Immobilization of Ca²⁺ by OH^- ions on calcium silicate hydrates

The interface of calcium silicate hydrate (C-S-H) plays a a crucial role in the mobility of ions through hardened cement paste and thus in the long term durability of concrete structures. Here, we combine experimental zeta potential measurements with non-equilibrium molecular dynamics simulations to elucidate the surface state of C-S-H under cement-relevant conditions. Our findings reveal that the co-adsorption of Ca²⁺ and OH^- ions overcompensates the negative surface charge of C-S-H, originating from deprotonated silanol groups, thereby forming a surface adsorption network. This mechanism is governed by the interplay between solution pH and calcium speciation, where higher pH values favor the formation of calcium-hydroxide solution complexes. Consequently, an increase in pH leads to a higher Ca/Si ratio in C-S-H and a greater adsorption of these complexes, further increasing the effective surface charge density. Conversely, a reduction in pH promotes ion desorption, leading to a progressive decrease in the Ca/Si ratio and decrease in the effective surface charge. These findings provide new insights into the long-term compositional evolution of C-S-H in cement paste, with implications for phase stability, mechanical performance, and concrete durability.