Enhancement of the mechanical properties and spatial uniformity of lightweight expanded polystyrene concrete using carbonated recycled fine aggregate

While environmentally beneficial, recycled fine aggregates (RFA) are limited by high porosity and poor interfacial properties, which also aggravate segregation behavior in lightweight expanded polystyrene (EPS) concrete. To synergistically address these dual challenges, this study provides the first systematic investigation of carbonation pretreatment (0.1–0.4 MPa, 12–48 h) for modifying RFA tailored specifically to EPS concrete applications. The effects were evaluated through a comprehensive approach that integrated a newly developed ultrasonic pulse velocity–density model with image analysis, allowing for the quantification of EPS spatial uniformity, a key methodological innovation in this study. Results show that optimal carbonation conditions (0.4 MPa, 48 h) reduced RFA water absorption by up to 31.7% while increasing aggregate density by 5.6%, improving concrete workability and enhanced compressive strength by up to 15.3% at a 50% replacement level. Notably, a nonlinear “reaction inhibition effect” was identified, indicating that excessive surface densification can constrain further treatment efficiency. Carbonated RFA substantially improved EPS spatial uniformity, reducing segregation indices by 46–52%, as validated by the proposed quantitative model. Overall, this work demonstrates a synergistic waste-to-resource strategy, in which carbonated RFA simultaneously enhances both the mechanical performance and uniformity of lightweight EPS concrete, contributing to the development of advanced low-carbon construction materials.