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
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 at a constant concentration of the ions in the gel pore.
We found strong competition for surface sorption sites, with notable differences in the retention of the
cations under study and a remarkable enhance of the adsorption in C-A-S-H with respect to C-S-H.