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Geopolymer is a term covering a class of synthetic aluminosilicate materials with potential use in a number of areas, but predominantly as a replacement for Portland-based cements. The name Geopolymer was first applied to these materials by Joseph Davidovits in the 1970s, although similar materials had been developed in the former Soviet Union since the 1950s under the name Soil Cements. Geopolymers are generally formed by reaction of an aluminosilicate powder with an alkaline silicate solution at roughly ambient conditions. Much of the drive behind research is to investigate the development of geopolymers as a potential large-scale replacement for concrete produced from Portland Cement, due to geopolymers’ lower carbon dioxide emissions, greater chemical and thermal resistance and greater mechanical properties at both atmospheric and extreme conditions.
Metakaolin is a commonly used starting material for laboratory synthesis of geopolymers, and is generated by thermal activation of kaolinite clay. Geopolymers can also be made from natural sources of pozzolanic materials, such as lava or coal fly ash. Most studies have been carried out using natural or industrial waste sources of metakaolin and other aluminosilicates.
The majority of Earth’s crust is made up of Si-Al compounds. Davidovits proposed in 1978 that a single aluminium and silicon-containing compound, most likely geological in origin, could react in a polymerisation process with an alkaline solution. The binders created were termed "geopolymers", although now, the majority of aluminosilicate sources are by-products from organic combustion, like fly ash from coal combustion. These inorganic polymers have a chemical composition somewhat similar to zeolitic materials, however exist as an amorphous solid, rather than having a crystalline microstructure.
The chemical reaction that takes place to form geopolymers follows a multi-step process:
- Dissolution of Si and Al atoms from the source material due to hydroxide ions in solution,
- Reorientation of precursor ions in solution, and
- Setting via polycondensation reactions into an inorganic polymer.