Geopolymerization of halloysite via alkali-activation: Dependence of microstructures on precalcination

Halloysite has a chemical composition very similar to that of kaolinite, but it possesses a special nanotubular structure and surface reactivity. This work focuses on the geopolymerization of halloysite via alkali-activation, and investigates the effects of precalcination temperatures ranging from 450 °C to 1000 °C on the microstructures and mechanical properties of halloysite-based geopolymer. The products obtained were characterized using a combination of techniques including X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, N2 physisorption analysis, and nuclear magnetic resonance. The results show that halloysite underwent minimal structural changes when calcined at 450 °C, and its alkali-activation product showed poor mechanical properties due to the formation of hydrosodalite with a porous structure. The dehydroxylation that occurred at calcination temperatures between 650 °C and 850 °C could improve the reactivity of halloysite with alkaline solution. The formation of geopolymer with a compact structure resulted in increased compressive strength. When halloysite was calcined at 1000 °C, geopolymerization also occurred, but to a lesser extent because of the formation of nanosized γ-Al2O3. This resulted in a decreased compressive strength in the product. These results indicate that the microstructures and properties of halloysite-based geopolymers are greatly dependent upon the precalcination of halloysite and that the optimal temperature is around 750 °C to ensure a high degree of dehydroxylation with high reactivity, which is favorable for geopolymerization.