Basics of Concrete - Fly Ash
In 1910, fly ash was first used as a pozzolan in concrete in the Los Angeles aqueduct. Since then, fly ash has become one of the most used mineral admixtures added to concrete in the world. Although fly ash has many uses, in this section we will learn about this "green" product and its benefits in relation to ready-mix concrete.
What is fly ash? Simply put, fly ash is a byproduct in the form of finely divided residue that results from the combustion of pulverized coal and is transported from the combustion chamber by exhaust gases.
Where does it come from and how is it processed? Fly ash is a byproduct resulting from the combustion of pulverized coal in coal-fired electric generating plants. In these plants, coal is pulverized and blown into a burning chamber, causing it to ignite, this in turn heats the boiler tubes. The light ash particles that are suspended in the exhaust gases are known as "fly ash". In an effort to produce clean energy, these particles are removed from the combustion gases before leaving through the facility's smoke stack into the atmosphere. The fly ash is removed from the exhaust through mechanical collectors, known as bag houses, or through electrostatic precipitators. This material is then handled and stored in silos
(or other method of storage), much like Portland cement.
What is the chemical composition of fly ash? Often coals are blended at power plants to achieve the greatest fuel efficiency. Obviously the types and relative amounts of mineral matter in the coal being burned determine the chemical composition of fly ash.
Fly ash particles are composed of glass with crystalline matter, carbon, and traces of lime. The reactivity of fly ashes are controlled by its fineness, silica / alumina contents, and glass content. Fly ash has been divided into two classes based on the chemical composition.
The two are types of fly ash are Class C and Class F.