Effect of temperature on Sr and Cs incorporation and mass transport mechanisms in geopolymer cement wasteforms.

Geopolymer wasteforms are a promising alternative to Portland cement for immobilisation of intermediate-level radioactive waste, particularly caesium-137 and strontium-90. Successful implementation relies on ensuring wasteform stability and radionuclide retention under elevated temperatures expected in a geological disposal facility. This study evaluates radionuclide retention and mass transport mechanisms of Cs- and Sr-loaded geopolymers leached at elevated temperatures (35, 50, and 90 °C). Monolithic leach tests confirmed that while Cs consistently exhibits a significantly larger Cumulative Fraction Leached compared with Sr across all temperatures, all geopolymers maintained acceptable leachability indices (Li > 6). Increasing the temperature to 90 °C resulted in a pronounced increase in release rates, with Cs release nearly tripling and Sr release increasing by an order of magnitude, primarily due to accelerated kinetics and increased solubility of secondary phases such as SrCO3 at elevated temperatures. Micro- and nanostructural analysis (XRD, FTIR, NMR) confirmed that the K-A-S-H gel framework remained stable, suggesting that bulk structural integrity is not compromised by these temperatures. Mass transport modelling revealed that Cs and Sr release is dominated by a complex, multi-parametric combination of diffusion and surface exchange kinetics (DSEM), with diffusion becoming increasingly dominant at higher temperatures. However, the temperature dependency does not conform to the Arrhenius principle, suggesting the process is governed by multiple overlapping mechanisms rather than a single activation energy across the investigated temperature range. Together, these findings show that geopolymers exhibit excellent thermal stability and maintain superior radionuclide retention performance compared to conventional cementitious systems under simulated repository temperature extremes.