In-depth analysis of macro-properties and micro-mechanism of eco-friendly geopolymer based on typical circulating fluidized bed fly ash

Geopolymer is an eco-friendly building material that significantly reduces energy consumption and CO2 emissions. The increase in circulating fluidized bed fly ash (CFBFA) production presents an opportunity to use CFBFA in geopolymer synthesis, mitigating environmental hazards and promoting a sustainable, low-carbon cement industry. However, the complex transformation of amorphous aluminosilicates in CFBFA into geopolymer gels and the impact of calcium (Ca) composition on the macro-properties and micro-mechanism of geopolymers are not well understood, hindering large-scale utilization of the CFBFA-based geopolymer. In this study, low-Ca CFBFA (LCFA) and high-Ca CFBFA (HCFA) were used as precursors, with NaOH as the activator. The workability, mechanical properties, and microstructure evolution of the geopolymer samples were systematically compared. The LCFA-based geopolymer slurry exhibited ideal workability. The high Ca content in HCFA increased the water demand of the slurry, thereby reducing its workability. Compared to LCFA-based geopolymer samples, although Ca-containing components in HCFA accelerated the geopolymerization processes, some gels transformed into irregular crystalline phases during the later stages of curing, compromising the dense structure and slowing the development of mechanical properties. The evolution of the Si-Al coordination structure showed that LCFA-based geopolymers favored the development of Si-rich gels (Q4(1Al) and Q4(2Al)), whereas HCFA-based geopolymers exhibited a preference for Al-rich gels (Q4(3Al) and Q4(4Al)). The results of this research can provide valuable insights for the synthesis of CFBFA-based geopolymers.