Reaction kinetics and rheological behaviour of meta-halloysite based geopolymer cured at room temperature: Effect of thermal activation on physicochemical and microstructural properties

In this paper, four samples of meta-halloysite from widely available Cameroon halloysite clay calcined at 600, 650, 700 and 750 °C, respectively, and the resultant geopolymer binders were systematically characterized. Isothermal conduction calorimetry (ICC) was used to measure the reaction kinetics of meta-halloysite geopolymers at room temperature during 48 h. The increase in reaction rate and in heat released correlates with the thermal activation temperature. It was typically found that the increase of thermal activation temperature (from 600 to 750 °C) enhances the amorphous or reactive phase content from MH600 to MH750 samples. This results in an improvement of rheological behaviour and setting time of the fresh meta-halloysite based geopolymer pastes. The hardened meta-halloysite based geopolymers were amorphous, compact and dense according to X-ray diffractometry (XRD) and Scanning Electron Microscopy (SEM). The compressive strength of resultant products increased with the activation temperature, up to 750 °C (74 MPa at 180 days). Therefore, the best temperature in view of the highest strength of meta-halloysite geopolymer cured at room temperature is 750 °C, although the lower increase in mechanical performance (7.3%) gained between GPMH700 and GPMH750 is limited. Thus, thermally activated halloysite clay appears to be a promising candidate for geopolymer synthesis.