Raman, Infrared, and Chemical Characterization of Fly Ash-Generated Spherules

The majority of studies on silicate glass spherules containing fly ash deal only with the determination of their chemical composition. Nearly 70 vol.% of fly ash is comprised of silicate glass spherules. Here, we report spectroscopic properties of silicate glass spherules using the laser micro-Raman and Fourier transform infrared techniques coupled with refractive index measurements, X-ray diffraction, and electron probe micro-analysis to better ascertain their physical and chemical properties. Glass spherules show similar refractive indices (1.499–1.510) and a bell-shaped diffraction pattern with 5–10 vol.% of crystallites observed on microscopic and submicroscopic scales. The bulk chemical composition of fly ash spherules is predominantly silica-rich (SiO₂: 70.96–74.13 wt.%) with a subordinate amount of Al₂O₃ (0.11–0.69 wt.%), FeO_(Total) + MgO (3.6–4.94 wt.%), and CaO + Na₂O + K₂O (20.83–22.62 wt.%). The infrared spectra suggest the presence of a dissolved –OH– bearing fluid phase in the studied fly ash spherules. The spectra also show symmetric stretching peaks of C–O–C due to the atmospheric CO₂ adsorption at 2350 cm^–1. The Raman spectra show broad amorphous and/or short-ordered phases.