Carbonation of hydrated cement:The impact of carbonation conditions on CO₂ sequestration, phase formation, and reactivity

The efficiency of hydrated cement carbonation is often ascribed to the total amount of CO2 stored by the material. However, the morphology and composition of silicate phases upon carbonation are equally important to unseal the pozzolanic potential of cement recycling for sustainable growth. Therefore, the current study investigates the impact of aqueous- and dry-carbonation (AC and DC) on polymorph formation, silicate microstructure, and porosity of CEM-I as well as CEM–III–derived pastes. The limited diffusion of calcium ions during DC gives rise to the formation of a carbonate shell comprised of calcite, aragonite, and vaterite and results in a 15% lower carbonation efficiency compared to AC. Manual milling was required to expose the amorphous silicate gel and allow for optimal reactivity. AC in 0.1 M sodium hydroxide was found to be the most efficient carbonation method for both reactivity and CO2 sequestration. Sodium hydroxide allowed for an increased CO2 capture and thereby promoted rapid carbonation and the formation of a highly porous silicate phase. The presence of sodium hydroxide promotes decalcification but is postulated to limit the pozzolanic reactivity. Although a high calcium content allows for high CO2 sequestration in CEM-I, for better pozzolanic reactivity a silica-rich material like CEM-III is recommended.