Self-activation of recycled fine aggregates in alkali-activated slag recycled concrete: Hydration behavior, microstructure and two-stage kinetic modeling

To achieve the synergistic utilization of construction waste recycling and low-carbon cementitious materials, this study investigates the hydration behavior, mechanical properties, and self-activation mechanism of alkali-activated slag recycled concrete (AASRC) under the combined action of recycled coarse aggregate (RCA) and recycled fine aggregate (RFA). Through experimental groups designed with varying alkali activator dosages and substitution rates of RCA and RFA, the influence of recycled aggregates on the macroscopic mechanical properties, hydration behavior, and microstructure evolution of AASRC was systematically analyzed. The results indicate that the alkali activator dosage is the primary factor governing the strength development of AASRC, while an appropriate amount of RFA significantly promotes early hydration reactions and strength formation. Microstructural characterization reveals that the residual old mortar and alkaline components in RFA facilitate the generation of C-A-S-H gels and optimize the pore structure, thereby exhibiting a distinct "self-activation effect." Based on the experimental findings, a two-stage hydration kinetic model considering nucleation-growth and thermo-chemical coupling was established. This model quantitatively explains the mechanism by which RFA enhances early strength through Ca²⁺ leaching-driven processes and local thermal activation positive feedback, effectively characterizing the hydration evolution of AASRC. This study provides a theoretical basis for the engineering application of green, low-carbon recycled concrete.