Fly Ash Brick Manufacturing Process | Composition of Fly Ash Bricks

Civil Engicon Team
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Update: April 07, 2023

Fly Ash Brick Manufacturing Process | Composition of Fly Ash Bricks

Fly ash brick manufacturing process involves mixing fly ash with water, cement, sand and other additives, and then molding the mixture into bricks. The bricks are then dried and cured before they are ready for use. This process is known as the "wet" process.
Fly ash is a by-product of burning coal in power plants and it contains fine particles that can be used as a substitute for some of the cement in the brick mixture. This makes fly ash bricks a sustainable alternative to traditional clay bricks, as they require less energy and raw materials to produce.
The composition of fly ash bricks can vary depending on the specific mix of materials used. So, they contain 50-60% fly ash, 25-30% sand or stone dust, and 10-15% cement. Other additives such as lime, gypsum or aluminum powder may also be added to improve the brick's strength, durability and workability.
Fly ash bricks are known for their high strength, low water absorption and good insulation properties. They are also resistant to fire, pests and weathering, making them a popular choice for construction projects. Fly ash bricks can be made in different sizes and shapes to suit specific building requirements.

Fly Ash Brick Manufacturing Process : Civil Engicon

What is Fly Ash?

Fly ash is a fine gray powder which particles are mostly spherical and glassy. It is a byproduct (combustion product) in coal-fired power stations generally known as boiler. Since fly ash has pozzolanic properties, It means it reacts with lime to form cementitious compounds. It is commonly known as a supplementary cementitious material. In the past fly ash was released into the atmosphere so it was causing air pollution.  

What is a Fly Ash Brick?

A fly ash brick is a building material which is used in masonry works of construction. It contains fly ash, sand or stone dust, lime, gypsum and water. It is made by hydraulic compression machine at 28 MPa and cured for 24 hours in a 66 °C steam bath. After then it is toughened with an air-entraining agen, the bricks can last for more than 100 freeze-thaw cycles. Fly ash having similar property to cement so, it is called self-cementing brick.

Mix Proportion or Composition of Fly Ash Brick:

The following three compositions are used in the manufacturing of fly ash bricks:

1. Mix Proportion-1 for Manufacturing Fly Ash Brick (Sludge Lime):

  • Fly Ash [55 to 60%]
  • Sand or Stone Dust [20 to 25%]
  • Sludge Lime [15 to 22%]
  • Gypsum [5%]

It is a cost effective method to manufacture Fly Ash Brick because sludge lime which is a waste material too is used instead of hydrated lime.

2. Mix Proportion-2 for Manufacturing Fly Ash Brick (Hydrated Lime):

  • Fly Ash [60 to 65%]
  • Sand or Stone Dust [20 to 25%]
  • Hydrated Lime [8 to 12%]
  • Gypsum [5%]

This is a popular composition to make quality full fly ash brick. This method is costlier than sludge lime method and cheaper than cement method.

3. Mix Proportion-3 for Manufacturing Fly Ash Brick (Cement):

  • Fly Ash [50 to 60%]
  • Sand or Stone Dust [32 to 40%]
  • Cement [8 to 10%]

In this method cement is used instead of lime and gypsum. It is costlier method because cement is costly compare to lime and gypsum so it is rarely adopted to manufacture fly ash brick.

Manufacturing Process of Fly Ash Brick:

  • Fly ash bricks are manufactured by hydraulic press or vibratory press.
  • The raw materials in a required proportion are mixed in the pan mixer to have a semi dry uniform mix.
  • Then the semi dry mix is placed in the separate moulds of hydraulic/vibratory press to shape the bricks.
  • Moulded bricks are air dried for one/two days in a shed depending upon the weather conditions and then curing for 14-21 days.
  • The bricks produced are sound, compact and uniform in shape

Merits & Demerits of Fly Ash Brick:

Merits of Fly Ash Brick:

  1. Fly ash bricks are light weight so that its transportation becomes easy.
  2. Fly ash bricks are uniform in shape and size compare to burnt clay bricks, so they require less mortar in brick work and finishing work.
  3. Since fly ash brick is made by machine method hence its better than burnt clay bricks made by hand method.
  4. Fly ash bricks are environment friendly because it is waste material from thermal power plant.
  5. Fly ash brick is a green building product and recommended in LEED & TERI-GRIHA systems.
  6. It saves agricultural land which by replacing clay bricks.
  7. It takes less energy intensive compared to clay bricks. Also it helps in keeping clean environment.

