Fly Ash Bricks – Composition, Uses and Comparison

Fly Ash Bricks | Advantages of using Fly Ash Bricks | Comparison between Fly ash bricks and Clay bricks | Fly ash brick is a gift for today’s construction industry and works in a variety of ways to benefit the environment and consumer.

What is Fly Ash Bricks

Fly ash brick (FAB) is a building material made up of class C or class F fly ash and water. It is used to make masonry units. The bricks can sustain more than 100 freeze-thaw cycles after being compressed at 28 MPa (272 atm) and cured for 24 hours in a 66 °C steam bath before being toughened with an air entrainment agent.

The brick is described as “self-cementing” due to the high concentration of calcium oxide in class C fly ash.

The process saves energy, reduces mercury pollution in the environment, and is often 20% less expensive than traditional clay brick production.

Bricks, blocks, coarse aggregates, fine aggregates, limestone, sealant, plaster, steel, wood, etc., Out of all building materials, bricks play the predominant role in the field of construction.

While building any residential or commercial structure, it becomes utmost important to choose the right building materials to be used.

The material is also termed as “green product” as it reduces environmental pollution. It is obtained by burning coal. Rather than discarding the material, fly ash has been put to some good and efficient use. Fly ash concrete requires less curing which makes it more workable.

The best feature of the bricks made of fly ash is that their strength improves over a period. Moreover, fly ash bricks reduce the chances of thermal cracking.

Developed and patented by Dr. Bhanumathidas and Kalidas, the bricks are available as Class C and Class F. Fly ash bricks are lighter in weight too which makes them the best and first choice of the builders.

One more reason for you to select fly ash brick for construction is the bricks are sound-proof in nature, hence, reducing outdoor noise and sounds. Rather than spending extra money to make your house sound-proof, cut that extra price and invest it in decorating it.

The durability of these bricks prevents natural disasters like earthquake, hurricanes, etc., Builders generally prefer fly ash concrete for the smooth and superior quality of finishing it provides to the structure which is very difficult to obtain while using red bricks.

With the revolving and changing technologies in the industry of construction, fly ash bricks have coup d’état red bricks. The environment-friendly bricks are cost-effective, durable, light-weight, sound-proof, space efficient and hence, is being chosen by many top developers across the globe for construction purposes.

This article is the part of 8 Essential Construction and Building Materials in India.

Fly Ash Bricks
Fly Ash Bricks

What is Fly Ash Brick?

Fly ash brick is a brick made by using Fly ash of class ‘C’, Sand/Stone Dust, Lime, and Gypsum in appropriate proportions mixed in water.

Ordinary Portland Cement can also be used in place of Lime and Gypsum. Owing to the high concentration of Calcium Oxide in class ‘C’ fly ash, the brick is described as “self-cementing”.

Fly Ash Bricks Raw Material

Fly Ash brick is a product of basic cement clinker materials i.e. FLY ASH, STONE DUST/SAND, LIME, GYPSUM and BONDING AGENT.

The mix is so ideally worked out to produce bricks of higher strength with consistency as well as uniformity.

Fly ash50 – 70%
Sand15 – 20%
Lime & Gypsum15 – 20%
Cement05 – 08%

Fly Ash Bricks Properties

Properties of Fly Ash Bricks including Size of Fly ash brick, Weight of Fly ash brick, the compressive strength of Fly ash brick and Water absorption of Fly ash brick.

  • Size : Machine Made Modular size 20 cm x 10 cm x 10 cm
  • Weight: 2.5 Kg to 3 Kg.
  • Compressive strength : 100 to 120 Kg/cm2
  • Water absorption: 15 to 20%

Advantages of using Fly Ash Bricks

  1. Fly ash bricks are lightweight material compared to clay bricks, so it is suitable for multi-storey buildings, less weight means less stress on the building, hence safety assured.
  2. Fly ash bricks absorb less heat than normal bricks, therefore it keeps your building cool even in summer, hence most suitable for Indian conditions.
  3. Due to its uniform and even shape, less mortar is required in construction. Also, plastering can be avoided if used for the compound wall or godowns.
  4. Plaster of Paris can be applied directly without a backing coat of plaster
  5. The compressive strength of Fly ash bricks are high compared to normal bricks, Therefore, less wastage occurs during transportation.
  6. Fly ash bricks are less porous, therefore they absorb very little water, whereas burnt clay bricks absorb more water during construction. Thus Fly ash bricks save money on the water during construction and even keep your building strong during rainy seasons.

Fly Ash is a burnt residue of pulverized coal (bituminous or sub-bituminous) and is siliceous in nature.

In past few decades, R&D efforts were undertaken and it has been proved that this material can be utilized in number of ways in building construction products as well as in civil works with adequate durability.

Major areas of fly ash utilization are Building materials such as Bricks, Blocks, Tiles, etc. Also used in Grouting, Engineered fills for low-lying land spaces for human settlement, use in Road Construction and Construction of ash dykes and embankments.

Sr. No.
1.Weight of Bricks2.75 – 3.25 Kg / BrickDepend upon Sand quality
2.Compressive Strength50-80 kg/cm2
3.Water Absorption12-16%Depend upon product recipe


with new technologies upto 95% of total raw material can be free ash. So really the cost of production is mainly finance cost of the system – once paid for the cost of bricks and larger blocks in manufacturing will be negligible.

