Geopolymers
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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.

photocatalytic geopolymer coating

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 [1][2] 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 [3][4]. 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 (Na2SiO3) [5][6]. 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 [7][8]. 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 [9].
Kaolin has high concentrations of SiO2 and AlO3 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 (Al2Si2O7) is made from kaolin clay. An amorphous kaolinite produced by treatment at 500 to 800 °C was used to convert kaolin to metakaolin [10][11]. 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 [12]. In strong alkali solutions, metakaolin dissolves and releases Al and Si rapidly, producing geopolymer, zeolite, and other compounds depending on the reaction environment [13]. 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 [9][14][15]. Fly ash is used in the development of geopolymers because of its naturally high concentrations of SiO2 and Al2O3; low SiO2 and Al2O3 content is insufficient for alkali activation [16].
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 [17]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.

References

  1. Duxson, P.; Fernández-Jiménez, A.; Provis, J.L.; Lukey, G.C.; Palomo, A.; van Deventer, J.S.J. Geopolymer Technology: The Current State of the Art. J. Mater. Sci. 2007, 42, 2917–2933.
  2. Blissett, R.S.; Rowson, N.A. A Review of the Multi-Component Utilisation of Coal Fly Ash. Fuel 2012, 97, 1–23.
  3. Falah, M.; Mackenzie, K.J.D. Photocatalytic Nanocomposite Materials Based on Inorganic Polymers (Geopolymers): A Review. Catalysts 2020, 10, 1158.
  4. Siyal, A.A.; Shamsuddin, M.R.; Khan, M.I.; Rabat, N.E.; Zulfiqar, M.; Man, Z.; Siame, J.; Azizli, K.A. A Review on Geopolymers as Emerging Materials for the Adsorption of Heavy Metals and Dyes. J. Environ. Manag. 2018, 224, 327–339.
  5. Davidovits, J. Geopolymers. J. Therm. Anal. 1991, 37, 1633–1656.
  6. Saloma; Hanafiah; Elysandi, D.O.; Meykan, D.G. Effect of Na2SiO3/NaOH on Mechanical Properties and Microstructure of Geopolymer Mortar Using Fly Ash and Rice Husk Ash as Precursor. AIP Conf. Proc. 2017, 1903, 050013.
  7. Al Bakri, A.M.; Kamarudin, H.; Bnhussain, M.; Nizar, I.K.; Mastura, W. Mechanism and Chemical Reaction of Fly Ash Geopolymer Cement—A Review. J. Asian Sci. Res. 2011, 1, 247–253.
  8. Castillo, H.; Collado, H.; Droguett, T.; Vesely, M.; Garrido, P.; Palma, S. State of the Art of Geopolymers: A Review. E-Polymer 2022, 22, 108–124.
  9. Adewuyi, Y.G. Recent Advances in Fly-Ash-Based Geopolymers: Potential on the Utilization for Sustainable Environmental Remediation. ACS Omega 2021, 6, 15532–15542.
  10. Zidi, Z.; Ltifi, M.; ben Ayadi, Z.; Mir, L.E.L.; Nóvoa, X.R. Effect of Nano-ZnO on Mechanical and Thermal Properties of Geopolymer. J. Asian Ceram. Soc. 2020, 8, 1–9.
  11. Cao, R.; Fang, Z.; Jin, M.; Shang, Y. Study on the Activity of Metakaolin Produced by Traditional Rotary Kiln in China. Minerals 2022, 12, 365.
