Effect of curing conditions on the microstructure and mechanical performance of geopolymers derived from nanosized tubular halloysite

As a natural clay mineral, halloysite possesses a distinctive nanotubular structure and surface reactivity. Effects of curing temperatures and curing environments on the microstructure and the development of compressive strength of alkali-activated halloysite-based geopolymers were investigated in this study. Fourier-transform infrared spectroscopy, X-ray diffraction, and nuclear magnetic resonance were used to characterize the chemical structure of obtained geopolymer products, while N-2 physisorption and scanning electron microscopy were used to characterize the microstructure of obtained geopolymers. Halloysite-based geopolymers cured at ambient temperatures had a compacted and homogeneous microstructure, and thus exhibited high compressive strength. However, elevating the curing temperature produced geopolymers containing more unreacted halloysite and larger pores, due to more rapid polymerization, water evaporation, and zeolite formation, which decreased the compressive strength of the final product. In addition, curing at certain temperatures (60-80 degrees C) in a moist environment favored the early compressive strength of halloysite-based geopolymers. The results showed that owing to the high geopolymerization reactivity of halloysite, alkali-activated halloysite-based geopolymers are sensitive to the curing conditions, and that curing at ambient temperature in a moist environment was beneficial to the development of geopolymeric compressive strength. (C) 2020 Elsevier Ltd. All rights reserved.