The long-term development in the micro- and macro-properties of geopolymers made from different fly ashes

This paper systematically presented the trend of mechanical properties and micro-characteristics of fly ash-based geopolymer pastes up to 730 days (d) and analyzed the reason causing the divergent tendencies among the geopolymers synthesized with five Class F fly ashes (FFA). The test results showed that the geopolymer specimens had continuously increased compressive strength due to the sustained geopolymerization, although the rate of rise experienced three stages, including rapid growth, gentle consolidation, and slight increase. The microscopic detections revealed that the decrease in the Si/Al molar ratio, Na/Al molar ratio, and porosity of geopolymers were linearly related to the enhancement of compressive strength in 730 d, while increased amorphous phase content (APC) of geopolymers only contributed to that in 28 d. Besides, the differentiation of five FFA caused the divergence of compressive strength in the early stage, but the convergence was observed starting from 210 d, which was explained well by the corresponding divergence and convergence of micro-characteristics. This research proposed that, under dry ambient environments, the mechanism of medium and long-term FFA-based geopolymerization reaction for enhancing the compressive strength did not seem to be on the newly generated geopolymer gel with low Si/Al molar ratio but possibly on the recombination and optimization of Si–Al molecular structure between the existing geopolymer gel and newly formed Al-rich gel that released from residual FFA particles.