Immobilisation of Uranium^vi in Cementitious Materials: Evidence for Structural Incorporation in Calcium–Silicate–Hydrates and Solid Solution Formation

Abstract

Cementitious materials are an important component in the multi-barrier concepts developed in many countries for the safe storage of low and intermediate level (L/ILW) radioactive waste in deep geological repositories. Reliable thermodynamic models able to predict the interaction between radionuclides and cementitious materials in the long term are important for performance assessment. The development of such models requires a sufficiently detailed macroscopic and molecular-level understanding of the uptake processes involved. This study applied wet chemistry and luminescence spectroscopy to determine speciation of U^VI in cementitious materials. Batch sorption experiments on hardened cement paste (HCP) and its main constituent calcium–silicate–hydrate (C–S–H) phases were used to quantify U^VI uptake and investigate the effect of changes in solution chemistry and solid composition on the nature of the U^VI sorbed species. Complementary to wet chemistry, laser-induced luminescence spectroscopy studies on U^VI doped HCP and C–S–H pastes has provided information on local coordination of sorbed U^VI species. Wet chemistry studies and luminescence investigations showed that in HCP samples, U^VI was predominantly bound to C–S–H. U^VI uptake by HCP and C–S–H phases was found to be fast and sorption distribution ratios (R _d values) were very high, indicating strong retention by the solids. In addition, a strong dependence of pH and CaO:SiO₂ ratio was observed. Luminescence spectra of U^VI-doped C–S–H phases were similar to U^VI in the natural uranyl silicate soddyite, indicating comparable coordination of bound U^VI, thus suggesting that sorbed U^VI is incorporated in the interlayer of the C–S–H structure. Decay profile analysis combined with factor analysis of series of spectra of U^VI – C–S–H suspensions, recorded with increasing delay times, revealed the presence of two luminescent sorbed U^VI species in agreement with the wet chemistry data. At least the stronger sorbing species is incorporated in the C–S–H structure whereas the weaker sorbing species might be either an incorporated species or a surface complex. The high recrystallization rates found for C–S–H phases together with the information obtained from the wet chemistry and luminescence spectroscopy studies suggest that U^VI uptake by C–S–H phases may be described by a solid solution model. A sub-lattice ideal solid solution model with three C–S–H end-members and three U^VI-bearing end-members has been developed.