Metakaolin and Fly Ash-based Matrices for Geopolymer Materials: Setting Kinetics and Compressive Strength

This work aims to study the effects of the curing temperature (40 to 80° C) and the composition of the activating solution (SiO2/Na2O molar ratio of 1 to 1.7) on the process of geopolymerization of fly ash and metakaolin-based matrices. A unique combination of three experimental techniques was used for this purpose: rheology, electrical conductivity, and calorimetry. Correlation among the results indicates that increasing the curing temperature of metakaolin-based mixtures promotes the dissolution of the amorphous phases and therefore favors the formation of the geopolymer phase. However, too rapid dissolution leads to early gelation, which slows down further dissolution and prevents the formation of the geopolymer phase. This explains the existence of a peak compressive strength of about 55 MPa at 60 °C. Mixtures based on fly ash behave differently: increasing the temperature up to 80 °C impacts favorably the dissolution of the vitreous phase and the formation of the geopolymer phase, which results in a continuous increase in the compressive strength up to 57 MPa. Moreover, for both sources of aluminosilicate, it is found that SiO2/Na2O molar ratio of 1.2 provides the suitable alkalinity and the right content of soluble silicate necessary for a good geopolymerization rate.