Mechanical property and structure of alkali-activated fly ash and slag blends

Mechanical property and structure of alkali-activated fly ash (FA)/ground granulated blast furnace slag (GGBFS) blends were investigated via compressive strength testing, X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy/energy dispersive spectroscopy. It is shown that the incorporation of slag into solid precursors can improve compressive strength of the geopolymer binders and the optimal slag content ratio which will result in the highest strength is 0.8. GGBFS is relatively more reactive than FA in alkaline activation. The binder is predominantly a class of Al-substituted sodium calcium silicate hydrate (N-C-A-S-H) gel phase, which distributes around the solid particles homogeneously. Combining the results obtained from the glass diffraction maximum of XRD and the wavenumber of T-O-Si bands displayed in FTIR, it suggests that the degree of polymerization of geopolymer binders decreases and increases. This means that the microstructure of the binder is more complex. The microstructure and chemical composition of the binders are between those of aluminosilicate gel formed in silicate-activated FA and those of calcium silicate hydrate gel formed in silicate-activated GGBFS. The major reasons contributing to this structure are the numerous calcium cations which will lead to a depolymerisation influence on the network, and/or the majority of linear chains of Si-O-Si supplied by GGBFS.