The manufacturing process is fully automatic with state of art technology. Though a new age product introduced in the market, Fly Ash bricks are very well accepted by the organized sectors in heavy industries, high rise buildings, large townships, colonies, etc. because of unique features and merits.

A good quality ash brick or block will have the following characteristics:

1. High on resource efficiency – less than 10% mined / virgin material like sand, aggregate, lime and cement – all put together should be less than 10% of total input material.

2. High on energy saving potential – thermal conductivity of less than 0.65 W/m2 K – it can save up to 25% aircon load which is about 60% of electricity consumption

3. High on compressive strength – above 6.5Mpa ( 65 kg/cm2)

4. Lightweight – a brick should be less than 2.9 kg and 6″ block less than 12 kg and 8″ block less than 15 kg ideally

To ensure these parameters are maintained simple quality checks in a lab can be done on the batch on a random sample basis.



  • Higher Compressive strength as compared to any conventional bricks.
  • Lower water absorption as compared to conventional bricks.
  • Higher resistance to Salinity & water seepage.
  • No salt-pitting tendencies / No efflorescence.


  • Smooth surfaces and uniform sizes.
  • Better thermal properties.
  • Thinner masonry joints with higher uniformity.
  • Higher acceptability for exposed brick masonry work.


  • Lesser nos. of bricks consumption because of regular full sizes.
  • Wastage component lower at site.
  • Lesser cement mortar consumption because of thinner joints.
  • Saving on plastering due to lesser plaster thickness.
  • Almost 30% lower cost per unit of construction.

Here we have compared both the bricks on the basis of their characteristics which will certainly help you to choose the better one.

Here is the Quick Comparison between two Popular types of Bricks i.e., fly ash bricks vs red bricks.

Fly ash bricksClay bricks
The colour of fly ash brick is the same as the manufacturing is carried out using machinery in a controlled condition.Not in uniform colour as the colour of brick depends upon the type of soil and quality control during production.
Uniform in shape and smooth in the finish as the manufacturing is carried out using machinery.Clay bricks are uneven in shape as mostly clay bricks are made manually.
As the surface is even and joints are thinner, no plastering required in normal cases.As the surface finish is not even, plastering is necessary.
Fly ash bricks are lighter as the main composition is fly ash.Clay bricks are heavier as compared to fly ash bricks.
Fly ash bricks are less porous.Clay bricks are more porous.
The cost of fly ash brick is approximate 30% lower than clay brick.The cost of clay brick is higher than fly ash brick.
This is made by using the by-product of Thermal Power Plant, which saves the environment from being polluted.Continuing use of clay bricks in the construction industry will lead to extensive loss of fertile topsoil. This could be a devastating environmental hazard.

Technical Comparison Between Fly Ash Bricks Vs Clay Bricks (CONVENTIONAL RED BRICKS)

In this Section, we will explain Difference Between Clay Brick And Fly Ash Brick. You can check following table to find Difference Between Fly Ash Brick and Clay Brick.

Strength40 – 65 Kg/cm270 – 120 Kg/cm2
Shape & SizeNon-Uniform & IrregularUniform & Regular
Water Absorption20 – 25 %10 – 15 %
Breakage / Wastage8 – 10 %3 – 4 %
Mortar ConsumptionHighLess
Density1500 – 1700 Kg/cm31200 – 1350 Kg/cm3
Nos. of Bricks for100 sq.ft. wall of 9” thick1100 Nos925 Nos.
Mortar Joint Thickness15 – 18 mm8 – 10 mm
Plaster Thickness15 – 20 mm10 – 12 mm

From the above comparison, it is very much clear that fly ash bricks are superior to clay bricks and one can use fly ash bricks for construction of house without any hesitation.

how expensive fly ash then clay bricks?

The cost of fly ash bricks depends upon the availability of fly ash in that region. However, it is seen that fly ash bricks are cheaper than clay bricks in most of the regions due to the scarcity of clay.

What are all the raw materials used in making a fly ash brick?

Fly ash (50 -70%) , Sand (15-20%) , Lime & Gypsum (15-20%) , Cement (05-08%) – Check Details Here

What is the Fly Ash Brick Size In Inches?

Fly Ash Bricks is Cuboid in Shape. Cuboid Have Length, Width and Depth or Height. So, The Fly Ash Brick Size In Inches is 4 Inches x 4 Inches x 8 Inches.

What is the Weight Of Fly Ash Brick ?

Weight Of Fly Ash Brick is Approx 2 Kg to 3 Kg.

What Is The Fly Ash Brick Size In MM?

The Fly Ash Brick Size In MM is 200 mm x 100 mm x 100 mm

What is Fly Ash Brick Making Cost?

Fly Ash Brick Production Cost is Rs. 2.5 Per Brick.

What is Fly Ash Brick Cost Per Piece ?

Rs. 6 Per Brick

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About the Author
Er. Mukesh Kumar
Er. Mukesh Kumar is Editor in Chief and Co-Fonder at Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.