Fly ash: Types, Properties, Advantages & Disadvantages

Fly ash


Fly ash is a microscopically partitioned residue of thin coal combustion and is carried by groove gases and collected by electrostatic precipitators.

Flyash is a by-product of coal-fired power generating plants and consists of fine particles that rise with flue gases.

In an industrial context, flyash usually refers to the ash produced during coal combustion.

Flyash increases strength, decreases permeability, reduces corrosion of strong steel, increases sulfate resistance and reduces alkali-aggregate reaction.

Flyash is particularly used to make high strength and high-performance concrete.

In India alone, we produce about 75 million tonnes of flyash per year.

High precision, low carbon content, good reaction, good flyash are the essence.


One method is for integrating some percentage of flyash with cement clinker for the production of PPC and the other method is to use flyash as a mixture when making concrete.

Types of Fly ash:

Class F:

Flyash generally produced by burning anthracite or bituminous coal is classified as class ‘F’ fly ash.

Commonly produced flyash by burning anthracite or bituminous coal usually has less than 5% lime.

This flyash is pozzolanic in nature and contains less than 20% lime (CaO).

Class C:

Flyash normally manufactured by burning lignite or sub-bituminous coal is classified as Class ‘C’ flyash.

It comes with pozzolanic and cementation properties.

Fly ash is usually made by burning lignite or sub-bituminous coal.


It usually contains more than 20% lime.

The content of alkali and sulfate (SO4) is generally higher in Class C flyash.

Physical properties of fly ash:

1. The fineness of fly ash:

According to ASTM, the beauty of flyash is to be tested in both dry n wet s.

The flyash sample is sieved in a 45-μm strainer and the proportion of retaining on the 45-μm strainer is calculated.

In addition, beauty is also measured by the Lachelier method and the Blaine specification

2. The specific gravity of fly ash:

The specific gravity of flyash varies from a low value of 1.90 for sub-bituminous ash to a high value of 2.96 for iron-rich bituminous ash.

3. Size and shape of fly ash:

Since flyash is a really fine material, the particle dimension is between 10 and 100 μm.

The flyash is usually shaped like a circular glass.

4. Colour:

The colour of fly ash is determined by the chemical and mineral elements.

The materials of lime in flyash provide tan and light-weight colour whereas brown colour is applied by the presence of iron content material.

A dark brown to black colour is usually attributed to an enhanced un-burn material.

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Advantages of fly ash:

  1. The heat of hydration can be reduced by replacing cement with flyash.
  2. The amount of water is reduced for the given functionality or better workability at the same water content.
  3. Bleeding rate decreases as work efficiency increases.
  4. Superior long time strength and durability performance.
  5. Lower water content results in less shrinkage and perforation.
  6. Low permeability and improved resistance to sulfate attack.

Disadvantages of fly ash:

  1. A few hours after pouring there is a change from liquid to solid, but the curing process may take longer.
  2. Fly ash can prolong the time it takes for concrete to set up.
  3. Flyash reduces the amount of air ingress, and high concrete admixture in flyash often requires more air-entraining penetration.
  4. The colour of concrete is more difficult to control with flyash than blends with only Portland cement.

Uses of fly ash:

  • Mass solid block.
  • Ready-mix and precast applications.
  • Includes marine environment.
  • For use with alkali-reactive aggregates.
  • Water retaining structure.
  • Self-compacting concrete.
  • Tunnelling work.
  • Road Stabilization.
  • Concrete concrete.

Also, read 1. Superplasticizer 2. Waterproofing Membrane 3. Portland Cement

Frequently Asked Questions:

What is in fly ash?

Fly ash contains substantial amounts of silicon dioxide (SiO2) (both amorphous and crystalline), aluminum oxide (Al2O3) and calcium oxide (CaO).

Why do we use fly ash in concrete?

The use of fly ash in concrete improves the practicality of plastic concrete and the strength and durability of hardened concrete.

How Fly ash is produced?

Fly ash is produced by coal-fired electric and steam generating plants.

Why is it called fly ash?

During combustion, mineral impurities in the coal fuse into the suspension and exit the combustion chamber with exhaust gases.
As the fused material increases, it cools down and freezes into spherical glass particles called fly ash.


Fly ash is a useful component of modern concerts.

Due to its pozzolanic nature, flyash act as a useful material of cement.

This cement concrete cannot be justified in view of heavy test results and also successful practical cases of the use of flyash in concrete.

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