All about civil construction knowledge- PARAM VISIONS

What is fly ash? - Where do we use fly ash in construction?

 1. What is fly ash?

Fly ash is produced as a by-product while burning pulverized coal in electric or steam generation plants. Typically, pulverized coal is blown into the combustion chamber with air where it ignites generating heat & mineral residue.

The residual by-product contains coarse as well as fine-sized ash particles. The coarse particles, known as bottom ash or slag settle into the bottom of the combustion chamber. 

The fine & lighter ash particles remain suspended in the flue gas. These suspended particles are separated from the exhaust gas by electrostatic emission control devices. Such collected fine ash particles from flue gas are known as fly ash.

The fly ash is generally spherical in shape with particle sizes ranging between 10 to 100 microns. They are light to dark grey in color depending upon the percentage of chemical & mineral contents.

Generally, they are cement grey in color as shown in the below image.

The color of the fly ash having 👇

A higher percentage of lime content 👉 light grey.

A higher percentage of carbon content 👉 dark grey.

A higher percentage of iron content 👉 brownish grey.

As the composition of an individual coal source differs from one another, the fly ash color changes as per the constituents present in the burned coal.

2. Where do we use fly ash in construction?

Fly ash is used 

1. In the manufacturing of portland cement.

2. In the production of construction materials such as fly ash bricks, concrete blocks, door frames, fencing poles, pavers, etc.

3. As a base material with sand while installing the pavers.

4. In embankment construction.

5. As a geopolymer component.

6. As a raw material in manufacturing lightweight building blocks.

7. As a catalyst along with silicon hydroxide.

8. As a soil stabilizer along with lime.

9. To fill the voids in the base layer of the asphalt roads.

10. In the production of low-grade concrete.

11. As a substitute material for a percentage of cement in the PCC pavements.


3. What are the advantages & disadvantages of using fly ash in concrete?


1. Enhances the workability of concrete.

2. Dense concrete with a higher level of compaction can be achieved.

3. Setting time of concrete can be varied by adding the specified percentage of fly ash to the mixture.

4. Reduces heat of hydration of cement in the concrete.

5. Enhances the permeability of concrete.

6. Saves money as fly ash is a cheaper substitute when compared to cement.

7. Reduces the shrinkage cracks in the concrete.

8. Controls bleeding of concrete to a certain level.

9. Fly ash helps to reduce the W/C ratio of concrete.

10. Helps to achieve uniform & smooth top surface finishings for the concrete.


1. Salt present in the fly ash has a tendency to create efflorescence.

2.  Fly ash extends the strength-gaining period of concrete. 

3. In wet climatic areas, fly ash-based products attract moss & fungal growth over the unplastered surface.

4. Not good at regulating freeze & thaw climatic variations.

5. Fly ash does not have uniform properties which makes it difficult to control the quality of concrete.

6. Due to the slower strength gain of concrete, we cannot dismantle the formwork quickly.

To go through all types of building materials used in construction, click here.

Thank you for going through this article. Have a good day 😄.


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