. The new insight presented here is critical development of geopolymers for radioactive waste disposal.