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Geopolymers: Ceramic-Like Inorganic Polymers

J. Davidovits

Journal of Ceramic Science and Technology, Vol. 8, No. 3 (2017), pp. 335-350

DOI: 10.4416/JCST2017-00038

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Abstract

Geopolymers are ceramic-like inorganic polymers produced at low temperature, generally below 100 °C. They consist of chains or networks of mineral molecules linked with covalent bonds. The raw materials are mainly minerals of geological origin, hence the name \"geopolymer\". They comprise several molecular units for example: silico-oxide (Na,K)-(-Si-O-Si-O-) for (Na,K)-poly(silicate) or (Na,K)-poly(siloxonate), silico-aluminate (Na,K)-(-Si-O-Al-O) for (Na,K)-poly(sialate), ferro-silico-aluminate (Na,K)- (-Fe-O-Si-O-Al-O-) or (Na,K)-poly(ferro-sialate), alumino-phosphate (-Al-O-P-O-) for poly(alumino-phosphate), formed in a geopolymerization process. We focus here on the reactivity of calcined kaolinite, an aluminosilicate oxide Si2O5Al2O2, metakaolin, which led to the discovery of geopolymers 40 years ago. A distinction is made between two synthesis routes: alkaline medium (Na+, K+, Li+, Ca++, Cs+ and the like) and acidic milieu (phosphoric acid, organic carboxylic acids). The alkaline route is the most important, so far. NMR spectroscopy provides data on the molecular structure and polymeric character. The geopolymerization mechanism starts with polycondensation of oligomers into small ribbon-like molecules. This intermediary stage involves several Si-OH groups together with H2O molecules. It is referred to as NASH or KASH by some cement scientists and generalized to the final geopolymer structure. The poly(sialate) final structure consists of well-polymerized individual elementary nanoparticles of 5 to 40 nm in size.

Keywords

Geopolymer, polymerization mechanism, applications, properties, molecular structure.

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