Spectroscopic evaluation of U^VI-cement mineral interactions: ettringite and hydrotalcite

Portland cement based grouts used for radioactive waste immobilization contain high replacement levels of supplementary cementitious materials, including blast-furnace slag and fly ash. The minerals formed upon hydration of these cements may have capacity for binding actinide elements present in radioactive waste. In this work, the minerals ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O) and hydrotalcite (Mg₆Al₂(OH)₁₆CO₃·4H₂O) were selected to investigate the importance of minor cement hydrate phases in sequestering and immobilizing U^VI from radioactive waste streams. U L_III-edge X-ray absorption spectroscopy (XAS) was used to probe the U^VI coordination environment in contact with these minerals. For the first time, solid-state ²⁷Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was applied to probe the Al coordination environment in these U^VI-contacted minerals and make inferences on the U^VI coordination, in conjunction with the X-ray spectroscopy analyses. The U L_III-edge XAS analysis of the U^VI-contacted ettringite phases found them to be similar (>∼70%) to the uranyl oxyhydroxides present in a mixed becquerelite/metaschoepite mineral. Fitting of the EXAFS region, in combination with ²⁷Al NMR analysis, indicated that a disordered Ca- or Al-bearing U^VI secondary phase also formed. For the U^VI-contacted hydrotalcite phases, the XAS and ²⁷Al NMR data were interpreted as being similar to uranyl carbonate, that was likely Mg-containing.