Low flammability, foam-like materials based on bio-based, renewable ammonium alginate and sodium montmorillonite clay were fabricated through a simple, environmentally-friendly freeze-drying process in which water is used as solvent. These materials exhibit mechanical properties similar to those of rigid PU foams or balsa; the compressive modulus (1-97 MPa) and density (0.047-0.174 g/cm(3)) increase with increasing solids content, with an associated change from a layered to network microstructure structure. Calcium ions, either added directly or indirectly via CaCO3/gluconolactone (GDL) are an effective crosslinking agent for alginate, with the GDL route providing the greatest enhancement of mechanical properties. The thermal stability of the aerogels is also enhanced by the presence of crosslinking. The alginate/clay aerogels possess inherently low flammability, as measured by cone calorimetry, with heat release values decreasing as the proportion of clay in the composites is increased. (C) 2012 Elsevier Ltd. All rights reserved.