Optimization of carbonation parameters for recycled fine aggregate utilizing the Taguchi method and absorbent-assisted treatment.

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.