Direct evidence for the mechanism of early-stage geopolymerization process

Geopolymer, an alternative construction material to cement, acquires strength and endurance through the geopolymerization process. However, the study on the early-stage geopolymerization is rare since it occurs rapidly and involves liquid phase, viscous slurry, and gel-like material, making the observation of this process complicated. This research addresses the challenge of observing this process by providing real-time evidence through in situ techniques: Fourier-transformed infrared (FTIR) spectroscopy, X-ray absorption spectroscopy (XAS), and environmental scanning electron microscopy (E-SEM). These methods consistently confirm the formation of aluminosilicate oligomers, short chains of silicate and aluminate species, in the polycondensation Stage I within the initial 5 minutes. This stage involves the reaction of Si-O and Al-O monomers, released during a dissolution process, with available Na2SiO3 from alkaline solutions in the system. In the subsequent polycondensation Stage II, aluminosilicate oligomers polymerize, creating a three-dimensional network of aluminosilicate chains through the crosslinking of Si-O-Si and Si-O-Al bonds. This process continues beyond the initial 5 minutes, gradually nearing completion around 30 minutes, as consistently confirmed by various experimental techniques. Rheological studies on the geopolymer paste flow rate support these interpretations. The study provides compelling direct evidence into the mechanisms of early-stage geopolymerization, significantly contributing to the research field and paving the way for widespread utilization in geopolymer applications.