Geopolymer

Geopolymers are inorganic, typically ceramic, materials that form long-range, covalently bonded, non-crystalline (amorphous) networks. Geopolymers are a sub-class of alkali-activated cements. They are mainly produced by a chemical reaction between a chemically reactive aluminosilicate powder (e.g. metakaolin or other clay-derived powders, natural pozzolan, or suitable glasses), and an aqueous solution (alkaline or acidic) that causes this powder to react and form into a solid monolith. The most common pathway to produce geopolymers is by the reaction of metakaolin with sodium silicate, which is an alkaline solution, but other processes are also possible.

Commercially produced geopolymers may be used for fire- and heat-resistant coatings and adhesives, medicinal applications, high-temperature ceramics, new binders for fire-resistant fiber composites, toxic and radioactive waste encapsulation, and as cementing components in making concrete. The properties and uses of geopolymers are being explored in many scientific and industrial disciplines: modern inorganic chemistry, physical chemistry, colloid chemistry, mineralogy, geology, and in other types of engineering process technologies.

The original raw materials used in the synthesis of geopolymers were mainly rock-forming minerals of geological origin, hence the name: geopolymer was coined by Joseph Davidovits in 1978 These materials and associated terminology were then popularized over the following decades via his work with the Institut Géopolymère (Geopolymer Institute).

One can distinguish between two synthesis routes, respectively:

• in alkaline medium (Na⁺, K⁺, Li⁺, Cs⁺, Ca²⁺…), and;
• in acidic medium (phosphoric acid: H₃PO₄).

The alkaline route is the most important in terms of R&D and commercial applications and will be described below. Details on the acidic route have been published by Wagh in 2004, by Perera et al. in 2008, and by Cao et al. in 2005.