Retention of waste cocktail elements onto characterised calcium silicate hydrate (C-S-H) phases: A kinetic study under highly saline and hyperalkaline conditions

C-S-H phases are a major part of hardened cement used as potential construction material of a repository for high-level radioactive waste. Therefore, C-S-H phases are of great interest with respect to their structural and kinetic sorption properties to elucidate their contribution to the long-term safety of such a repository. Using TG-FTIR, XRD and zeta potential measurements, synthesised C-S-H phases were analysed. The retention of a waste cocktail consisting of U(VI), Eu(III), Cs(I), and iodide as a mixture onto C-S-H phases were analysed over 112 d by using inductively coupled plasma mass spectrometry (ICP-MS). Under hyperalkaline (pH > 12.5) and highly saline (2.55 M) conditions, no state of equilibrium was reached for Cs(I) and iodide with respect to their immobilisation on C-S-H phases after 112 d. For U(VI) and Eu(III) an equilibration time of seven days is required to achieve a more or less steady state with high retention (> 85%). The experiments demonstrate that Ca(II) and Na(I) from pore water solution play an important role in the immobilisation of waste cocktail (WC) elements.