This study evaluates indirect carbonation for CO₂ sequestration and high-purity CaCO3 production using FBC fly ash (HFA) and bottom ash (HBA). NH₄Cl extraction showed higher calcium recovery than DI water, with CaCO3 yields of 10.2 wt.% (HFA) and 8.4 wt.% (HBA). CO2 capture capacities were 45.0 kg and 36.8 kg per ton of ash. Vaterite formation occurred under low pH conditions. Carbonation residues had lower compressive strength but improved durability. This process supports sustainable waste management, circular economy principles, and CCUS by utilizing industrial by-products and generating high-purity CaCO3 for environmental and construction applications.