Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Extended X-ray absorption fine structure (EXAFS) spec-troscopy has been used to investigate the coordination envi-ronment of U(VI) in cementitious materials. The EXAFS measurements were carried out on U(VI)-doped samples prepared under varying conditions, such as samples from sorption, hydration and diffusion experiments, and using different cementitious materials, such as crushed hydrated hardened cement paste (HCP) and calcium silicate hydrates (C-S-H). The samples had U(VI) loadings ranging between 1 700 µg/g to 45 000 µg/g. Applying principal component analysis (PCA) on 13 EXAFS spectra (each spectra corre-sponding to a minimum of 5 different scans) of the low loading samples, we obtained one single species indicating a similar U(VI) coordination environment for both HCP and C-S-H samples. This result confirms that C-S-H phases control the uptake of U(VI) in the complex cement matrix. The radial structure of this species is similar to a U(VI) surface complex or to U(VI) in uranyl silicate minerals (two axial O atoms at 1.82 ± 0.02 Å; four equatorial O atoms at 2.25 ± 0.01 Å; one Si atom at 3.10 ± 0.03 Å). At high U(VI) loading, PCA revealed a second U(VI) species, with a radial structure similar to that of U(VI) in calcium uranate (two axial O atoms at 1.94 ± 0.04 Å; five equatorial O at-oms at 2.26 ± 0.01 Å; four Ca atoms at 3.69 ± 0.05 Å and five U atoms at 3.85 ± 0.04 Å).
This study suggest that, at low U(VI) loading, U(VI) is bound to C-S-H phases in HCP while at high U(VI) load-ing, the immobilization of U(VI) in cementitious materials is mainly controlled by the precipitation of a calcium uran-ate-type phase.