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. For this, three series of samples were synthesized, which comprise samples with Ca/Si molar ratios of 0.8, 1.2, and 1.6, representing different cement degradation stages, and varying 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 Rd 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 and 1H-29Si cross-polarization MAS NMR spectroscopy. AlO4 tetrahedra were identified to occupy bridging positions of the Si chain and cross-linking positions. Elevated temperatures during solid phase formation were found to increase the crystallinity of the material 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 effected by elevated temperatures. To simulate salinity changes of the pore water, the effect of increased ionic strengths (up to 2.6 M) 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 increased ionic strengths on U(VI) retention was insignificant.

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. For this, three series of samples were synthesized, which comprise samples with Ca/Si molar ratios of 0.8, 1.2, and 1.6, representing different cement degradation stages, and varying 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 Rd 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 and 1H-29Si cross-polarization MAS NMR spectroscopy. AlO4 tetrahedra were identified to occupy bridging positions of the Si chain and cross-linking positions. Elevated temperatures during solid phase formation were found to increase the crystallinity of the material 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 effected by elevated temperatures. To simulate salinity changes of the pore water, the effect of increased ionic strengths (up to 2.6 M) 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 increased ionic strengths on U(VI) retention was insignificant.