Geopolymer
A geopolymer is a vague pseudo-chemical term used to describe inorganic, typically bulk ceramic-like material that forms covalently bonded, non-crystalline (amorphous) networks, often intermingled with other phases.
[1] 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.
The term geopolymer was coined by Joseph Davidovits in 1978 due to the rock-forming minerals of geological origin used in the synthesis process.
[9] 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.
[citation needed] 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.
The fundamental unit within a geopolymer structure is a tetrahedral complex consisting of silicon or aluminum coordinated through covalent bonds to four oxygens.
[10] These oligomers are named by some geopolymer chemists as sialates following the scheme developed by Davidovits,[2] 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.
The aqueous chemistry of aluminosilicate oligomers is complex,[12] 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[13] The reaction process broadly involves four main stages: 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.
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.
[citation needed] Production of geopolymer cement requires an aluminosilicate precursor material such as metakaolin or fly ash, a user-friendly alkaline reagent[19][promotional source?]
These conditions are not considered so user-friendly as when more moderate pH values are used, and require careful consideration of chemical safety handling laws, regulations, and state directives.
Conversely, geopolymer cement recipes employed in the field generally involve alkaline soluble silicates with starting molar ratios ranging from 1.45 to 1.95, particularly 1.60 to 1.85, i.e. user-friendly conditions.
Some of the first patented applications of geopolymer-type materials - actually predating the coining of the term geopolymer by multiple decades - relate to use in automobile spark plugs.
[34] Because geopolymer artifacts can look like natural stone, several artists started to cast in silicone rubber molds replicas of their sculptures.
[43] It has also been claimed that the Roman lime-pozzolan cements used in the building of some important structures, especially works related to water storage (cisterns, aqueducts), have chemical parallels to geopolymeric materials.
