Geopolymer

A geopolymer is an inorganic, often ceramic-like material, that forms a stable, covalently bonded, non-crystalline to semi-crystalline network through the reaction of aluminosilicate materials with an alkaline or acidic solution. Many geopolymers may also be classified as alkali-activated cements or acid-activated binders. 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 re-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.

The term geopolymer was coined by Joseph Davidovits in 1978 due to the rock-forming minerals of geological origin used in the synthesis process. These materials and associated terminology were popularized over the following decades via his work with the Institut Géopolymère (Geopolymer Institute).

Geopolymers are synthesized in one of two conditions:

The alkaline route is the most important in terms of research and development and commercial applications. Details on the acidic route have also been published.

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 or repairing concretes. Due to the increasing demand for low-emission building materials, geopolymer technology is being developed as a lower-CO₂ alternative to traditional Portland cement, with the potential for widespread use in concrete production. The properties and uses of geopolymers are being explored in many scientific and industrial disciplines such as modern inorganic chemistry, physical chemistry, colloid chemistry, mineralogy, geology, and in other types of engineering process technologies. In addition to their use in construction, geopolymers are utilized in resins, coatings, and adhesives for aerospace, automotive, and protective applications.

In the 1950s, Viktor Glukhovsky developed concrete materials originally known as "soil silicate concretes" and "soil cements", but since the introduction of the geopolymer concept by Joseph Davidovits, the terminology and definitions of the word geopolymer have become more diverse and often conflicting. The word geopolymer is sometimes used to refer to naturally occurring organic macromolecules; that sense of the word differs from the now-more-common use of this terminology to discuss inorganic materials which can have either cement-like or ceramic-like character.

A geopolymer is essentially a mineral chemical compound or mixture of compounds consisting of repeating units, for example silico-oxide (-Si-O-Si-O-), silico-aluminate (-Si-O-Al-O-), ferro-silico-aluminate (-Fe-O-Si-O-Al-O-) or alumino-phosphate (-Al-O-P-O-), created through a process of geopolymerization. This method of describing mineral synthesis (geosynthesis) was first presented by Davidovits at an IUPAC symposium in 1976.

Even within the context of inorganic materials, there exist various definitions of the word geopolymer, which can include a relatively wide variety of low-temperature synthesized solid materials. The most typical geopolymer is generally described as resulting from the reaction between metakaolin (calcined kaolinitic clay) and a solution of sodium or potassium silicate (waterglass). Geopolymerization tends to result in a highly connected, disordered network of negatively charged tetrahedral oxide units balanced by the sodium or potassium ions.