Geopolymers and Geopolymer-Derived Composites

Geopolymers are aluminosilicate inorganic polymers, cured at ambient temperatures from a liquid precursor. The strained nature of the 5-coordination aluminum cation polyhedra is identified as the reason why metakaolin-based geopolymer ceramics are made from solution, rather than with high temperature diffusion. The microstructure of alkali based geopolymers is nanoporous and nanoparticulate. This enables carbothermal reduction and carbothermal nitridation under flowing argon or nitrogen, in the presence of excess carbon to produce nanoparticles of silicon carbide (SiC), silicon nitride (Si3N4) or SiAlON from geopolymers. Crystallized geopolymer composites have been developed having tailorable thermal expansion coefficients. Intrinsic mechanical properties as well as those of geopolymer composites are summarized for composites containing both ceramic, polymeric as well as biological reinforcements. Amorphous glass frit-containing geopolymer composites undergo amorphous self-healing upon heating above the melting point of the glass. Amorphous self-healed geopolymer (ASH-G) refers to composites below ~950°C where the geopolymer is still amorphous, while ASH-C denotes amorphous self-healed ceramic where the geopolymer matrix has crystallized.