Novel acid-based geopolymer synthesized from nanosized tubular halloysite: The role of precalcination temperature and phosphoric acid concentration

Halloysite is a hydrated polymorph of kaolinite but possesses a distinctive nanosized tubular structure and surface reactivity. The microstructure and mechanical performance of geopolymers derived from calcined halloysite via acid-activation were investigated in this study. The acid-activation products were characterized using a combination of techniques including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Acid-activation of halloysite calcined at 450 °C by 6 mol/L and 10 mol/L phosphoric acid solution produced metavariscite crystals. When the phosphoric acid concentration was increased to 14 mol/L, the acid-activation product became more compact due to the formation of geopolymer that were mainly composed of Al–O–P networks. Calcination of halloysite at 750 °C led to an active form of halloysite, which was favored by geopolymerization. Higher phosphoric acid concentration led to the incorporation of more Si and P in the geopolymer matrix with Si–O–P–O–Al networks, and thus greater compressive strength. Halloysites calcined at 1000 °C barely reacted with phosphoric acid, due to the nanocrystalline formation and the decreased Si–OH content in the calcined halloysite, which had low reactivity unfavorable for geopolymerization. These results indicated that the microstructure and mechanical performance of halloysite-based geopolymers are highly dependent on the precalcination temperature of halloysite and the phosphoric acid concentration. Suitable precalcination temperature and high concentration of phosphoric acid facilitate high degree of geopolymerization, which is conducive to the formation of an acid-activated halloysite-based geopolymer with excellent mechanical performance.