Sustainable Cellulose–Silica Aerogels from Wheat Husk for Thermal and Fire Protection

To address the resource-intensive and environmental drawbacks of petroleum-based thermal-insulating foams, valorizing agro-waste to produce multifunctional insulation materials is a promising route. Herein, wheat-husk agro-industrial by-product was used to fabricate cellulose–silica composite aerogels (CSA) through [BMIM]Cl-mediated cellulose dissolution–regeneration, coupled with sodium-silicate impregnation followed by in situ formation of silica and hexamethyldisilazane (HMDS) surface silylation. Different from conventional strategies that introduce pre-synthesized silica nanoparticles via blending or post-impregnation, this in situ strategy enables intimate hydrogen-bonded interfacial contact and uniform dispersion of silica within the three-dimensional cellulose network. The structure–property links among cellulose solid content, porous-skeleton evolution, mechanical, thermal-transport, flame-retardant and wetting performances were systematically explored. Raising cellulose content densified the interpenetrating cellulose–silica network, which is speculated to enhance intermolecular non-covalent interactions; CSA-10 achieved 1.14 MPa compressive stress under 6% strain. Owing to the hierarchical porous architecture suppressing gas–solid dual-path heat transfer, aerogels maintained low thermal conductivity (0.035–0.041 W·m−1·K−1). CSA-6 exhibits fast self-extinguishing performance, with 55% residue at 800 °C. HMDS grafting introduced trimethylsilyl groups, delivering water contact angles of 129.3–146.9°. This work achieves high-value valorization of wheat-husk waste and provides candidate multifunctional aerogels for energy-saving building insulation.