"Geopolymers – made like a cement, but high temperature stable, like a ceramic"
“Geopolymers” are polysialates which are charge-balanced, aluminosilicate, ceramic inorganic polymers. Such materials are made from an aluminosilicate source such as metakaolin, (dehydrated clays) and alkali metasilicate “waterglass” solution. A typical stoichiometric chemical composition is M2O•Al2O3•4SiO2•11H2O where M may be Group I elements, usually of Na, K or Cs. The inorganic polymer is made by high shear mixing with the liquid suspension undergoing dissolution, polycondensation or precipitation and setting under ambient conditions.
The ceramic-like product shares the brittle nature of ceramics, but can be reinforced with ceramic, metal or polymer particulates, platelets, chopped fibers, uniaxial fibers, or fiber weaves, yielding a tough, flaw-tolerant composite which is resistant to fire and acid corrosion. Biological reinforcements can also be used due to the ambient setting temperatures of the geopolymer matrix.
Geopolymers also have refractory adhesive properties up to 1,000°C whereupon they crystallize into ceramics of tailorable, crystallographic thermal expansion. Under inert nitrogen or argon gases at 1400°C to 1600°C, geopolymers undergo carbothermal reduction or nitridation forming soft agglomerates of ultra- high temperature refractory powders such as SiC, Si3N4, or SiAlON nanoparticles. Geopolymers are a potential significant partial solution to global warming. Alkali activated materials, such as slag-fly ash binders are sometimes referred to as “geopolymers” by the civil engineering community. However, nuclear magnetic resonance (MAS-NMR) spectroscopy of these alkali activated materials indicate that they are primarily composites of cementitious, e.g. calcium silicate hydrate phases which are not stable at high temperatures.