Fly ash is a well-known supplementary cementitious material (SCM) in the construction field nowadays. Fly ash is harvested in the electrostatic precipitator as a residue left from the burning coal in coal powerplants.

Coal powerplants release 2 types of ashes during the run. One is fly ash and the other is bottom ash. As implies in the name, fly ash is the fine and light particles emit with the smoke. Bottom ash contains coarse and denser byproducts of burning coal which collects at the bottom of the furnace.

In the presence of lime and water, fly ash forms a compound similar to cement. That makes fly ash popular as a prime material in blending cement. During the last 50 years, the use of fly ash in concrete grow dramatically due to many benefits.

What is fly ash

Fly ash

Coal is formed as a result of decomposing organic matter for millions of years under high pressure and thermal stress. Due to that, the composition of fly ash can be different. Also, the burning process and additives used in the power plant can make the fly ash composition vary.

There are 2 common different types of fly ash available. They are class C and class F. Both types contain similar chemical compounds but in different proportions. The combination of silicon dioxide, aluminum oxide, and iron oxide is exceeding 70% by weight in class F fly ash. In class C fly ash the similar compounds are only more than 50% by weight.

Class C vs. Class F fly ash

Class CClass F
Makes from lignite or sub-bituminous coalMakes from burning anthracite or bituminous coal
More CaSO4, free lime,
C3A, calcium-rich glass, MgO
More glass, aluminosilicate glass, quartz
SiO2 + Al2O3 + Fe2O3 ≥ 50%SiO2 + Al2O3 + Fe2O3 ≥ 70%
Pozzolanic and cementitiousMore pozzolanic
Less effective for ASR mitigationGood for ASR mitigation
Class C vs. Class F fly ash

Fly ash holds a similar range of particle size to cement. The particles are round in shape. When mixed in concrete, these spherical particles will increase the workability of the concrete.

The main advantage of the fly ash is its cementitious ability. In the presence of cement, hydrates fly ash partially behave as a binder material. Class C fly ash holds the most cementitious ability. When cement hydrates in the concrete, it forms calcium silicate hydrate and lime in the medium. This calcium silicate hydrate is the strength of the developing compound and lime is the byproduct. when fly ash is present in the concrete it reacts with this residue lime content and produces similar calcium silicate hydrate to develop extra strength in the concrete.

Applications for Fly Ash

Fly ash is a common material these days. Due to its availability and low cost, applications of fly ash are numerous.

Mixing with cement

Pozzolanic Portland cement (PPC) is fly ash blended cement categorized as Type II cement. The cement contains 30 % fly ash by weight and ordinary Portland cement. Due to low Portland cement consumption, PPC is economical and generates low heat of hydration.

Mixing with concrete

Fly ash can mix with the concrete even at the batching plant. 15% – 35% of fly ash replacement is general for structural concrete. More than 50% replacement is used in mass concrete structures like concrete gravity dams as well as roller compacted concrete pavements.

Adding fly ash completely change the mix design proportions. So the behavior of the concrete will change according to that. Special attention should be paid to the properties of the concrete when fly ash is in the mix.

Even though the ultimate strength is achieved, fly ash introduction can lead to slow strength development. On the concerns of durability, there are plus sides as well as minus sides. Without proper knowledge and understanding, fly ash addition can lead to unnecessary problems in the mix.

Fly ash bricks

Both fly ash and bottom ash are popular for making bricks. Nearly 60% of fly ash is added to these bricks. Therefore those bricks are lighter than the conventional cement bricks.  Due to the higher proportion of fine material, fly ash added bricks are more impermeable as well as fire-resistive.

As a result of lightweight, overall structural load in the beams, columns, and foundations are reduced when using fly ash bricks in buildings. The advantage will reflect in the structural design.  In addition, Brick handling and transportation is easier with fly ash bricks.

Embankment construction and soil stabilization

Fly ash of class F and bottom ash are common for landfills. This ash can use to stabilize the soft soil grounds as well as road sub-base construction. Stabilization is effective in sandy, silty soils, but is also great for aggregate bases in pavement construction. Fly ash increases shear strength, bearing capacity, and resistance values while reducing the permeability. 


The accepted ASTM standard for the chemical composition of fly ash is ASTM C618. AASHTO M 295 is also applicable.  Typical concrete testing standards are adopted to test the behavior of fly ash added to concrete.

Benefits of fly ash using in concrete

Fly ash as a supplementary cementitious material

The number one benefit from the fly ash, when mixed in the concrete, is its additional strength. Due to this, we can reduce the cement consumption in the concrete. Parallel to that, the required water content is also reduced. The cementitious ability of fly ash conforming to EN 450 is considered in the mix designing too. The maximum amount of fly ash equals 30% cement is considered cementitious in the mix designing by modifying the water to cement ratio as follows.

