Synthesis and characterization of geopolymer prepared from circulating fluidized bed-derived fly ash

Abstract

The use of circulating fluidized bed fly ash (CFBFA) to prepare geopolymer as a substitute for cement not only reduces CO2 emissions, but also solves many environmental problems caused by the large amount of CFBFA stockpiles. The effects of alkali activator modulus (SiO2/Na2O molar ratio) and curing temperature on the mechanical properties of geopolymers were investigated, and the intrinsic relationship between mechanical properties and phase composition, micromorphology and chemical structure was established by XRD, SEM, FTIR, and NMR characterization, and the HCl extraction experiments were used to investigate geopolymerization mechanism. The results show that the compressive strength of geopolymer decreases as the alkali activator modulus increases from 1.0 to 1.8. The compressive strength of geopolymer within 7 days can be increased greatly when the curing temperature increased from ambient temperature to 60 °C, but it decreased significantly after 28 days. The 90 days compressive strength reaches 34.7 MPa under the curing temperature of 20 °C and the activator modulus of 1.0. Combined with the results of XRD, FTIR, and SEM, it was known that when the curing temperature reaches 60 °C, the geopolymer gels were transformed into the zeolite-like phase in the later stages of curing, leading to reduce in compressive strength, whereas ambient temperature curing was conducive to the growth of gels. NMR results show that the Q4(2Al)+Q4(1Al) structure was mainly dominated in the geopolymer in the early stage of curing under ambient temperature, and the Q4(4Al)+Q4(3Al) structure gradually increased when the curing exceeded 28 days. When the curing temperature was 60 °C, the structure gradually transformed to a single Q4(2Al) structure as the curing age increases.