Investigation of geopolymerization and setting process of metakaolin-based geopolymer based on low-field nuclear magnetic resonance relaxometry

Water plays a crucial role in determining the porous microstructure of concrete, as its transition from liquid to bound states directly influences the final structure. To advance the commercial application of geopolymer materials, both the setting and geopolymerization processes were monitored using low-field nuclear magnetic resonance (NMR) relaxometry technology. Initially, fresh geopolymer slurries with three different liquid-tobinder ratios were prepared and monitored from their fresh to hardened states using a low-field NMR relaxation instrument over 24 hours. The distribution of the T2 relaxation signal provided information on the state and content of water molecules, indirectly revealing the microstructural evolution of the metakaolin-based geopolymer. The results indicated a significant decrease in water signal intensity, dropping by nearly 80 % within the first two hours of the geopolymerization process. Although the initial water signal intensities varied, the trends in relative water signal intensity were almost identical during the first two hours across different liquid-tobinder ratios. Specifically, for the geopolymer specimen with a liquid-to-binder ratio of 0.70, the transformation of water in capillaries to water in the gel structure began after 60 minutes of curing. Furthermore, both the start time of this transition and the geopolymerization process exhibited delays as the liquid-to-binder ratio increased. The rate of microstructural change in the geopolymer was notably significant, and the time corresponding to this change closely aligned with the setting time determined by the Vicat penetration test. Additionally, four stages akin to cement hydration were identified for the metakaolin-based geopolymer, based on the Vicat penetration testing and temperature monitoring results: (1) Dissolution, (2) Setting, (3) Acceleration, and (4) Hardening.