Exploring the longevity of geopolymer waste forms co-hosting cationic and anionic radionuclides: A mechanistic investigation

Abstract

Geopolymers (GP) are increasingly recognized as a promising candidate for the primary engineered barrier to effectively immobilize a broad spectrum of radionuclides. This study delves into the development of metakaolin-based geopolymers capable of stabilizing a combination of highly radioactive cationic and anionic fission products. The addition of 2 wt% Cs and later a mixture of Cs, Eu, and Mo resulted in a reduction in compressive strength from 23.75 ± 1.03 to 8.05 ± 1.88 MPa. XRD and SEM-EDS analyses revealed the emergence of NaCl, Na₂CO₃. H₂O and Na₂MoO_4·2H₂O minor crystalline phases in the GP containing fission products. A 7-day standard product consistency test and a 90-day semi-dynamic leaching test ensured the chemical durability of structural elements (Si, Al, Na) and in situ injected elements (Cs, Eu, and Mo). Cumulative leaching tests and sorption results indicated an ion exchange mechanism for Cs with Na, complexation/sorption for Eu, while Mo was likely retained in the matrix through encapsulation. The absence of minor phases in the post-leaching FTIR and XRD patterns suggested fast initial dissolution of soluble phases, leading to higher leaching kinetics in the initial stages. The reported physico-mechanical properties, and short- and long-term leaching test values underscore the significant stabilization potential of the geopolymer for mixed radioactive species.