Uranium(VI) retention by calcium (alumino)silicate hydrates – Impact of temperature and ionic strength

Abstract

Calcium aluminosilicate hydrates (C-A-S-H) are the main hydration products of cement formulations with Al-rich additives such as blast furnace slags, fly ashes or calcined clays. We studied systematically the effect of Al substitution on both the structure of C-A-S-H phases and their U(VI) retention properties in comparison to C-S-H phases. To represent the different stages of cement degradation, three series of samples were synthesized, which included samples with Ca/Si molar ratios of 0.8, 1.2 and 1.6 and different Al/Si molar ratios of 0, 0.06 and 0.18 for each series. Furthermore, the impact of synthesis temperature (room temperature, 100 °C, 200 °C) on the C-(A-)S-H structure was studied. The U(VI) retention, studied either by sorption experiments or by direct synthesis of U(VI)-containing C-(A-)S-H phases, was found to be strong with log R_d between 4.9 and 5.6. Structural characteristics of the cementitious phases were obtained from powder X-ray diffraction (XRD) as well as ²⁹Si and ²⁷Al solid-state magic angle spinning nuclear magnetic resonance (SS MAS NMR) spectroscopy. AlO₄ tetrahedra were identified to occupy bridging positions of the silicate chains, thereby increasing the mean chain length and providing additional sorption sites for U(VI) at room temperature. Elevated temperatures during solid phase formation, however, lead to a stronger cross-linking of the silicate chains with the appearance of neoformed crystalline phases. Luminescence spectroscopy was applied to characterize the U(VI) binding. Various U(VI) species (interlayer or surface sorbed) are formed in different proportions, depending on the composition and structure of the C-(A-)S-H phases, which are partly affected by elevated temperatures. To simulate salinity changes of the pore water, the effect of enhanced ionic strengths and the presence of carbonate on the stability of the U(VI) retention by the C-(A-)S-H phases was studied by sorption and leaching experiments over extended timescales of up to 6 months. For C-(A-)S-H phases previously exposed to temperatures of up to 100 °C, the effect of medium ionic strengths (up to 2.6 M) on U(VI) retention was insignificant.