Carbonation Curing of Semi-Dry Flue Gas Desulfurization Ash for CO2 Sequestration: Microstructural Evolution and Strength Development of Alkali-Rich Industrial Waste

Semi-dry desulfurization ash (SDA) is generated in rapidly increasing quantities and remains underutilised, despite its high CaO content, which makes it a promising candidate for CO2 storage via carbonation curing. However, the carbonation behaviour and consolidation mechanism of standalone SDA compacts are not yet well understood. In this study, SDA compacts were prepared at water-to-solid (w/s) ratios of 1.5:10 and 1.83:10 and subjected to carbonation curing for 0–24 h under controlled CO2 conditions. Compressive strength, CO2 uptake, and microstructural evolution were assessed using XRD, TG–DTG, FTIR, and SEM. CO2 uptake increased with curing time and reached approximately 20% after 24 h, whereas compressive strength exhibited a non-linear response, peaking at 8.67 MPa after 6 h at w/s = 1.5:10 and declining thereafter. Phase and microstructural analyses indicate that strength development is governed by the transformation of Ca(OH)2 to CaCO3 polymorphs, with early densification followed by increased porosity as calcite coarsens. Sulphur-bearing phases (e.g., CaSO3·0.5H2O) remain largely inert under the tested conditions. These findings demonstrate that carbonation curing can significantly enhance CO2 fixation in SDA and generate low-strength construction materials while also highlighting the need to optimise mix design and curing parameters to mitigate strength loss at extended curing times.