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Fly ash is a heterogeneous by-product material produced in the combustion process of coal used in power stations. It is a fine grey coloured powder having spherical glassy particles that rise with the flue gases. As fly ash contains pozzolanic materials components which reach with lime to form cementatious materials. Thus Fly ash is used in concrete, mines, landfills and dams.

Chemical Composition of Fly Ash

The chemical composition of fly ash depends upon the type of coal used and the methods used for combustion of coal.

Table No 1:  Chemical composition of fly ash of different coals.

Sub bituminous
SiO2 (%)20-6040-6015-45
Al2O3 (%)5-3520-3020-25
Fe2O3 (%)10-404-104-15
CaO (%)1-125-3015-40
LOI (%)0-150-30-5

Physical Properties of Fly Ash

The physical properties of fly ash are,

1. Fineness of Fly Ash

As per ASTM, the fineness of the fly ash is to be checked in both dry n wet sieving. The fly ash sample is sieved in 45 micron sieve and the percentage of retained on the 45 micron sieve is calculated. Further fineness is also measured by LeChatelier method and Blaine Specific
Surface method.

2. Specific Gravity of Fly Ash

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

3. Size and Shape of Fly Ash

As the flyash is a very fine material, the particle size ranges in between 10 to 100 micron. The shape of the fly ash is usually spherical glassy shaped.

4. Colour

The colour of the fly ash depends upon the chemical and mineral constituents. Lime content in the fly ash gives tan and light colours where as brownish colour is imparted by the presence of iron content. A dark grey to black colour is typically attributed to an elevated un-burned content.

Classification of Fly Ash

The classification of fly ash is done differently as per codes used. They are

1. Type of Fly Ash as per IS Codes (IS 3812-1981)

A. Grade I

This grade of Fly ash is derived from bituminous coal having fractions SiO2+Al2O3+Fe2O3 greater than 70 %.

B. Grade II

This grade of Fly ash derived from lignite coal having fractions SiO2+Al2O3+Fe2O3 greater than 50 %.

2. Type of Fly Ash as per American Society for Testing and Materials (ASTM C618)

Depending on the type of coal and the resultant chemical analysis, ASTM has classified flyash into ,

A. Type C

Type C fly ash is produced from the combustion of lignite or sub bituminous coals, contains CaO higher than 10 percent and possesses cementitious properties in addition to pozzolanic properties.

B. Type F

Type F fly ash is produced from the combustion of bituminous or an anthracite coal contains CaO below 10 percent and possesses pozzolanic properties.

3. Type of Fly Ash based on boiler operations

A. Low temperature(LT) fly ash

It is produced when the combustion temperature is below 900o C

B. High temperature(HT) fly ash

It is generated out of combustion temperature below 1000o C

Mechanism of Fly Ash

The chemistry of hydration of Portland cement is that about 50% of Portland cement is composed of the primary mineral tri-calcium silicate, which on hydration forms calcium silicate hydrate and calcium hydroxide.

If we have Portland cement, and the fly ash is the pozzolana, it can be represented by silica because non-crystalline silica glass is the principal constituent of fly ash. The silica combines with the calcium hydroxide released on hydration of Portland cement. Calcium hydroxide in hydrated Portland cement does not do anything for strength, so therefore we use it up with reactive silica.

Slowly and gradually it forms additional calcium silicate hydrate which is a binder, and which fills up the space, and gives us impermeability and more and more strength.

Table No 2:  Hydration reaction of Portland cement and fly ash Portland cement

Hydration ProcessTricalcium
+Calcium Hydroxide
Portland cementC3S+H=C-S-H+CH
Portland Cement + Fly AshS

 Comparison of Requirements of Fly Ash in ASTM, EN and IS

Table No 3: Comparison of Requirements of Fly Ash in ASTM, EN and IS

PropertiesASTM C-618En-450En-197-IEn-3892-IIS 3812
2003- I
Sio2 minimum35
Reactive/soluble Sio2, min.252520
Sio2+Al2O3+Fi2O3 minimum7070
MgO, Maximum70
Total alkalis, max.1.51.5
SO3, maximum5323
Free CaO, maximum11
Total/reactive CaO, maximum101010
Fineness, 45 micron, maximum34401234
Blaines fineness m2
/kg min.
Cement activity 28 days75758080
Lime reactivity, N/mm24.5
Soundness, Le-Chatelier, mm10101010
Autoclave, Percent0.80.8

Uses of Fly Ash

The major uses of fly ash are listed below,

  1. Used in the manufacture of Portland cement.
  2. Typically used for embankment construction.
  3. Used as a soil stabilisation material.
  4. Fly ash is also used as a component in the production of flowable fill.
  5. Used as the filler mineral in asphalt road laying to fill the voids.
  6. Fly ash is used as component in geoploymers.
  7. Used in Roller compacted concrete dams.
  8. Used in the manufacture of fly ash bricks
  9. When flyash is treated with silicon hydroxide, it acts as a catalyst.

Read More: Fly Ash Bricks and Comparison with Clay Bricks – Composition, Manufacture

About Fasi Ur RahmanVerified

Fasi is a Civil Engineer associated with Tumkur Smart City Project. He is the author, editor and partner at