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 green building material research due to its low carbon footprint and superior performance. This study seeks to enhance the impact resistance of recycled aggregate concrete (RAC) by elucidating the synergistic mechanisms of alkali activation , nano-modification, and fiber reinforcement . To this end, four mix designs , incorporating NaOH and NaOH-Na₂SiO₃ systems with 2 % nano-SiO₂(NS), were developed and assessed through setting time, compressive strength , drop hammer impact tests, and XRD/SEM analyses. The NaOH-Na₂SiO₃ system exhibited a 23.5 % increase in compressive strength over NaOH, achieving 28.41 MPa, while NS refined pore structures , elevating strength to 32.2 MPa; XRD/SEM analyses confirmed mechanisms of pore refinement and interfacial enhancement. In the optimized system, the NT12-C5 formulation, incorporating polypropylene fiber (PPF) and recycled carbon fiber (RCF), exhibited superior impact resistance, with NS enhancing interfacial bonding between carbon fiber and the matrix, resulting in a 47.8 % increase in initial crack impact energy. The Weibull model validated the reliability of impact performance. Furthermore, life cycle assessment revealed that Soil Solidification Rock Recycled aggregate concrete (SSRRAC) substantially reduced carbon emissions compared to ordinary Portland cement (OPC), while maintaining competitive economic costs. This study’s innovations include: (1) synergistic optimization of low-carbon AAC performance using NaOH-Na₂SiO₃ and NS; (2) optimized PPF/RCF formulations promoting the reuse of waste carbon fiber; and (3) application of the Weibull model to overcome conventional statistical constraints. Collectively, these findings establish a theoretical and practical foundation for the global development of sustainable building materials. • NaOH-Na₂SiO₃ composite boosts RAC compressive strength by 23.5 %. • Nano-SiO₂ refines pores, reaches 32.2 MPa, enhances fiber bonding. • NT12-C5 shows best impact resistance with less variability. • Weibull model validates dynamic performance reliability.