Bio-based composite aerogels with high mechanical strength, flame retardancy, and thermal insulation via hydroxyapatite-mediated ion release

Biomass-based aerogels derived from polysaccharides and proteins have attracted considerable interest because of their environmental compatibility, cost-effectiveness, and inherent biodegradability. However, their carbon- and hydrogen-rich chemical structures render them intrinsically flammable, necessitating effective flame-retardant modification to satisfy stringent fire-safety requirements. Herein, an eco-sustainable strategy is reported for constructing biomass-derived composite aerogels that simultaneously integrate high mechanical strength, efficient thermal insulation, and enhanced fire safety. High-strength flame-retardant aerogels were fabricated from sodium alginate (SA), sodium lignosulphonate (LS), and hydroxyapatite (HAP) through a glucono-δ-lactone (GDL)-regulated in situ ion-release strategy. The gradual hydrolysis of GDL induces controlled acidification of the precursor system, promoting the sustained release of Ca 2+ from HAP and the homogeneous ionic cross-linking of SA chains into a stable three-dimensional “egg-box” network. The mechanically optimized SL3H0.6G0.8 aerogel achieved the highest compressive modulus of 9.32 MPa, corresponding to approximately 13 times that of pristine SA aerogel (0.69 MPa). In contrast, SL3H0.9G0.8 exhibited the most balanced overall performance, retaining a lightweight porous structure and a low axial thermal conductivity of 0.04553 W m −1  K −1 while delivering superior thermal stability, flame retardancy, and smoke-suppression capability. The enhanced fire resistance originates predominantly from a synergistic condensed-phase mechanism involving LS-derived carbonaceous char formation, phosphate-assisted dehydration and carbonisation, and reinforcement by thermally stable calcium/phosphate-rich inorganic residues. The resulting organic-inorganic protective layer effectively suppresses heat transfer, volatile release, and smoke evolution during combustion. Consequently, SL3H0.9G0.8 achieved a UL-94 V-0 rating and a limiting oxygen index of 37.5%, together with a peak heat release rate of 104.56 W g −1 and a total smoke production of 0.23 m 2 . This work establishes a sustainable route for the green fabrication of mechanically robust, thermally insulating, and fire-safe biomass-based aerogels, offering considerable potential for energy-efficient building applications.