A Study on the Impact Resistance and Hybrid Fiber Reinforcement of Recycled Aggregate Concrete Modified by Composite Activators

Alkali-activated concrete (AAC) is a focal point in sustainable construction research due to its low carbon emissions and excellent mechanical properties. This study systematically examines the mechanisms by which various alkali-activation systems influence the setting behavior and impact resistance of recycled aggregate concrete (RAC). Four representative mix designs were developed by comparing single NaOH and NaOH-Na₂SiO₃ composite systems, with and without 0.2% nano-silica modification. Initial experiments revealed that the NaOH-Na₂SiO₃ system increased the compressive strength of neat paste by 23.5% to 28.41 MPa compared to the single NaOH system, with nano-silica further refining pore structure and elevating strength to 32.2 MPa. Building on the superior NaOH-Na₂SiO₃ system, polypropylene fibers (PPF) and recycled carbon fibers (RCF) were incorporated, and their effects were analyzed using drop-weight impact tests and Weibull failure modeling. Results demonstrate that nano-silica enhances the interfacial bonding between carbon fibers and the matrix, with the NT12-C5 mix exhibiting the highest impact resistance.