Water/Cement = {Water/(Cement + k × fly ash)}

According to the cement type you are using this cementitious factor varies. For CEM I 32.5 corresponding k value is 0.2 and for CEM II it is 0.4 as given in the EN 206-1:2000.This cementitious value (k) varies with the chemical composition of fly ash. The generally accepted value is 0.2 for mix designs. It means that 1 kg of fly ash is equal to 200 grams of cement in the mix considering compressive strength.

Fly ash to increase setting time

Fly ash addition will increase the setting time of the concrete mix. This can be helpful for large concrete pours to avoid cold joints. When the concrete is fresh for a good amount of time, working the concrete is easy. You don’t have to rush on vibrating and pouring the whole area quickly. Having a reasonable time to work on the concrete will improve the quality of the product and reduce safety risks.

Reducing the heat of hydration and peak temperature using fly ash

This is the number one reason why we use fly ash in mass concrete structures like concrete gravity dams. The heat of hydration and peak temperature are the most serious problems in mass concrete construction. Too much hydration heat will cause thermal cracks in the concrete. Keeping the peak hydration temperature under control is the best way to avoid these thermal cracks. Adding fly ash reduces the use of cement and it increases the setting time. This reduces the hydration heat as well it allows the concrete more time to dissipate the heat.

You can see how fly ash lower the peak hydration temperature using concreteworks program.


Because of the cementitious ability of fly ash, we can optimize the amount of cement using in the mix. Cement is the most expensive ingredient in a common concrete mix. So, replacing about 20% – 30% makes the mix more economical as well as sustainable decision.

Increase workability

Fly ash particles are fine and almost round in shape. These particles will add extra workability to the concrete by lubrication the mix. Adding fly ash increase the pump-ability and flow-ability of the concrete. This will helps in reducing the required water content to achieve the target slump value in the mix designing.

Also, better workability will reduce the risk of honeycombing in the concrete. Further, it will increase the ability to cast sharp edges with better surface quality.

Less segregation and bleeding

Fly ash increase the density of the mix. Dense mixes are less likely to segregate than leaner mixes. With fly ash reduce the tendency for aggregates to sink and bleeding. It affects to increase the homogeneity of the concrete.

Less drying shrinkage

Cement is the cause of shrinkage in concrete. Mortar shows higher shrinkage than concrete because of the high content of cement. When fly ash is mixed, the required cement content is reduced. It directly lowers the shrinkage potential of the mix.

Reduce permeability

Fly ash present in the concrete act as a cementitious material as well as filer material. The void structure in the concrete effects by the introduction of fly ash. Fly ash particles will fill up the pores in the concrete making it denser and more impermeable. This makes the concrete more resistant to aggressive chemicals. Therefore fly ash mixed concrete shows better results against chemicals such as sulfates and chlorides.

Additionally, reinforcement corrosion is reduced in fly ash mixed concrete due to low permeability in the concrete. This will improve the service life of the structure.

Increase the wearing resistance

Pavements and parking areas need better wearing resistance. The use of fly ash mixed concrete is the most suitable for these constructions. The improved surface quality of the concrete and dense mix will result in good wearing resistance with fly ash addition.


Every coin has two sides. It’s the same for fly ash too. The use of fly ash in concrete has advantages as well as disadvantages. It’s mandatory to understand the difficulties you may end up with when using fly ash.

Firstly, fly ash affects concrete in many ways. There are several parameters going to effects due to the addition of fly ash. This complex the work. So small builders, housing contractors, and the general public may not be familiar with the fly ash and its impacts. Surely, professional involvement is necessary for concreting works with fly ash addition.

Another main disadvantage with fly ash is low early strength and slow strength gain. Of course, this is helpful in some cases like mass concrete pours. When it comes to structural concretes, the slow rate of strength badly affects the progress. It increases the formwork removal time and curing period. Although you can introduce lime and accelerators to improve strength gaining rate, it will complex the work in other ways. When concreting in cold-weather conditions this strength gain is critical to generate enough heat for the rest of hydration. Too cold concrete will not achieve the desired strength in-time.

Fly ash reduces the pour volume in the concrete microstructure. This will make the concrete vulnerable to freeze-thaw attack. So you need to consider adding air-entraining admixtures into the concrete.

Finally, fly ash is hazardous. Inhaling the fly ash dust will cause throat irritations as well as lung diseases. Eye and skin contact also can cause severe health problems.


Fly ash contributes to the concrete in many ways. It improves the quality and durability of the concrete while reducing the overall cost of the mix. Due to the complex behavior, fly ash added concrete should be handled by knowledgeable personals. Proper mix design and correct handling techniques will always improve the effectiveness of fly ash in concrete.


Kalhara Jayasinghe is a civil engineer currently engage with hydropower construction works in Sri Lanka. He has completed his bachelor's degree & master's in structural engineering from the University of Peradeniya and achieved chartered engineer title in 2019 from the Institute of Engineers Sri Lanka.

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