Fly ash

Fly ash is a microscopically partitioned residue of thin coal combustion, it is carried by groove gases and collected by electrostatic precipitates, particularly used to make high strength concrete and high performance concrete.

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

Here we will learn about fly ash, types of fly ash & much more.

Introduction to Fly Ash:

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

In industry, it usually refers to the ash produced during coal combustion, India produces about 75 million tonnes of fly-ash per year.

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

Types of Flyash:

1.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).

2.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.

It is usually made by burning lignite or sub-bituminous coal usually contains more than 20% lime.

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

Flyash properties:

1. fineness:

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

The fly ash 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. specific gravity:

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:

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.

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 it changes from liquid to solid but the curing process may take longer.
  2. It can prolong the time as concrete takes more time to set.
  3. It reduces the amount of air ingress, and high concrete admixture, 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 concrete and precast applications.
  • Includes marine environment.
  • For use with alkali-reactive aggregates.
  • Water retaining structure.
  • Self-compacting concrete.
  • Tunneling work.
  • Road Stabilization.
Also read: Geosynthetics, Gypsum, Superplasticizer & Geotextile


Fly ash is a useful component of modern concerts, due to its pozzolanic nature, it act as a useful material of cement.

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

Hello, I'm Rahul Patil founder of, I had studied B.E. Civil. This blog provides authentic information regarding civil structures, equipment, materials, tests & much more.

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