Geopolymers are a new class of ceramic-like materials which are 3-dimensionally bonded, aluminosilicate, inorganic polymers and cured at ambient temperatures. They are made from calcined clay and an aqueous, alkaline, metasilicate solution precursor. A typical composition is centered around M2O•Al2O3•4SiO2•13H2O where M is an alkali metal such as Na, K or Cs. The strained nature of the 5-coordination aluminum cation polyhedra is identified as the reason why metakaolin-based geopolymer ceramics are made from highly alkaline solution, rather than with high temperature diffusion as is typical for ceramics. Geopolymers are made under high shear mixing, are scalable into large batch sizes and can be 3D printed. While cements are 2D-bonded inorganic polymers of silicates and aluminates forming calcium silicate hydrates and calcium aluminates, geopolymers are 3D, covalently-bonded, aluminosilicate polymers which set at room temperature. Geopolymers essentially behave as an acid-resistant, refractory glue which can be reinforced with a variety of sand, gravel, chopped basalt fibers or platelets. Geopolymer composites exhibit significant graceful failure and fracture toughness such as three times that of alumina. Geopolymer composites can adhere to metals and form an acid-resistant coating as well as undergo self-healing. Geopolymers can be heated up to ~1,000°C after which they crystallize into an aluminosilicate ceramic, such as nepheline or leucite. Geopolymers can be made into porous water purification filters for removing heavy metals such as arsenic, cadmium, mercury etc. Current work is in progress to make geopolymers by valorization of mine tailings. Geopolymers may be a rapidly setting, acid resistant, thermally stable, alternative to cements and concrete for applications in geothermal wells.

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