Sustainable fly ash-derived silica reinforced nanocellulose-based porous composites for mechanically robust and fire-safe thermal insulation in building applications

The development of sustainable and fire-safe thermal insulation materials is critical for enhancing building energy efficiency and safety. This study reports the fabrication of porous composites reinforced with fly ash-derived silica and nanocellulose/poly (vinyl alcohol) via directional freezing and vacuum-assisted impregnation. By adjusting the silicate modulus, the interfacial interactions and pore-wall reinforcement were controlled, resulting in improved mechanical strength, thermal resistance, and flame retardancy. The optimized composite exhibits a compressive strength of 22.6 MPa and achieves a UL-94 V-0 rating, indicating excellent structural stability and fire safety. Although its thermal conductivity (∼0.287 W/(m·K)) is higher than that of ultralight insulation materials, the composite provides moderate thermal resistance together with high mechanical robustness and improved fire safety, making it more suitable for fire-safe thermal barrier applications rather than ultralow-k insulation. Compared to conventional insulation materials, the developed composite demonstrates a balanced combination of mechanical robustness, thermal resistance, and fire protection. This work provides a sustainable strategy for converting industrial solid waste into multifunctional fire-safe thermal barrier materials and offers new insights into the design of multifunctional bio-based porous composites for sustainable construction.