Evaluating the Effectiveness of CO₂ Curing Versus Conventional Water Curing in Fly Ash–Blended Concrete

This study evaluates the performance of carbon dioxide (CO₂) curing in comparison with conventional water curing for concrete embedding coal fly ash as a partial replacement of Ordinary Portland Cement. Cement mortar cubes were prepared with varying fly ash replacement ratios (0–25%) and subjected to CO₂ curing for 6, 12, and 24 hours, as well as conventional water curing for up to 28 days. Compressive strength development, carbonation depth, and durability-related characteristics were assessed. The results demonstrate that CO₂ curing significantly enhances early-age compressive strength by accelerating carbonation reactions and refining the pore structure. Specimens subjected to 24 hours of CO₂ curing achieved strength levels comparable to those obtained after several days of conventional curing. An optimum fly ash replacement level of approximately 5% yielded comparable or improved strength under both curing regimes. At higher fly ash contents (≥15%), a reduction in strength was observed due to dilution of cementitious phases, although CO₂ curing partially mitigated this loss. Phenolphthalein testing confirmed effective carbonation penetration, indicating successful CO₂ sequestration within the cement matrix. The study concludes that integrating CO₂ curing with partial cement replacement using coal fly ash offers a viable pathway toward sustainable, low-carbon concrete with enhanced early-age performance.