Demerits of Fly Ash Brick:

  1. Its mechanical bonding strength is weak but this can be improved by mixing marble or stone dust and contentious  materials.
  2. Only modular size can be produced so there is limitation of size of bricks.
  3. In building air exchange is poor due to less air voids compare to clay bricks.

Comparison Between Clay Bricks or Red Brick and Fly Ash Bricks (Red Bricks Vs Fly Ash Bricks):

Clay Brick or Red BrickFly Ash Brick
It required extra energy to be burnt.No need to burn it.
Not in uniform colour because colour depends upon he quality of soil used.Colour of fly ash brick remains same as manufactured. 
Mostly clay bricks are made by hand so it is not uniform in shape.Its shape is good and finishing is smooth because it is made by machine process.
Clay bricks are more porous.Fly ash bricks are less porous.
Clay bricks are heavier than fly ash bricksFly ash bricks are lighter than clay bricks due to presence of ash in it.
As the surface finish of brick is not even so plastering is required. As the surface of brick is even and joints are thinner so that no plastering required in normal cases.
Cost is higher compare to fly ash bricks.Cost is approx 30% less than clay bricks.
Production of clay bricks can destroy fertile soil so environmental hazard will be created.Since it is made by waste product of thermal power plant so it reduces environmental pollution. 


What is Fly Ash Bricks Size?

Fly ash brick size is same as clay bricks, the standard size of fly ash brick is 190 mm x 90 mm x 90 mm (Length x Width x Height). Fly ash brick size may vary depending upon the requirements of purchasers and manufacturer. Fly ash bricks are also available in modular and non-modular size as per IS: 1077:1992.

What is the Minimum Compressive Strength of First Class Fly Ash Bricks?

The Minimum Compressive Strength of First Class Fly Ash Brick is 10 N/mm² or 105 kg/cm².

What is the Minimum Compressive Strength of Second Class Fly Ash Bricks?

Minimum Compressive Strength of Second Class Fly Ash Brick is 7.5 N/mm² or 75 kg/cm².

How to Increase the Compressive Strength of Fly Ash Bricks?

The Compressive Strength of Fly Ash Brick can be increased by using extra chemicals like cement, marble dust or stone dust.

Can We Use Fly Ash Bricks as Load Bearing Bricks?

Fly ash brick is not a load bearing brick. So, it cannot be used as load bearing bricks.

Do Fly Ash Bricks need Curing Before Using in Masonry Works?

Since clay bricks are more porous compare to fly ash bricks (less porous because of hydraulic press), so the clay bricks are soaked in water for 24 hours before using it but fly ash bricks are not required that much time. It can be used without soaking too.

Do Fly Ash Bricks Tested As Well As Clay Bricks?

Procedures of testing Fly Ash Bricks are similar to Clay Bricks.

Do AAC Bricks Lighter (Weight) and Stronger than Fly Ash Bricks?

AAC bricks are lighter in weight and strong in strength compare to fly ash brick or clay brick.

What is Problem with Fly Ash Bricks?

There is a problem with fly ash brick, it gets strength slowly-slowly.

Compressive Strength of Fly Ash Brick in kg/cm²?

The compressive strength of first class fly ash brick is minimum 105 kg/cm².

Compressive Strength of Fly Ash Brick in N/mm²?

The compressive strength of first class fly ash brick is minimum 10 N/mm².

What is Fly Ash Brick Manufacturing Machine Price?

Its price ranges from 11 Lacs to 15.5 Lacs. It may change time to time and region to region.

What is the Price of a Fly Ash Brick?

Fly Ash Brick price ranges from 7 to 10It may change time to time and region to region.

What is the Weight of a Fly Ash Brick?

Weight of a fly ash brick is approx 2.6 kg. 


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