Study on rheological and mechanical properties optimization and mechanism of Nano-CaCO₃ regenerated aggregate concrete

Recycled Aggregate Concrete (RAC) suffers from deteriorated rheological properties and insufficient mechanical strength due to the defects of the original aggregates, which is the core bottleneck restricting its large-scale engineering application. Existing studies mostly focus on the optimization of a single property by nanomaterials, and the relationship between dosage and performance lacks systematic analysis, making it difficult to balance construction performance and strength. This study, through a multi-factor orthogonal experiment (nanocalcium carbonate (NC) dosage of 0–2.5%, recycled aggregate substitution rate of 0–100%, water-binder ratio of 0.35–0.45), combined with SEM, XRD, and MIP multi-scale characterization techniques, systematically revealed the synergistic modification mechanism of NC on the rheological and mechanical properties of RAC. The results showed that a 1.0–1.5% NC dosage could simultaneously achieve a 23.6–31.2% reduction in yield stress, a 18.9–25.3% reduction in plastic viscosity, and a 20.3–26.7% increase in 28-day compressive strength, breaking through the technical limitation that “modified materials are difficult to balance construction fluidity and mechanical performance”. The optimal mix ratio determined by orthogonal optimization (1.2% NC, 50% recycled aggregate substitution rate, 0.40 water-binder ratio) reduced the yield stress and plastic viscosity to the appropriate range for construction, and the 28-day compressive strength reached 52.8 MPa (an increase of 24.5% compared to the benchmark group). Mechanism analysis indicated that NC achieved a performance leap through “porosity filling—hydration nucleation—interface strengthening” triple synergy: refining the capillary pore structure (total porosity decreased by 12.7%), accelerating the cement hydration process (28-day hydration degree increased by 18.3%), and reducing the thickness of the aggregate-mixing cement interface transition zone (ITZ) from 45 to 28 μm, from the microscopic level. This study established an experimentally validated quantitative correlation framework of “nanomaterial dosage—recycled aggregate characteristics—concrete macroscopic properties”, which demonstrates good predictive performance within the experimental range (NC dosage 0.8–1.6%, recycled aggregate replacement rate 30–70%, water-binder ratio 0.38–0.42), with regression coefficients R² > 0.92 for both yield stress and compressive strength predictions, providing technical support for the precise application of NC in construction waste resource utilization projects based on experimental optimization and mechanism-supported performance study.