Geopolymers represent novel material types at the interface of glass, ceramics, and materials based on traditional inorganic bonds. Geopolymers utilize waste materials as source material and activate the materials with alkaline activators to act as binders. Metakaolin is categorised as an aluminosilicate material because it contains variable amounts of alumina and silica. Geopolymers offer benefits due to their ease of synthesis and low emissions of greenhouse gases such as CO2, SO2, and NOx.
1. Geopolymer Aluminosilicate Materials
Geopolymers result from the interactions of inorganic elements like coal fly ash and incinerator ash, slags such granulated blast (steel) or furnace (iron) slag, and clays like metakaolin or calcined clay [
30,
31] with an alkaline activator. Geopolymers focus on utilizing waste products to create value. Other industrial wastes included glass, melt-quenched aluminosilicates, natural minerals such as kaolinite and natural zeolite, volcanic ash, and mine tailing, waste ceramics, and catalyst residues, as well as mixtures of these materials [
9,
32]. Fly ash and metakaolin are the most frequent aluminosilicates or raw materials employed by researchers to construct traditional geopolymer adsorbents. Geopolymers incorporate waste materials as source materials and an alkaline activator to serve as a binder. Commonly, the alkaline liquid utilised in geopolymerization is a mixture of sodium hydroxide (NaOH) and sodium silicate (Na
2SiO
3) [
33,
34]. Geopolymerization is a heterogeneous chemical reaction involving solid aluminosilicate oxides and alkali metal silicate solutions under very alkaline conditions and low temperatures that produces amorphous to semicrystalline polymeric structures composed of Si-O-Al and Si-O-Si bonds [
35,
36]. Geopolymerization entails a very rapid chemical reaction under alkaline circumstances with Si and Al minerals, resulting in a three-dimensional polymeric chain and ring structure composed of Si-O-Al-O bond [
37].
Kaolin has high concentrations of SiO
2 and AlO
3 depending on the place of extraction. Kaolin is then extracted and subjected to the calcination process, which seeks to produce a material with pozzolanic features and high reactivity. Metakaolin (Al
2Si
2O
7) is made from kaolin clay. An amorphous kaolinite produced by treatment at 500 to 800 °C was used to convert kaolin to metakaolin [
38,
39]. The Al (VI) in kaolinite is converted into Al (IV) and Al (V) in this process to generate amorphous aluminium silicate. Spinel, mullite, and other crystals are formed when a high temperature is maintained constantly throughout the calcination process. Al (IV) and Al (V) will be changed into Al (VI) through this procedure [
40]. In strong alkali solutions, metakaolin dissolves and releases Al and Si rapidly, producing geopolymer, zeolite, and other compounds depending on the reaction environment [
41]. Metakaolin has substantially greater activity than kaolin in the same environment, which expands the application range of metakaolin, especially as geopolymer material.
Fly ash is a solid fine residue formed of particles expelled from the boilers of coal-fired power plants with flue gases [
37,
42,
43]. Fly ash is used in the development of geopolymers because of its naturally high concentrations of SiO
2 and Al
2O
3; low SiO
2 and Al
2O
3 content is insufficient for alkali activation [
44].
Slag is a by-product of the production of wrought iron and steel. As by-products of metallurgical operations or incineration processes, many slags are formed. In slag-blended systems, the geopolymerization reaction rate rises with increasing slag and activator concentrations [
45].
Table 1 summarises the most recent published research on aluminosilicate materials. The table includes the aluminosilicate materials and the research findings. This summary shows that researchers are focusing more on using geopolymer aluminosilicate materials for concrete and cement applications and less on using the geopolymer materials for coating applications.
This entry is adapted from the peer-reviewed paper 10.3390/coatings12091348