Multi-scale Engineered Tunnel Muck Concrete for Enhanced Freeze-thaw Resilience in High-altitude Environments: Synergistic Effects of Nano Silica and Vitrified Beads

Concrete infrastructure in high-altitude regions suffers from accelerated degradation under low-pressure and high-frequency freeze-thaw cycles, which conventional air-entraining agents often fail to mitigate. This study presents a sustainable multi-scale modification strategy for enhancing the freeze-thaw resistance of concrete made with recycled tunnel muck, tailored for the unique durability demands of high-altitude, cold-region infrastructure. By integrating nano silica and vitrified beads, the approach achieves a synergistic material enhancement: nano silica chemically densifies the matrix and refines the interfacial transition zone, while vitrified beads act as solid air-entraining agents that physically dissipate internal stresses and provide stable microstructural insulation. Mechanically, the modified tunnel muck concrete exhibits significantly reduced strength and mass loss along with improved dimensional stability under low-pressure, high-frequency freeze-thaw cycles where conventional air-entraining agents lose effectiveness. Microstructural evolution reveals that, unlike air-entraining agents-based systems prone to void collapse and hydration discontinuity under such extreme conditions, the synergistic action of nano silica and vitrified beads mitigates damage through pore refinement, internal stress buffering, and interfacial densification. Collectively, these findings demonstrate a robust and scalable material strategy for climate-resilient, resource-circular infrastructure in cold alpine regions.