The uptake of radium by calcium silicate hydrates and hardened cement paste

In the present study the interaction of Ra(II) with calcium silicate hydrates (C-S-H) as well as with fresh and degraded hardened cement pastes (HCP) has been investigated using batch-type sorption tests. Synthetic C-S-H phases with varying CaO:SiO₂ (C:S)mol ratios, relevant to non-degraded HCP and degraded HCP, were prepared in the absence of alkalis (Na(I), K(I)) and in an alkali-rich artificial cement pore water (ACW). The Ra(II) sorption kinetics, the effect of the solid to liquid ratio and the C:S mol ratio on the Ra(II) uptake were investigated. In addition, the reversibility of Ra(II) sorption was tested using dilution tests. Comparison with data on the uptake of Sr(II) by C-S-H phases obtained from previous in-house studies showed that the same model concept could be applied to interpret the immobilization of Sr(II) and Ra(II) in these systems. Ra(II) sorption on C-S-H in the absence of alkalis could be interpreted in terms of a Ra²⁺-Ca²⁺ cation exchange model. The Ra²⁺-Ca²⁺ exchange selectivity coefficient, ^Ra_CaK_C, was determined to be (6±4). There are indications that, in the presence of alkalis, the Ra(II) sorption by C-S-H phases can only be described with the help of an ion-exchange model involving Ra²⁺, Ca²⁺ and the monovalent alkalis.

The cation exchange model developed for C-S-H phases in the absence of alkalis was successfully applied to predict the sorption of Ra(II) on degraded HCP, assuming that the Ra(II) sorbs solely on the C-S-H fraction of the HCP. Model predictions and experimental data were found to agree well in view of the simplified sorption model used.