Mechanical property and microstructure of geopolymer concrete based on fly ash and slag

Abstract:

To analyze the impact of mix proportions on the performance of geopolymer concrete (GC), the effects of the ratio of fly ash (FA) to granulated blast furnace slag (GGBS), the sodium silicate modulus, and the content (mass fraction) of sodium silicate on the mechanical properties of geopolymer concrete were studied. Further, the microstructure of geopolymer concrete was investigated by means of scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and mercury intrusion porosimetry (MIP) tests. Results show that with the increase in curing age, the degree of polymerization was higher, the amount and types of gel observed in SEM images increased, and the macroscopic strength of geopolymer concrete was higher. The decrease in the modulus of sodium silicate resulted in the decrease in Si/Al ratio and increase in Na⁺, which promoted the formation of gel and made the geopolymer concrete exhibit higher mechanical properties. A decrement in the ratio of fly ash to slag, i.e. the increase in the amount of Ca in the system, promoted the generation of gel. When the content of sodium silicate increased from 20% to 25%, Si/Al ratio increased from 1.31 to 3.37, and the microstructure density and macroscopic strength of geopolymer concrete samples were improved. However, when the content of sodium silicate was excessive, Si/Al ratio decreased from 3.37 to 2.00, and the formation of gel was difficult, showing a lower strength of the geopolymer concrete samples. Reducing the modulus of sodium silicate and increasing the proportion of slag can both promote the polymerization, which is helpful to the improvement of the mechanical properties and the compactness of the microstructure of geopolymer concrete.