What is Fly Ash ?

Fly ash is considered as one of the most important and advantageous waste material in construction industry. It is residue produced during the combustion of coal or any other form of coal. Nearly 30% of the coal is converted into ash on burning out of which 75% is in fine fly ash form and rest 25% in the form of coarse bottom ash.

Properties of Fly ash

• The particles of flyash are spherical in shape with size ranging from 0.5 pm to 100 pm.
• The main constituent of flyash is silicon dioxide, which is present in 2 forms – Amorphous and Crystalline.
• Certain environmental toxics are also present in fly ash such as arsenic, barium, beryllium baron, chromium, lead, manganese and other heavy metals.
• The chemical properties of flyash mainly depends on the chemical content of the burnt coal.
• Flyash produced by burning of anthracite or bituminous coal is called as ‘Class F’ flyash. It has only 5% of CaO due to which it only has pozzolanic properties.
• The flyash produced by burning of lignite or any other sub-bituminous coal may have CaO content more than 10%, hence this calss has both the properties of pozzolanic and cementitious.

Uses of Flyash

In Manufacturing Portland Pozzolanic Cement ( PPC ) :

Because of its pozzolanic properties, flyash is widely used in the manufacturing of portland pozzolanic cement. Compared to OPC, PPC has less rate of early strength gain, but due to its pozzolanic action, the ultimate strength of PPc is more than OPC. Due to the absorption of Ca(OH)₂ in the pozzolanic action, PPC is more resistant against chemical attack.  

In Manufacturing Fly Ash Bricks :

Along with pozzolanic properties, flyash also has ceramic properties due to which it is widely used in the manufacturing of fly ash bricks. Bricks produced from flyash are generally superior in quality and have high compressive strength compared to ordinary clay bricks.

In Manufacturing Asphalt Concrete :

Asphalt concrete is composite material consisting of an asphalt binder and mineral aggregate. Because of its hydrophobic nature, use of flyash in maunfacturing of asphalt concrete makes it more resistant against damage caused by water. Moreover it also improves the strength of asphalt concrete.

In Soil Stabilization :

It is the process in which the the engineering properties of soil are improved. Mainly use of ‘Class C’ flyash increases the shear strength of soil, thus increasing the bearing capacity of soil.

In Admixtures :

Fly ash is also used in the form of admixtures during the preparation of concrete. Generally use of flyash as an admixture reduces bleeding, segregation and shrinkage of cement.

Disadvantages of Fly Ash

• Since it is mainly produced of combustion of coal, it contains inorganic compounds which are harmful to environment.
• As it has low rate of strength gain, fly ash cannot be used in the construction where early removal of formwork is required.
• It also has longer setting time so it cannot be used in the cold seasons for construction purpose.