Spectroscopic investigations of U(VI) speciation In cementitious materials

In the Swiss radioactive waste management program, cement is used as a matrix for long-lived intermediate-level waste (ILW), in which Uranium is an important radionuclide. Calcium Silicate Hydrates (C-S-H) are one of the major components of Hardened Cement Paste (HCP). A molecular-level understanding of Uranium uptake processes occurring in cementitious materials is essential to improve long-term predictions in safety analysis. U(VI) uptake by C-S-H (CaO/SiO2 = 1.07) in Artificial Cement pore-Water (ACW) and HCP has been investigated using Time Resolved Laser Fluorescence Spectroscopy (TRLFS) and X-Ray Absorption Spectroscopy (XAS) in order to determine the chemical environment of retained and precipitated U(VI) species in cementitious matrices. Phase X (CaUO4(H2O)x) soddyite and a uranophane (Ca(H3O)2 have been chosen as relevant reference compounds.
The main results are:
1) Preliminary TRLFS and XAS results are in a good agreement concerning U(VI) speciation in cementitious systems
2) TRLFS spectra of U(VI) in supernatant ≠ TRLFS spectra of U(VI) in cementitious pastes, i.e. free U(VI) species in ACW ≠ sorbed U(VI) species in cementitious pastes
3) TRLFS and XAS spectra of U(VI) sorbed species in HCP and C-S-H pastes are similar, i.e. C-S-H phases are responsible of U(VI) immobilization in HCP
4) For U(VI) high loading in HCP and U(VI) precipitated in ACW, i.e. U(VI) environment closed to a Ca-uranate phase with long U-Oaxial distances (1.86 ± 0.02) Ǻ
5) For U(VI) low loadings in HCP and C-S-H in ACW, i.e. uranophane-like structure with short UOaxial distances (1.83 ± 0.02) Ǻ