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

To address poor rheological properties and low mechanical strength of recycled aggregate concrete (RAC) caused by recycled aggregate (RA) defects, this study investigated effects of nano-CaCO₃ (NC) on RAC's rheological (yield stress, plastic viscosity) and mechanical (compressive, splitting, flexural strength) properties under varying NC content (0%–2.5%), recycled aggregate replacement rates (0%–100%), and water-to-binder ratios (0.35–0.45). Orthogonal experiments optimized mix design, and SEM, XRD, MIP elucidated modification mechanisms. Results showed 1.0%–1.5% NC reduced yield stress by 23.6%–31.2% and plastic viscosity by 18.9%–25.3%, while increasing 28-day compressive strength by 20.3%–26.7%. Optimal mix (1.2% NC, 50% recycled aggregate replacement, 0.40 water-to-binder ratio) yielded RAC with construction-appropriate rheological properties and 28-day compressive strength of 52.8MPa (24.5% higher than reference). NC enhanced performance via pore refinement (filling effect), promoted hydration (nucleation effect), and improved aggregate-mortar interface (ITZ modification). This study provides theoretical and technical support for NC application in RAC engineering.