Microscopic mechanism of radionuclide Cs retention in Al containing C-S-H nanopores

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