Carbon Free Inorganic Polymers “Geopolymers”
Polymers are macromolecules which consist of a succession of several small and repetitive chemical entities called monomers. Poly- means “many” and –mer means “part” or “segment”. Regarding the chemistry of polymers, two types are commonly known; the organic and inorganic polymers.
The organic polymers are essentially formed of carbon and hydrogen-based monomers as well as some other elements like oxygen (O), chlorine (Cl), nitrogen (N), Florine (F), etc. while the inorganic polymers, which are widely known as geopolymers, form long-range, covalently bonded amorphous carbon free networks composed mainly of silicon (Si) and aluminum (Al). Clays (also called aluminosilicates) are inorganic substances rich in Al and Si. Clay is abundant in nature and thus it can be considered as the main source to obtain the raw materials necessary to synthetize geopolymers. In order to increase the reactivity of the raw material powder to form geopolymers, the clays have to be calcinated. During the calcination process, the crystallinity of the clay is destroyed Once the clay is calcined, it is mixed with an “activation solution” which is either a silicon and sodium-rich highly basic solution or a phosphorus-rich highly acidic solution. The aggressive medium, in terms of pH, allows the aluminosilicate powder to dissolve and consequently silicate and aluminate monomers are produced. The last step consists on the growth of the material by polymerisation reaction of dissolved species creating the final structure of the geopolymer.
Theoretically, any material rich in silicon and aluminum can be a good candidate as a raw material for the elaboration of geopolymers. Wastes of some industries are also considered, for instance, fly ash which is a byproduct generated during combustion of coal in thermal power plants and granulated blast furnace which is a cementing by-product in steel industry. Those powders are highly amorphous due to the rapid cooling underwent during their production. The exact mechanism of the formation of geopolymers is still unknown because of the high speed of the reaction and the simultaneity of the steps.
Thanks to the availability of the raw materials, geopolymers are being used increasingly in construction thanks to their elaboration at low temperatures, rheological and mechanical properties (easily shaped) that are very similar in their performances to Ordinary Portland Cement (OPC)  . Technological properties are not the only argument that makes geopolymers a promising alternative to OPC, but also the environmental aspect. In recent decades, manufacturers have sought to minimize the energy cost and reduce the environmental impact by designing and developing new materials. In this scope, geopolymers have emerged as a promising alternative to conventional materials such as cementitious materials and ceramic coatings. Various theories have been also proposed attempting to link aspects of geopolymerisation technology to the construction of ancient structures, most particularly the Pyramids of Egypt suggesting that pyramids were possibly ‘poured’ as synthetic stone blocks even if the veracity of such arguments is still under an intense debate . geopolymers can be also used in various practices for the adsorption and immobilization of toxic metals, or in applications requiring thermal and “thermo-physical properties” such as thermal conductivity and resistance of high temperatures and fire.
Structure of a geopolymer 
Structure of an organic polymer.
Geopolymer bricks and ornamental tile.
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 A. Aboulayt, M. Riahi, M. Ouazzani Touhami, H. Hannache, M. Gomina, R. Moussa, Properties of metakaolin based geopolymer incorporating calcium carbonate, Adv. Powder Technol. 28 (2017) 2393–2401. doi:10.1016/j.apt.2017.06.022.
 J.S.J. van D. John L. Provis, Geopolymers Structure, processing, properties and industrial applications, woodhead Publishing, Oxford, 2009.
 V.F.F. Barbosa, K.J.D. MacKenzie, C. Thaumaturgo, Synthesis and characterisation of materials based on inorganic polymers of alumina and silica: Sodium polysialate polymers, Int. J. Inorg. Mater. 2 (2000) 09–317. doi:10.1016/S1466-6049(00)00041-6.
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