Retention mechanisms of stable- and radioactive- Cs and Sr in geopolymer materials: Insights into gel structural evolution and leaching behavior

Abstract

Geopolymers are potentially useful materials for radioactive wastes stabilization or immobilization. This study explores the retention mechanisms of stable- and radioactive-Cs and Sr in fly ash (FA)/ground granulated blast furnace slag (GGBS) blended geopolymer materials by examining the evolution of phases evolution and gel structures during geopolymer curing and subsequent element/radionuclide leaching behavior in Milli Q water (MW) and simulated Sellafield underground water (UW). Results show that Cs+ incorporation altered gel structures, while Sr2+ disrupted the geopolymerization process and generated SrCO3. Diffusion and dissolution mechanisms governed the leaching of Cs+ from the geopolymer, with geopolymer undergoing gel structural transformations during leaching in both MW and UW, while Cs + may facilitate Al release in the UW. Diffusion mechanism primarily drove the leaching of Sr2+ from geopolymer into Milli Q water, involving localized structural reorganization. Leaching in the UW probably involved gel transformation/reorganization and SrCO3 dissolution. Cs was more mobile than Sr regardless of whether stable or radioactive nuclides were applied, and compared with conventional cement stabilisers, geopolymer exhibited better immobilization. The leaching results and immobilization mechanisms of geopolymer for stable and radioactive nuclides were not always completely aligned, warranting further direct investigation using radioactive nuclides. This study provides a perspective on product evolution both during geopolymer incorporation and subsequent leaching of Cs and Sr, and highlights the potential of geopolymer materials to effectively stabilize and immobilize Cs and Sr-containing nuclear wastes.