Lamellar-structured anisotropic sodium alginate/montmorillonite aerogels with mechanical robustness and...

Lightweight, flame-retardant, and thermally insulating bio-based materials are required for thermal protection in buildings; however, sodium alginate (SA) aerogels are mechanically fragile and prone to drying shrinkage. Herein, anisotropic SA/montmorillonite (SA/MMT) aerogels were fabricated via freeze-casting, post-freeze ionic crosslinking in a low-temperature CaCl₂/EtOH bath, and air drying. The key innovation lies in the MMT-assisted stabilization of the freeze-cast lamellar skeleton, which preserves a highly porous framework during air drying while enhancing mechanical robustness and fire resistance. Increasing the MMT/SA ratio from 0 to 80 wt% reduced radial shrinkage from 13.6% to 4.2% while maintaining porosity above 95% and increased the axial specific modulus from 65.4 to 174.5 MPa cm³ g^-1. The LOI increased from 29 to 81.2, whereas pHRR and THR decreased from 28.8 to 13.0 W g^-1 and from 2.91 to 1.41 kJ/g, respectively. The optimized SA/MMT-80 aerogel exhibited a density of 0.09 g/cm³ and a thermal conductivity below 0.05 W/(m K). XPS and Raman analyses of the post-combustion residues revealed a stabilized carbonaceous-inorganic residual barrier, with the I_D/I_G ratio decreasing from 2.82 to 1.76, while TG-FTIR indicated a possible complementary gas-phase dilution effect associated with the released CO₂. These results establish a structurally stabilized air-drying route for multifunctional SA-based aerogels.