Selenite Sorption on Hydrated CEM-V/A Cement in the Presence of Steel Corrosion Products: Redox vs Nonredox Sorption

Abstract

Reinforced cementitious structures in nuclear waste repositories will act as barriers that limit the mobility of radionuclides (RNs) in case of eventual leakage. CEM-V/A cement, a ternary blended cement with blast furnace slag (BFS) and fly ash (FA) could be qualified and used in nuclear waste disposal. Chemical interactions between the cement and RNs are critical but not completely understood. Here, we combined wet chemistry methods, synchrotron-based X-ray techniques, and thermodynamic modelling to explore the redox interactions and non-redox sorption processes in simulated steel-reinforced CEM-V/A hydration systems using selenite as a molecular probe. Among all the steel corrosion products analysed, only the addition of Fe0 can obviously enhance the reducing ability of cement towards selenite. In comparison, steel corrosion products showed stronger reducing power in the absence of cement hydrates. Sele-nium K-edge X-ray absorption spectroscopy (XAS) revealed that selenite immobilization mechanisms included non-redox inner-/outer-sphere complexations and reductive precipita-tions of FeSe and/or Se(0). Importantly, the hydrated pristine cement showed a good reducing ability, driven by ferrous phases and (bi)sulfides (as shown by sulfur K-edge XAS) originated from BFS and FA. The overall redox potential imposed by hydrated CEM-V/A was deter-mined, hinting to a redox shift in underground cementitious structures.