Structural reorganisation of class F fly ash in alkaline silicate solutions

Class F fly ash was leached in alkaline solutions (pH∼13.95) of varying concentrations of soluble silicates at 20 °C. The elemental compositions of the leached solutions were studied by inductively coupled plasma with optical emission spectroscopy (ICP–OES). X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) were used to examine the bulk properties of the leached solids. The surface morphology and compositions were characterised by scanning electron microscopy (SEM) coupled with an energy dispersive spectrometer (EDS). It was found that when the soluble silicate dosage was low (<∼200 mM), dissolution was inhibited by secondary precipitates on the fly ash particle surface, which might be scattered precipitates or films of polysialates depending on the dosage. Significant structural alteration, however, was observed if the fly ash was treated with alkaline solutions of high soluble silicate dosages (>∼200 mM). The greatly enhanced aluminosilicate dissolution followed by the subsequent precipitation was found to be responsible for the formation of a new aluminosilicate gel phase as observed from the ICP–OES analysis, FTIR study and SEM micrographs.