The role of water during the middle or later periods of Class F fly ash-based geopolymerization

Abstract

The free water in the interconnected pores plays a significant role in supporting the development of fly ash-based geopolymerization reactions during the middle and later periods of the process. In this paper, the free water loss and mechanical properties of geopolymers within the different curing methods were tested. Through the analysis of X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), and low-field nuclear magnetic resonance (NMR) results, the differential tendencies of micro-characteristics were observed. In the period of 28–56 days, the main reactive modes probably evolved into the continuous generation of oligomers that increase the amorphous phase content (APC) and the polycondensation between formed geopolymer binders and oligomers that promotes the optimization of Si/Al molar ratio and porosity. The varied free water content led to the change in the alkalinity of the pore solution that acted on the geopolymerizaiton, and the physical actions of water, including softening, lubrication, and drying shrinkage, also affected the mechanical performance of specimens. The research found that the remaining water content composed of the trapped and free water showed a threshold of at least 6.40 %, lower than which the geopolymerization would be suspended. In general, the phenomenon of differentiation and regression of mechanics indicates that the variation of water content with limits can only impact the geopolymerization in a relatively short period.