What is Fly Ash?

Power plants fueled by coal produce a significant quantity of the electricity we consume in the world today. But in addition to electricity, these plants produce a material that is fast becoming a vital ingredient for improving the performance of a wide range of concrete products. That material is fly ash. Fly ash is also produced as a by product from industrial plants using pulverized coal or lignite as fuel for the boilers.

Coal is not all carbon. Coal also contains quantities of non-combustible minerals. When coal is consumed in a power plant to generate electricity, it is first ground to the fineness of powder. Blown into the power plant’s boiler, the carbon is consumed — leaving molten particles rich in silica, alumina and calcium. These particles solidify as microscopic, glassy spheres that arecollected from the power plant’s exhaust before they can “fly” away — hence the product’s name: Fly Ash. Fly ash particles are glassy, spherical shaped “ball bearings” — typically finer than cement particles.

Fly ash (also known as Pulverised fuel ash/chimney ash/hopper ash) constitutes about 80 percent of the total ash generated in the power plant. Thebalance about 20 percent of ash gets collected at the bottom of the boiler and is taken out by suitable technologies and is referred as bottom ash (shown on left). Bottom ash – a heavier ash particle that falls to the “bottom” of power plant boilers – can be used in structural fill applications and as aggregate for manufacturing concrete blocks.

When fly ash alone or alongwith bottom ash is carried to storage or deposition lagoon or pond in the form of water slurry and deposited, it is termed as pond ash. Whereas if fly ash, alone or alongwith bottom ash is carried to a storage or deposition site in dry form and deposited, it is termed as mound ash.

Fly ash can be used in a variety of structural and low strength fill applications. It can be used as a mineral filler for paints, shingles, carpet backing and other products. It can be used in manufacturing mortars and stuccos. It even has various agricultural applications. But the largest application for fly ash is in the production of concrete.

The substitution rate of fly ash for portland cement will vary depending upon the chemical composition of both the fly ash and the portland cement. The rate of substitution typically specified is a minimum of 1 to 1 ½ pounds of fly ash to 1 pound of cement. It should be noted that theamount of fine aggregate will have to be reduced to accommodate the additional volume of fly ash. This is due to fly ash being lighter than the cement.