<p>This study addresses the dual issues of natural sand depletion and the surge in construction and demolition waste by improving recycled fine aggregate (RFA) through optimised CO₂ curing for use in both Portland pozzolanic cement (PPC) and one-part geopolymer mortars. Using the Taguchi L9 design, carbonation parameters—time, temperature, relative humidity, and CO₂ concentration were statistically optimised to enhance process efficiency. The optimal condition of 24 h at 30 °C, 65% RH, and 15% CO₂ markedly improved specific gravity and bulk density while lowering water absorption, confirming effective pore refinement in RFA. Silica gel and lime-assisted treatments further enhanced carbonation efficiency. The 3% silica gel-pre-soaked aggregate (CST2) achieved superior CO₂ uptake and surface densification, while lime pre-treatment also produced notable improvements. The reported CO₂ uptake values were quantitatively derived from thermogravimetric analysis by converting the mass loss associated with CaCO₃ decomposition into equivalent CO₂ content, enabling direct evaluation of carbonation efficiency. In PPC mortars, optimised RFA restored the strength loss typical of untreated aggregates, while in geopolymer mortars, CST2 and lime-treated RFA yielded up to 44% higher compressive strength, reflecting better binder–aggregate compatibility. Microstructural analyses (TGA, XRD, FTIR, SEM/EDS) confirmed the formation of a CaCO₃ layer and an improved ITZ structure, while leachate tests indicated reduced pH and lower chromium release. Overall, the study offers a sustainable pathway for CO₂ utilization and high-performance RFA production for durable, low-carbon cementitious systems.</p>