Significance of the formation of pentahedral aluminum in the reactivity of calcined kaolin/metakaolin and its applications

The specific behaviour of kaolinites heated between 600 and 800 °C (metakaolinite) makes this material very appropriate to be used as: i) ecological pozzolanic additive to ordinary Portland cement (OPC), ii) precursor of geopolymers, iii) carrier/supplier of metal nanoparticles with bactericidal/virucidal activity, and iv) catalytic reactions. The strong reactivity of the metakaolinite (mixed 2SiO2.Al2O3 oxide) is mainly associated with the presence of a large fraction (>50 %) of five-coordinated aluminum in its disordered structure. The thermal treatment of the metakaolinite favours the formation of the stable mullite below 1250 °C. The alkaline activation of metakaolinite produces geopolymers with remarkable mechanical properties, which reduce the CO2 emission associated to the OPC cement manufacture. This metakaolinite-derived geopolymers also permit the homogeneous and safe stockage of contaminants radioactive Cs and Sr cations. Finally, the retention of Ag and Cu nanoparticles at the metakaolin surface has been successfully used as support as well as dispenser of antimicrobial/virucidal agents. This fact opens the possibility to use metakaolinite as support for another kind of metal nanoparticle (i.e. Pt, Rd, Au, Pd, etc) in catalytic applications as well as in health. This critical review provides an overview of the importance of the metakaolin structure in its main applications.