  12. Rocha, J.; Klinowski, J. Physics and Chemistry of Minerals, 295i and 27A1 Magic-Angle-Spinning NMR Studies of the Transformation of Kaolinite; University of Cambridge: Cambridge, UK, 1990; Volume 17.
  13. De Rossi, A.; Simão, L.; Ribeiro, M.J.; Novais, R.M.; Labrincha, J.A.; Hotza, D.; Moreira, R.F.P.M. In-Situ Synthesis of Zeolites by Geopolymerization of Biomass Fly Ash and Metakaolin. Mater. Lett. 2019, 236, 644–648.
  14. Zhuang, X.Y.; Chen, L.; Komarneni, S.; Zhou, C.H.; Tong, D.S.; Yang, H.M.; Yu, W.H.; Wang, H. Fly Ash-Based Geopolymer: Clean Production, Properties and Applications. J. Clean. Prod. 2016, 125, 253–267.
  15. Kalombe, R.M.; Ojumu, V.T.; Eze, C.P.; Nyale, S.M.; Kevern, J.; Petrik, L.F. Fly Ash-Based Geopolymer Building Materials for Green and Sustainable Development. Materials 2020, 13, 5699.
  16. Prochon, P.; Zhao, Z.; Courard, L.; Piotrowski, T.; Michel, F.; Garbacz, A. Influence of Activators on Mechanical Properties of Modified Fly Ash Based Geopolymer Mortars. Materials 2020, 13, 1033.
  17. Humad, A.M.; Kothari, A.; Provis, J.L.; Cwirzen, A. The Effect of Blast Furnace Slag/Fly Ash Ratio on Setting, Strength, and Shrinkage of Alkali-Activated Pastes and Concretes. Front. Mater. 2019, 6, 9.
  18. Abbas, R.; Khereby, M.A.; Ghorab, H.Y.; Elkhoshkhany, N. Preparation of Geopolymer Concrete Using Egyptian Kaolin Clay and the Study of Its Environmental Effects and Economic Cost. Clean Technol. Environ. Policy 2020, 22, 669–687.
  19. Albidah, A.; Alghannam, M.; Abbas, H.; Almusallam, T.; Al-Salloum, Y. Characteristics of Metakaolin-Based Geopolymer Concrete for Different Mix Design Parameters. J. Mater. Res. Technol. 2021, 10, 84–98.
  20. Ionescu, B.A.; Lăzărescu, A.-V.; Hegyi, A. The Possibility of Using Slag for the Production of Geopolymer Materials and Its Influence on Mechanical Performances—A Review. Proceedings 2020, 63, 30.
  21. Jaya, N.A.; Abdullah, M.M.A.B.; Li, L.-Y.; Sandu, A.V.; Hussin, K.; Ming, L.Y. Durability of Metakaolin Geopolymers with Various Sodium Silicate/Sodium Hydroxide Ratios against Seawater Exposure. AIP Conf. Proc. 2017, 1887, 020063.
  22. Guzmán-Carrillo, H.R.; Manzano-Ramírez, A.; Garcia Lodeiro, I.; Fernández-Jiménez, A. ZnO Nanoparticles for Photocatalytic Application in Alkali-Activated Materials. Molecules 2020, 25, 5519.
  23. Guzmán-Aponte, L.A.; de Gutiérrez, R.M.; Maury-Ramírez, A. Metakaolin-Based Geopolymer with Added TiO2 Particles: Physicomechanical Characteristics. Coatings 2017, 7, 233.
  24. Yaakob, S.M.; Rabat, N.E.; Sufian, S. Effects of Na: Al and Water: Solid Ratios on the Mechanical Properties of Fly Ash Based Geopolymer. IOP Conf. Ser. Mater. Sci. Eng. 2018, 458, 012011.
  25. Wang, L.; Geddes, D.A.; Walkley, B.; Provis, J.L.; Mechtcherine, V.; Tsang, D.C.W. The Role of Zinc in Metakaolin-Based Geopolymers. Cem. Concr. Res. 2020, 136, 106194.
  26. Zailan, S.N.; Mahmed, N.; Abdullah, M.M.A.B. Photocatalytic Behaviour of TiO2-Geopolymer Paste under Sunlight. IOP Conf. Ser. Mater. Sci. Eng. 2020, 957, 012006.
  27. Strini, A.; Roviello, G.; Ricciotti, L.; Ferone, C.; Messina, F.; Schiavi, L.; Corsaro, D.; Cioffi, R. TiO2-Based Photocatalytic Geopolymers for Nitric Oxide Degradation. Materials 2016, 9, 513.
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