Spectroscopic evaluation of UVI-cement mineral interactions: ettringite and hydrotalcite

Spectroscopic identification of Ca-bearing uranyl silicates formed in C-S-H systems

Portland cement-based grouts used for radioactive waste immobilisation contain a Ca- and Si-rich binder phase, known as calcium-silicate-hydrate (C-S-H). Depending on the blend of cement used, the Ca/Si ratio can vary considerably. A range of C-S-H minerals with Ca/Si ratios from 0.6 to 1.6 were synthesised and contacted with aqueous U(VI) at 0.5 mM and 10 mM concentrations. Solid-state ²⁹Si MAS-NMR spectroscopy was applied to probe the Si coordination environment in U(VI)-contacted C-S-H minerals and, in conjunction with U L_III-edge X-ray absorption spectroscopy analysis, inferences of the fate of U(VI) in these systems were made. At moderate or high Ca/Si ratios, uranophane-type uranyl silicates or Ca-uranates dominated, while at the lowest Ca/Si ratios, the formation of a Ca-bearing uranyl silicate mineral, similar to haiweeite (Ca[(UO₂)₂Si₅O₁₂(OH)₂]·3H₂O) or Ca-bearing weeksite (Ca₂(UO₂)₂Si₆O₁₅·10H₂O) was identified. This study highlights the influence of Ca/Si ratio on uranyl sequestration, of interest in the development of post-closure safety models for U-bearing radioactive waste disposal.

Foreword to the special virtual issue on Actinide physics and chemistry with synchrotron radiation

Foreword to the virtual special issue of Journal of Synchrotron Radiation on Actinide Physics and Chemistry with Synchrotron Radiation.