Uptake of 226Ra in cementitious systems: A complementary solution chemistry and atomistic simulation study

The uptake of 226Ra to various cement hydration phases such as calcium-silicate-hydrates (C-S-H), ettringite (AFt) and monosulfate (AFm) as well as hardened cement pastes (HCP) made from ordinary Portland cement and low pH cement, respectively, was studied in batch-type sorption experiments under anoxic conditions. Besides sorption kinetics, the effects of solution composition and solid to liquid ratios were analyzed. Uptake kinetics were generally fast, leading to sorption equilibrium in less than 30 days. In particular, a strong retention of 226Ra by C-S-H phases was observed with a dependency of distribution coefficients on the Ca/Si - ratio of the C-S-H and the alkali content in solution. Atomistic simulation provided evidence that, in addition to Ra sorption on surface sites, the uptake of Ra due to exchange for Ca in the C-S-H interlayer space is a plausible uptake mechanism, in particular at low Ca/Si-ratios. Experiments performed on HCP revealed a correlation between sorption distribution coefficients of 226Ra and C-S-H content and composition. Experiments on C-S-H alteration showed that upon carbonation previously bound Ra is predominantly released into the pore solution, although some Ra can be retained by newly formed calcite.