Sustainable Performance and Interfacial Characteristics of Fully Recycled Concrete with Combined Recycled Concrete, Brick, and Ceramic Aggregates

The rapid growth of construction and demolition (C&D) waste calls for sustainable recycling in concrete production. However, most studies address single-source recycled aggregates, leaving the behavior of mixed recycled systems insufficiently understood. To address this gap, this study investigates concrete in which natural aggregates are fully replaced by recycled concrete aggregate (RCA), and part of the RCA is further substituted with waste ceramic tile (WCT) or recycled crushed brick (RCB) at controlled proportions. Aggregate type and blending ratio were systematically examined, and Vickers microhardness was used to characterize the interfacial transition zone (ITZ). This study quantitatively establishes a micro–macro correlation between ITZ microhardness profiles and mechanical performance in fully recycled concretes with hybrid aggregate systems. Results show that high replacement levels (100% WCT and 100% RCB) reduced density by 5.6% and 7.0%, respectively, and increased water absorption, reaching 17.0% for 100% RCB. Optimal ultrasonic pulse velocity (UPV) occurred at intermediate blending ratios—50% for WCT and 75% for RCB. Mechanically, a 25% WCT substitution enhanced splitting tensile strength by 50%, whereas a 100% RCB substitution reduced compressive strength by 10.6%. Microhardness profiling revealed ITZ widths of about 70 μm for RCA and WCT, and 80 μm for RCB, consistent with corresponding macro-scale trends. These quantitative findings demonstrate that the rational blending of recycled aggregates can fine-tune the microstructure and performance of fully recycled concrete, providing insight into the high-value recycling of C&D waste.