Two-Stage Deterioration Mechanisms in Recycled Aggregate Concrete: From Pore Interface Degradation to Aggregate Interface Defect Control

Recycled aggregate concrete (RAC) provides an effective approach for the large-scale reuse of construction and demolition waste. This study systematically investigates the performance degradation mechanism of RAC and proposes targeted suggestions for enhancing its performance. The key findings are summarized as follows: (1) The macroscopic performance of RAC is consistently inferior to that of natural aggregate concrete (NAC). Specifically, the compressive and tensile strengths of RAC decrease by 3–50% with the increase in recycled aggregate (RA) replacement rate. (2) At the macroscopic scale, the inherent defects of RA (e.g., cracks and attached mortar) are the primary drivers of RAC performance degradation. At the microscopic scale, early-stage deterioration is mainly attributed to the high porosity (20%~27%) of RAC and the weak interfacial transition zones (ITZ) between aggregates and paste. At the same time, later-stage degradation is induced by the formation of multiple weak transition zones within the matrix. (3) For practical engineering applications, the principle of “source strengthening, process optimization, and hierarchical application” should be adhered to, aiming to improve the performance and promote the utilization efficiency of RAC. These findings establish a cross-scale theoretical foundation for the performance enhancement of RAC, thereby contributing to the more efficient resource utilization of C&D waste and advancing the sustainability of the construction industry.