Recalcification of carbonated cement paste

Carbonation lowers the pH, leading to decalcification, shrinkage, and densification of the pore structure. Recalcification, the process of reintroducing calcium ions into decalcified cementitious materials, is a promising approach for restoring carbonated cement pastes. However, its impact on carbonated cementitious materials remains unelucidated. This study demonstrates, for the first time, how recalcification not only restores the Ca/Si ratio of calcium-(aluminum)-silicate-hydrate (C-(A)-S-H) to levels comparable with intact gel but also fundamentally alters its nanostructure. Using solid-state 29Si NMR, we show that recalcification turned silica gel into cross-linked Q3(1Al) sites, introducing small capillary pores and reducing the surface area. The extent of microstructural changes depended on the initial degree of carbonation. Importantly, 29Si NMR suggested that recalcification is a diffusion-controlled process, similar to calcium leaching and carbonation. These findings highlight the potential of recalcification to restore the binding phase and improve the durability of carbonated cement pastes, with implications for the development of targeted repair techniques in the construction industry.