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: in alkaline medium (Na+, K+, Li+, Cs+, Ca2+…) in acidic medium (phosphoric acid: )
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.Wagh, A.S. (2004). Chemically Bonded Phosphate Ceramics – A Novel Class of Geopolymers. Proceedings of the 106th annual meeting of the American Ceramic Society, Indianapolis. See also, Chapter 13, Phosphate-based Geopolymers, in J. Davidovits' book Geopolymer Chemistry and Applications.Perera, D.S., Hanna, J.V., Davis, J., Blackford, M.G., Latella, B.A., Sasaki, Y. and Vance E.R. (2008). Relative strengths of phosphoric acid-reacted and alkali-reacted metakaolin materials. J. Mater. Sci., 43, 6562–6566.
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.
Composition
In the 1950s, Viktor Glukhovsky developed concrete materials originally known as "soil silicate concretes" and "soil cements",Gluchovskij V.D. (1959). "Gruntosilikaty" Gosstrojizdat Kiev, Patent USSR 245 627 (1967), Patent USSR 449894 (Patent appl. 1958, granted 1974). 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;Kim, D.; Lai, H.T.; Chilingar, G.V.; Yen T.F. (2006). Geopolymer formation and its unique properties, Environ. Geol, 51(1), 103–111. 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.
In the simplest form, an example chemical formula for a geopolymer can be written as Na2O·Al2O3·nSiO2·wH2O, where n is usually between 2 and 4, and w is around 11-15. Geopolymers can be formulated with a wide variety of substituents in both the framework (silicon, aluminium) and non-framework (sodium) sites; most commonly potassium or calcium takes on the non-framework sites, but iron or phosphorus can in principle replace some of the aluminum or silicon.
Geopolymerization usually occurs at ambient or slightly elevated temperature; the solid aluminosilicate raw materials (e.g. metakaolin) dissolve into the alkaline solution, then cross-link and polymerize into a growing gel phase, which then continues to set, harden, and gain strength.
Geopolymer synthesis
Covalent bonding The fundamental unit within a geopolymer structure is a tetrahedral complex consisting of silicon or aluminum coordinated through covalent bonds to four oxygens. The geopolymer framework results from the cross-linking between these tetrahedra, which leads to a 3-dimensional aluminosilicate network, where the negative charge associated with tetrahedral aluminium is balanced by a small cationic species, most commonly an alkali metal cation (Na+, K+ etc). These alkali metal cations are often ion-exchangeable, as they are associated with, but only loosely bonded to the main covalent network, similarly to the non-framework cations present in zeolites.
Oligomer formation
Geopolymerization is the process of combining many small molecules known as oligomers into a covalently bonded network. This reaction process takes place via formation of oligomers (dimer, trimer, tetramer, pentamer) which are believed to contribute to the formation of the actual structure of the three-dimensional macromolecular framework, either through direct incorporation or through rearrangement via monomeric species. These oligomers are named by some geopolymer chemists as sialates following the scheme developed by Davidovits, although this terminology is not universally accepted within the research community due in part to confusion with the earlier (1952) use of the same word to refer to the salts of the important biomolecule sialic acid.Provis, J.L. and Van Deventer, J.S.J. (2009). Introduction to geopolymers, in: Geopolymers: Structure, Processing, Properties and Industrial Applications, J.L. Provis & Van Deventer (eds.), Woodhead, Cambridge UK, pp. 1–11
The image shows five examples of small oligomeric potassium aluminosilicate species (labelled in the diagram according to the poly(sialate) / poly(sialate-siloxo) nomenclature), which are key intermediates in potassium-based aluminosilicate geopolymerization. The aqueous chemistry of aluminosilicate oligomers is complex,Swaddle, T.W., Salerno, J., and Tregloan, P.A. (1994). Aqueous aluminates, silicates, and aluminosilicates, Chemical Society Reviews, 23 (5), 319–325 and plays an important role in the discussion of zeolite synthesis, a process which has many details in common with geopolymerization.
Example of geopolymerization of a metakaolin precursor, in an alkaline medium
The reaction process broadly involves four main stages: Alkaline hydrolysis of the layered structure of the calcined kaolinite Formation of monomeric and oligomeric species In the presence of waterglass (soluble potassium or sodium silicate), cyclic Al-Si structures can form (e.g. #5 in the figure), whereby the hydroxide is liberated by condensation reactions and can react again Geopolymerization (polycondensation) into polymeric 3D-networks.
The reaction processes involving other aluminosilicate precursors (e.g. low-calcium fly ash, crushed or synthetic glasses, natural pozzolans) are broadly similar to the steps described above.
Geopolymer 3D-frameworks and water
Geopolymerization forms aluminosilicate frameworks that are similar to those of some rock-forming minerals, but lacking in long-range crystalline order, and generally containing water in both chemically bound sites (hydroxyl groups) and in molecular form as pore water. This water can be removed at temperatures above 100 – 200 °C. Cation hydration and the locations, and mobility of water molecules in pores are important for lower-temperature applications, such as in usage of geopolymers as cements.Barbosa, V.F.F; MacKenzie, K.J.D. and Thaumaturgo, C., (2000). Synthesis and characterization of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers, International Journal of Inorganic Materials, 2, pp. 309–317.Rowles, M.R. (2004). The Structural Nature of Aluminosilicate Inorganic Polymers: a Macro to Nanoscale Study, PhD Thesis, Curtin University of Technology, Perth, Australia. The figure shows a geopolymer containing both bound (Si-OH groups) and free water (left in the figure). Some water is associated with the framework similarly to zeolitic water, and some is in larger pores and can be readily released and removed. After dehydroxylation (and dehydration), generally above 250 °C, geopolymers can then crystallise above 800-1000 °C (depending on the nature of the alkali cation present).Duxson, P.; Lukey, G.C., and van Deventer, J.S.J. (2007). The thermal evolution of metakaolin geopolymers: Part 2 – Phase stability and structural development, Journal of Non-Crystalline Solids, 353, pp. 2186-2200.
Commercial applications There exists a wide variety of potential and existing applications. Some of the geopolymer applications are still in development, whereas others are already industrialized and commercialized. They are listed in three major categories:
Geopolymer cements and concretes Building materials (for example, clay bricks) Low- cements and concretes Radioactive and toxic waste containment
Geopolymer resins and binders Fire-resistant materials, thermal insulation, foams Low-energy ceramic tiles, refractory items, thermal shock refractories High-tech resin systems, paints, binders and grouts Bio-technologies (materials for medicinal applications) Foundry industry (resins), tooling for the manufacture of organic fiber composites Composites for infrastructure repair and strengthening Fire-resistant and heat-resistant high-tech carbon-fiber composites for aircraft interiors and automobiles
Arts and archaeology Decorative stone artifacts, arts and decoration Cultural heritage, archaeology and history of sciences
Research on geopolymers In recent years, growing research attention has been directed toward geopolymers. The following are examples of recent studies: Application of dilute geopolymer suspensions as a wood modification (mineralisation) treatment
Geopolymer cements
From a terminological point of view, geopolymer cementDavidovits, J., (1991). Geopolymers: Inorganic Polymeric New Materials, J. Thermal Analysis, 37, 1633–1656. See also Chapter 24 in Geopolymer Chemistry and Applications, Joseph Davidovits, Institut Géopolymère, Saint-Quentin, France, 2008, ISBN 9782951482050 (3rd ed., 2011). is a binding system that hardens at room temperature, like regular Portland cement.