Performance and Environmental Implications of CO2-Cured Nanoengineered Recycled-Aggregate Concrete with Treated Aggregates

The large-scale use of recycled aggregate (RA) in concrete is hindered by its high porosity and water absorption, which degrade mechanical and durability performance. This study introduces a novel approach that integrates nanocomposite modification and CO 2 curing to unlock the potential of recycled-aggregate concrete (RAC). Recycled coarse aggregate was treated and beneficiated with a colloidal nanosilica (CNS)–ultrafine fly ash (UFFA) composite and subjected to CO 2 curing to promote synergistic hydration–carbonation reactions. Thermogravimetric analysis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM)-energy dispersive spectroscopy (EDS) confirmed the formation of stable mineral phases, including vaterite, calcite, tobermorite [calcium silicate hydrate (C─S─H) gel], and magnesite polymorphs, indicating significant microstructural densification. RAC with 30%–50% modified RA and 3% CNS–15% UFFA exhibited superior phase stability and achieved a 19% reduction in global warming potential compared with the control mix. These findings highlight a scalable and sustainable pathway for producing high-performance RAC, demonstrating that nanoengineered CO 2 curing strategies can simultaneously enhance material quality and reduce environmental impact.