Multi-scale synergy of nano-quartz and carbon fiber in waste-based alkali-activated composites for sustainable and durable construction

The growing demand for low-carbon and sustainable construction materials has accelerated the development of alkali-activated composites (AACs) utilizing industrial by-products and waste materials. This study presents a multi-scale design approach for high-performance AACs using ground granulated blast furnace slag (GBFS) as the precursor and 100% waste marble powder (WMP) as a sustainable fine aggregate. The combined effects of nano-quartz (NQ, 0–2 wt.%) and carbon fiber (CF, 0–1 vol%) were systematically investigated to evaluate their synergistic interaction and their influence on mechanical performance, durability, and transport properties. The results demonstrate that composite behavior is governed by a non-linear multi-scale synergy, where NQ-induced matrix densification and CF-mediated crack-bridging mechanisms jointly control pore structure, stress distribution, and durability performance. The optimum mixture containing approximately 1% NQ and 0.5% CF achieved the highest performance, with compressive and flexural strength increases of ∼40% and ∼146%, respectively, compared to the reference mixture. This improvement is attributed to enhanced gel formation and silicate polymerization, producing a dense C–(A)–S–H/N–A–S–H network with improved fiber–matrix interaction. Durability performance was significantly enhanced through transport-controlled mechanisms, including reduced pore connectivity and limited moisture ingress. The optimized mixture exhibited near-zero strength loss under freeze–thaw cycles (−0.59%), reduced degradation under sulfate exposure (−6.74%), and improved thermal resistance up to 600 °C. SEM–EDS–XRD analyses confirmed that NQ enhanced gel densification, while CF effectively controlled crack propagation. These findings highlight the role of balanced multi-scale optimization in improving durability of sustainable AACs.