Carbonation–hydration interplay in CO2-cured Portland cement: Mechanistic insights into early-stage strength development

Carbonation curing has emerged as a promising strategy for simultaneous strength enhancement and CO 2 sequestration in cement-based materials. However, the fundamental interplay between carbonation and hydration, and its implications for strength development, remains insufficiently understood. Since the formation of CaCO 3 consumes reactive clinker phases, it is unclear whether carbonation promotes or hinders hydration during early-age curing. This study systematically investigates the carbonation–hydration interplay in CO 2 -cured OPC by controlling pre-curing durations (6–30 h) prior to CO 2 exposure. A combination of XRD-Rietveld analysis, FTIR, BSE imaging, and MIP was employed to quantify phase evolution, microstructure, and pore characteristics. Results show that moderate pre-curing (12–18 h) optimizes the balance between hydration and carbonation, leading to enhanced clinker reactivity, reduced total porosity, and significantly improved early compressive strength. In contrast, extended pre-curing (>24 h) or excessive carbonation reduces strength development due to calcium depletion, limited CO 2 diffusion, and pore coarsening. Microstructural and chemical analyses reveal a dual role of carbonation: it accelerates early hydration through Ca 2+ consumption and nucleation effects, while prolonged carbonation progressively suppresses hydration by forming dense carbonate products and altering pore connectivity. These findings demonstrate that carbonation and hydration exhibit both competitive and synergistic interactions, with their relative contributions governed by the timing of CO 2 exposure within the investigated pre-curing window. This work provides mechanistic insights into optimizing CO 2 -curing regimes for improved performance and carbon efficiency in cement-based materials.