Characterization and utilization of coal fly ash: a review

Fly ash is a by-product, obtained mainly from thermal power plants, composed of silicon dioxide (SiO 2 ) and aluminum oxide (Al 2 O 3 ). Other oxides present in fly ash include iron (III) oxide (Fe 2 O 3 ), calcium oxide (CaO), magnesium oxide (MgO) and so on. It may also contain some trace elements such as arsenic (As), zinc (Zn), lead (Pb), selenium (Se), gallium (Ga) and germanium (Ge) and some lesser amount of rare-earth elements. The particle size of fly ash normally ranges from 0.01 to 200 μm. Such variability in the particle size distribution of fly ash could influence the modification of fly ash into many useful products such as zeolites, geopolymers and other composite materials. The physical, chemical and mineralogical characteristics of fly ash combined with its easy availability make this an attractive raw material for various applications. In order to explore the potential use of fly ash, it is essential to look deeply into its characteristic properties. This review meticulously studies the characterization of fly ash by different modern techniques, including X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, thermogravimetric test, energy-dispersive X-ray spectroscopy, differential scanning calorimetry, secondary ion mass spectroscopy and Raman spectroscopy and attempts to correlate the same with its potential applications.