The Mineral phase composition of fly ashes from coal fired Thermal Plants is a function of nature of the coal, coal fineness, coal comminution system, boiler type & efficiency of collection of fly ash. The Source & Nature of coal and temperature of generation of the fly ash primarily determines the chemico – mineralogical composition and resultantly to a large extent influences the pozzolanic potential and reactivity of the generated fly ashes, it would also determine the type of crystallites formed, morphology and the chemical composition and percentages of the amorphous / glass contents of the fly ash.
It’s a of known fact that the pozzolanic reactivity of fly ash depends on glassy/ amorphous silicate phase of fly ash and specific surface area available for reaction with lime. The investigations carried out on synthetic coal ashes with sodium, sulphur and silica at different temperatures (900ºto 1500ºC) and varying residence times indicated that, formation of sodium silicates is favored by higher temperatures and longer residence times, the sodium sulfate particles were detected on the surface of the larger sodium silicate fly ash particles formed at lower temperatures. Fly ash particle formation was characterized by fragmentation followed by coalescence. Larger particles were formed at lower temperatures, indicating more complete coalescence with some cenosphere formation.
The synthetic fly ashes produced at laboratory scale by Joshi et al concluded that prolonged heat treatment promotes sintering and devitrification which in turn influences other properties. Iron in an otherwise pure alumino-siliceous ash reduces pozzolanic strength, whereas calcium in an alumino-siliceous fly ash results in a very reactive ash. The influence of CaO addition on granulated cinders was studied by Haobo H. and concluded that with increase in CaO content, the vitreous network of granulated cinders becomes disordered and is destroyed, the polymerization of network former reduces and the hydraulic activity of granulated cinder improves.
This fly ash scenario thus necessitates better understanding the influence of different minerals present in coal on the resultant fly ash properties, the fly ash Mineralogy can be altered by compositional as well as process parameters which alter the properties of fly ash, so as to Tailor make a fly ash with enhanced reactivity in its applications. Altering the Mineralogy of an available fly ash is another avenue available on hand to alter the mineralogy of the fly ash to maximize its utilisation.
This Paper refers some of the reported work on Mineralogical alteration with the Fly ashes available and altering the Mineralogy of fly ashes during generation. The Paper also discusses some of the avenues studied at the author’s lab to alter Mineralogy of available fly ash such as fly ash to synthetic slag, Mullite, Zeloite and Clinker as well as attempts on altering the fly ash Mineralogy at generation stage in a Coal Fired Power Plant. . This Paper was presented and published at 13th NCB Int Seminar