Influence of fibers on the multiscale properties of fly ash and slag based geopolymer foam concrete

To address the need for low-carbon, lightweight, high-strength, and thermally insulating wall materials in prefabricated construction, this study systematically investigated the effects of polyvinyl alcohol (PVA) and glass fibers on the properties of chemically foamed fly ash-slag geopolymer foam concrete (GFC) at different temperatures. Multi-scale characterization techniques, including XRD, SEM, and digital image correlation (DIC), were employed to analyze compositional, microstructural, and crack‑evolution behaviors.The results show that while fiber addition reduces fluidity and raises dry density, it markedly enhances both mechanical and thermal performance. At a fiber length of 9 mm and content of 0.3 %, PF/GF reinforced GFC achieved a dry density of about 710 kg/m³ (<2 % increase), a compressive strength near 5 MPa (>20 % increase), and a thermal conductivity of 0.07–0.08 W/(m·K) (about 20 % reduction). At ambient temperature, PVA fibers bond tightly with the GFC matrix, refine the pore structure, and substantially improve toughness and crack resistance. Above 400 ℃, however, GF reinforced GFC retains higher residual strength and structural integrity due to its significantly higher melting point compared to PVA fibers. DIC and microstructural analyses confirm that GF maintains effective crack‑bridging and a stable fiber‑matrix interface under high temperature, while PVA fibers degrade and lose their reinforcing capability. By integrating DIC with SEM, a multi-scale framework linking macroscopic failure to microstructural response was established, elucidating the damage mechanisms of GFC under thermo-mechanical loading. These findings provide theoretical and technical support for developing lightweight, high-strength, and heat-resistant sustainable building materials.