Geopolymers are inorganic, polyaluminosilicate or chemically-bonded ceramics centered around the nominal formula M₂O•Al₂O₃•4SiO₂•11H₂O where M = Group I elements and the amount of water is variable, depending on the particle size and specific surface area of the aluminosilicate clay. They are refractory, inorganic polymers formed from both aluminum and silicon sources containing AlO₄^-and SiO₄ tetrahedral units, under highly alkaline conditions at ambient temperatures. Therefore, they are a rigid, hydrated, materials containing group I, charge-balancing cations which result in an amorphous, cross-linked, impervious, acid-resistant, 3-D structure.^1,2 Geopolymer composites are stable to 1000°C above which they crystallize into ceramic composites. They can be reinforced with ceramic, metal, polymeric or biological particulates, chopped fibers, weaves or meshes. They can be prefabricated in polymeric molds or 3D/4D printed. Other new inorganic polymers are being identified, such as acid-based Al₂O₃•SiO₂•P₂O₅ made by high shear mixing metakaolin with phosphoric acid; magnesium potassium phosphate (MgKPO₄); magnesium borate (MgO•B₂O₃); yttrium silicates and zinc silicates.³

To produce 1 ton of geopolymer liberates only 0.25 tons of CO₂ whereas 1 ton of CO₂ is liberated for manufacturing 1 ton of cement. In civil engineering, the term “geopolymers” refers to the product resulting from high shear mixing of class F fly ash mixed with ground, granulated, blast furnace, slag, waste products. The solid is also amorphous or crystalline, but it is based on the calcium silicate hydrate (CSH), C(A)SH, KASH, NASH) binder phases, forming cements not geopolymer. In this structure, the silicate or aluminate tetrahedra form 2D layers sharing only two or sometimes three corners, and are separated by layers of Ca(OH)₂. CSH is the main binder phase in Portland cement. One main difference between the cements versus geopolymers is that geopolymers are chemically stable up to 1,000°C, after which they crystallize into ceramic, retaining some mechanical strength. Cements contain significantly more water and steadily decompose with increasing temperature, losing their mechanical strength.⁴

Geopolymers have wide potential applications as: fire-resistant structures or coatings, corrosion-resistant coatings; stronger and tougher replacements for cements and concretes; ceramic composites exhibiting “graceful failure” or pseudo-ductility; geopolymers containing glass frit can undergo amorphous self-healing when heated below 950°C (ASH-G) or behave as amorphous, self-healed ceramics when crystallized above 950°C; ASH-G composites for molten salt encapsulation for thermal energy storage or micro nuclear reactor applications; (a, b,g and neutron) nuclear radiation shielding; electromagnetic pulse interference (EMI) shielding; water purification filters; refractory glues between ceramics, metals, glass and/or wood; non-burnable building insulation; as a substitute for cements or concrete when made from revalorized mine tailings; removal of heavy metals (As, Hg) or PFAS from water.