Development of High-Performance Eco-Friendly Concrete Incorporating Recycled Fine Aggregates: Mechanical, Microstructural and Carbon Footprint Assessment

This study investigates the feasibility of incorporating recycled concrete aggregate (RCA) as a full replacement for fine aggregate in high-performance concrete (HPC). Five mixtures containing 0%, 25%, 50%, 75%, and 100% RCA were experimentally evaluated. Hardened density, compressive strength, splitting tensile strength, flexural strength, water absorption, microstructure (SEM), and carbon footprint were analyzed. Results showed a near-linear density reduction of 5.8% at 100% RCA due to lower specific gravity and higher porosity of RCA. Mechanical performance decreased with RCA content: compressive strength reduced from 78 MPa to 53 MPa (−32%), splitting tensile strength from 6.2 to 4.2 MPa, and flexural strength from 7.6 to 5.0 MPa. Water absorption increased by 42%, indicating increased permeability. SEM analysis revealed a weakened and porous interfacial transition zone (ITZ) surrounding RCA particles, explaining strength reduction. Carbon footprint analysis showed a slight increase in emissions with RCA due to cement dominance and RCA processing energy. Results confirm that RCA enhances material circularity but introduces a performance–durability trade-off, requiring optimized mix design and SCM integration for structural applications.