CO2 sequestration and microstructural engineering of recycled concrete aggregates for unbound pavement sub-base applications

The widespread use of recycled concrete aggregate (RCA) in pavement construction is limited by its high porosity and weak adhered mortar, which reduce its engineering performance and durability. This laboratory-scale study presents a novel integrated evaluation of accelerated carbonation treatment (ACT) by linking CO 2 sequestration, microstructural evolution, and engineering performance to assess the suitability of RCA for pavement base and subbase applications. RCA was subjected to ACT at CO 2 concentrations of 10% and 20% for 3 and 7 days under 20 psi gauge pressure, with each test performed in triplicate. Carbon sequestration was quantified using thermogravimetric analysis (TGA), while mineralogical and microstructural changes were characterized using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The effects of carbonation on major engineering properties were then evaluated. Carbonation facilitated the formation of stable calcite, pore refinement and densification of the adhered mortar, leading to significant engineering enhancement. The highest absolute CO 2 uptake of 51.08 kg/ton was obtained at 20% CO 2 after 7 days, which was 108.1% higher than that of untreated RCA. Water absorption reduced from 2.95 to 1.84%, specific gravity increased from 2.36 to 2.50, and aggregate impact value reduced from 34.05 to 23.62%, indicating improved particle integrity and durability. Decreasing Ca/Si ratio from EDS analysis further supported these improvements. The laboratory-scale studies reveal that ACT boosts the engineering performance and CO 2 sequestration ability of RCA. Thus, highlights the potential of RCA as a sustainable material for low-carbon and resource-efficient pavement applications.