Geopolymers are inorganic, typically ceramic, alumino-silicate forming long-range, covalently bonded, non-crystalline (amorphous) networks. Obsidian (volcanic glass) fragments are a component of some geopolymer blends. 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 new cements for 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 field of geopolymers is a part of polymer science, chemistry and technology that forms one of the major areas of materials science.
Polymers are either organic material, i.e. carbon-based, or inorganic polymer, for example silicon-based. The organic polymers comprise the classes of natural polymers (rubber, cellulose), synthetic organic polymers (textile fibers, plastics, films, elastomers, etc.) and natural biopolymers (biology, medicine, pharmacy). Raw materials used in the synthesis of silicon-based polymers are mainly rock-forming minerals of geological origin, hence the name: geopolymer. Joseph Davidovits coined the term in 1978 and created the non profit French scientific institution (Association Loi 1901) Institut Géopolymère (Geopolymer Institute).
According to T.F. Yen geopolymers can be classified into two major groups: pure inorganic geopolymers and organic containing geopolymers, synthetic analogues of naturally occurring macromolecules. In the following presentation, 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 mineral synthesis (geosynthesis) was first presented at an IUPAC symposium in 1976.
The microstructure of geopolymers is essentially temperature dependent: it is X-ray amorphous at room temperature, but evolves into a crystalline matrix at temperatures above 500 °C.
One can distinguish between two synthesis routes: in alkaline media (Na+, K+, Li+, Ca2+, Cs+ and the like); or in acidic media with phosphoric acid, organic carboxylic acids from plant extracts (acetic, citric, oxalic, and humic acids).
In the beginning of 2000s the alkaline route was the most important in terms of research and development and commercial applications and is described below. The acidic route is discussed elsewhere.
Definition
In the 1950s, Viktor Glukovsky, of Kiev, USSR, developed concrete materials originally known under the names "soil silicate concretes" and "soil cements", but since the introduction of the geopolymer concept by Joseph Davidovits, 1991, the terminology and definitions of 'geopolymer' have become more diverse and often conflicting. The examples below were taken from 2011 scientific publications, written by scientists with different backgrounds.
Definitions of the term geopolymer
For chemists
'...Geopolymers consist of a polymeric Si–O–Al framework, similar to zeolites. The main difference to zeolite is geopolymers are amorphous instead of crystalline. The microstructure of geopolymers on a nanometer scale observed by TEM comprises small aluminosilicate clusters with pores dispersed within a highly porous network. The clusters sizes are between 5 and 10 nanometers.'
For geopolymer material chemists
'...The reaction produces SiO4 and AlO4, tetrahedral frameworks linked by shared oxygens as poly(sialates) or poly(sialate–siloxo) or poly(sialate–disiloxo) depending on the SiO2/Al2O3 ratio in the system. The connection of the tetrahedral frameworks is occurred via long-range covalent bonds. Thus, geopolymer structure is perceived as dense amorphous phase consisting of semi-crystalline 3-D alumino-silicate microstructure.'