Adsorption and Diffusion of Na+, Cs+ and Ca+2 Ions in C-S-H and C-a-S-H Nanopores

Cementitious materials act as a diffusion barrier, immobilizing liquid and solid<br>radioactive waste and preventing their release into the biosphere. The retention capability of hydrated<br>cement paste and its main hydration product, C-S-H gel, has been extensively explored experimentally<br>for many alkali and alkaline earth cations. Nevertheless, the retention mechanisms of these cations at<br>the molecular scale are still unclear. In this paper, we have employed molecular dynamics simulations<br>to study the capacity of C-S-H to retain Cs, Ca and Na, analyzing the number of high-affinity sites on<br>the surface, the type of sorption for each cation and the diffusivity of these ions. We have also explored<br>the impact of aluminum incorporation in C-S-H at a constant concentration of the ions in the gel pore.<br>We found strong competition for surface sorption sites, with notable differences in the retention of the<br>cations under study and a remarkable enhance of the adsorption in C-A-S-H with respect to C-S-H.