Impact of different metakaolin mixtures on oligomer formation and geopolymer properties: Impurity effect

Abstract

The impact of metakaolin mixtures on geopolymer formation and corresponding properties was evaluated by synthesizing geopolymers from mixtures of different metakaolins and 5 M potassium silicate. Mixture reactivity was investigated by viscosity, thermogravimetric (DTA-TGA), and in situ infrared spectroscopy (FTIR) measurements. Furthermore, mechanical strength and porosity measurements were undertaken on consolidated materials. The results have shown that the aluminum molar concentration governs the setting time and oligomer formation energy. Indeed, the high aluminum content associated with the high purity of the metakaolins lead to a low formation energy of oligomer, whereas for the metakaolins containing more impurities, the energy required for oligomer formation was higher. Regardless of the formulation, the mechanical strength and porosity trends were similar. Network characteristics were assessed by amorphous material content and in situ infrared spectroscopy (FTIR) analysis. It was demonstrated that (i) for Si/Al < 1.5, an amorphous network is formed with a constant Si/Al ratio, and for (ii) Si/Al > 1.5, different networks are formed. The zeta potential values of the different metakaolin mixtures corroborated these findings. Zeta potential values of metakaolins are governed by the impurities present in the metakaolins, which limit the release of aluminous species from the metakaolins in solution, emphasizing that knowledge of raw materials is essential to understand the local networks formation.