The setting and hardening of geopolymer concrete based on low-field nuclear magnetic resonance and cyclic voltammetry methods

The hydration process has an important influence on the various properties of geopolymer concrete. However, current testing methods for hydration degree are complex and inconvenient, which limits the wide application of geopolymer concrete. For this reason, this study innovatively proposes new methods for non-destructive characterisation of the hydration degree based on low-field nuclear magnetic resonance and cyclic voltammetry techniques. Meanwhile, the setting and hardening characteristics of red mud-coal metakaolin geopolymer concrete with water-binder (W/B) ratios of 0.50, 0.55, 0.60 and 0.65, respectively, was systematically and comprehensively evaluated. The reaction mechanism of geopolymer concrete was revealed using a variety of testing methods such as SEM-EDS, thermogravimetric, and heat of hydration. The results show that the degree of hydration is significantly correlated with the intensity-weighted relaxation time and specific capacitance value, which can be expressed by pertinent equations. In addition, it is found that the hydration process of geopolymer concrete is divided into different stages, forming two gel phases, C-A-S-H and N-A-S-H. Increasing the W/B ratio promotes the generation of C-A-S-H gels but severely hinders the generation of N-A-S-H gels, overall adversely affecting the hydration process. This work will facilitate non-destructive monitoring of the status of geopolymer concrete during actual projects to ensure construction safety.