Rapid-Hardening Engineered Cementitious Composites with OPC-SAC Binder and Recycled Fine Aggregates: Mechanical Properties and Micro-Mechanisms.

To address the slow early-age strength development of conventional engineered cementitious composites (ECCs), which limits their applicability in rapid-hardening engineering, and to promote the efficient resource utilization of construction and demolition waste, this study proposes a recycled high-ductility early-strength ECC (RHE-ECC) prepared using an ordinary Portland cement (OPC)-sulfoaluminate cement (SAC) composite binder, with recycled fine aggregate (RFA) fully replacing natural fine aggregate (NFA) and PVA fibers incorporated. The effects of the SAC replacement level and water-binder ratio (W/B) on the workability and mechanical properties of RHE-ECC were systematically investigated. The mechanical performance differences between RFA and NFA systems under the SAC-OPC composite binder were compared, and the micro-mechanisms by which RFA regulates the multiple-cracking behavior of ECC were elucidated through XRD and SEM analyses. The results indicate that at a SAC replacement level of 25%, the RHE-ECC achieves a 1 d compressive strength of 19.3 MPa while maintaining a 28 d compressive strength of 47.9 MPa, establishing a favorable balance between rapid early-age strength gain and sustained long-term development. At a W/B of 0.27, the RHE-ECC attains a 28 d ultimate tensile strain of 3.13%. This study systematically investigates, for the first time, the synergistic effects of the OPC-SAC composite cementitious system and full RFA replacement on the strain-hardening behavior of ECC, revealing that the porous old mortar layer of RFA weakens the ITZ, thereby reducing matrix fracture toughness and promoting multiple cracking, which enhances tensile strain capacity. These findings provide a theoretical foundation and technical support for the application of green, high-ductility cementitious composites in rapid-hardening engineering.