Fly Ash in Construction | Pozzolans as Building material

Fly ash is a cementitious material obtained as the by-product of coal combustion. It is widely used in various applications in construction like fly ash bricks (FAB), fly ash concrete, fly ash in the pavement, soil stabilization, etc, due to its cementitious binding properties and environmental benefits it results. 

Fly Ash in Construction

The composition of fly ash contains certain oxides materials whose improper disposal can threaten the air, water, and soil. 


Properties of Fly Ash Used in Construction


Composition of Fly Ash

  • Fly ash is spherical in shape and it solidifies in suspension form in exhaust gases. It is composed of:

    1. Calcium Oxide ( CaO)
    2. Silica SiO2
    3. Alumina Al2O3
    4. Iron Oxide Fe2O3
  • Fly ash predominantly consists of fine particles that have colors ranging from grey to tan to reddish-brown.


Physical Properties of Fly Ash

  • The average size of particles of fly ash is 20 μm. 
  • The bulk density range from 0.54g/cm3to 0.86g/cm3.


Morphology of Fly Ash

When studying the morphology of fly ash, around 70 to 90 percent of the particles were solid glassy spheres. The remaining 10 to 30 percent contains Quartz, Mullite, Hematite, and Magnetite, a small portion of unburned carbon (Fig.2).

Fig.2. Morphology of Fly Ash



Classification of Fly Ash

ASTM C618-12a classifies fly ash into two classes:
  1. Class C
  2. Class F
The classes are divided based on the chemical composition and the source of origin.

Class C Fly Ash

  • It is produced by burning anthracite or bituminous coal
  • Only posses pozzolanic properties
  • Contain Calcium oxide  (CaO) more than 20 % and a minimum 50 % of  SiO2 +Al2O3 +Fe2O3 content. 


Class  F Fly Ash

  • It is produced by burning lignite or sub-bituminous coal
  • Have both pozzolanic and cementitious properties
  • Contain less than 10 % CaO and minimum 70 % of SiO2 +Al2O3 +Fe2O3 content.


Uses of Fly Ash in Construction

  1. Fly ash is used for the manufacture of pozzolana cement concrete.
  2. Fly ash is used in road and embankment construction for filling, stabilizing, and constructing sub-base.
  3. Fly ash can be used to manufacture fired, unfired, and steam cured bricks.
  4. Fly ash can be used to manufacture watertight and more resistant pipes compared to ordinary cement concrete pipes.
  5. Fly ash is used for the manufacture of ceramics like fence posts, railroad ties, electric line insulators.

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