Characterisation of thermally altered cement pastes. Influence on selenite sorption

Cementitious materials are an integral part of the engineered barrier system in the French design of a deep geological radioactive waste repository. A maximum increase in temperature to 70°C resulting from exothermic hydration reactions and heat generation by the radioactive waste could lead to significant changes in hydrated cement paste mineralogy, which in turn will affect the sorption of radionuclides. Cementitious materials were prepared with ordinary Portland Cement at 20 and 70°C using different temperature conditions of hardening, cure and alteration. Selenite uptake was measured in each system. The sorption of selenite was studied in a pH and Eh range where Se(IV) is stable. Sorption kinetics and sorption isotherms of selenite were determined at 20 and 70°C for up to 140 days in the concentration range of 2 × 10⁻¹¹ to 8 × 10⁻⁴ mol/l for initial selenite. With increasing temperature, the hydrogarnet phase appeared and the crystallinity of C–S–H phases increased (formation of afwillite). Ettringite could no longer be detected. With the temperature increase, the specific surface area was about four times lower than the specific surface area of the sample aged at 20°C. The distribution ratios (R_d) decreased with temperature as a result. At 20°C, there was an initial fast sorption process (surface process) followed by a slower process such as a diffusion process into the solid phases. R_d values of 3600 ± 400 l/kg were obtained for 20°C altered cement suspensions at 30 days and for selenite concentration ranging from 8 × 10⁻¹² to 6 × 10⁻⁷ mol/l. At 70°C, the sorption kinetics were similar and R_d values were found to be 1290 ± 80 l/kg. Saturation of the sorption site(s) was observed for selenite concentrations greater than approximately 2 × 10⁻⁷ mol/l.