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Control of Polycondensation Reaction Generated from Different Metakaolins and Alkaline Solutions

A. Gharzouni, A. Gharzouni, I. Sobrados, E. Joussein, S. Baklouti, S. Rossignol

Journal of Ceramic Science and Technology, Vol. 8, No. 3 (2017), pp. 365-376

DOI: 10.4416/JCST2017-00040

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Abstract

The purpose of the present study is to control the polycondensation reaction of various geopolymer samples based on six metakaolins and two potassium alkaline solutions with different reactivities. First, metakaolin characterization revealed three levels of reactivity, which increase essentially with the increase in the degree of purity, amorphous phase and water demand value. The formation of geopolymer samples was then investigated. In situ thermal analysis showed that depending on the metakaolin surface reactivity, the availability of dissolved species decreases the energy required for oligomer formation to approximately 1.8 kJ/mol. However, a highly reactive alkaline solution favors the dissolution and decreases this energy to approximately 0.6 kJ/mol, even in the case of low-reactive metakaolins. In addition, in situ FTIR spectroscopy revealed that the metakaolin impurities are responsible for the generation of several networks. However, the geopolymer network is favored in the case of a highly reactive alkaline solution. Further, structural information was obtained with in situ 27Al NMR. It was proven that the reactivity of metakaolin and, more significantly, the reactivity of alkaline solutions ensure higher conversion rates of Al(VI) and Al(V) species to Al(IV), which may reach 80 %. Better compressive strengths (> 60 MPa) were obtained for high conversion rates.

Keywords

Metakaolin, alkaline solution, kinetics, oligomer formation, 27Al NMR

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