Skip to main content
Reginald Morrison
Facilities manager
Asked a question 2 years ago

fly Ash Percentage in PPC Cements

Different Cement companies have maintain different % of Flyash in cement. what are the advantage and disadvantage of Higher / Lower % of flyash in cement.

Where am I?

In TheConstructor you can ask and answer questions and share your experience with others!

PPC: Approximately 15 to 30% of ordinary Portland cement is replaced with fly ash in Portland pozzolana cement. The compressive strength of Portland pozzolana cement is more for long term use compared to ordinary Portland cement. Ordinary Portland cement is one of the widely used cement. 

For residential construction, we prefer Portland pozzolana cement as compared to ordinary Portland cement. We use ordinary Portland cement only for commercial purposes. Three grades of cement are available 33, 43, and 53. 33 grade of cement means the compressive strength of cement after 28 days of curing should be at least 33 mega Pascal not more than that value. 

43 grade of cement was the compressive strength of cement after 28 days of curing should be at least 43 Newton per mm square or more than that. Similarly, 53 Grade of cement means compressive strength of cement after 28 days of curing should be at least 53 Newton per mm square or more than that value.

Thank you.

Fly ash adding following advantages

Protect against sulphur attack. Increase concrete life. Reduction in cost. Reduce CO2 emission. Good work as a DPC. Reduction in shrinkage & plastic cracks.


Lower earlier strength. Less bonding compare to cement. Fly ash is not a cemetesius material. Requir High curing period.

in general, 20-40% fly ash replace by cement in concrete.

Seth Morgan
Construction Superintendent

Sir, Nearly 15-30% ordinary portland cement is replaced by Fly ash in portland pozzolana cement(PPC). Some times, it's percent may increase. The fly ash is used to lower the cost and it also inhance the performance of portland pozzolana cement.

Kent Welch
Planning and development surveyor

Fly ash is a better pozzolanic admixture. It doesn't possess any cementitious property. They chemically react with Calcium Hydroxide at room temperature to form cementitious compounds.

Fly ash is a by-product or a waste product also called as pulverized fuel ash is artificial pozzolanic material. It becomes reactive in presence of lime.

fly ash ingredient - 10- 25 % by mass of Portland Pozzolana Cement. Used in vast concreting Used in cold weather concreting Used in the making of Geopolymer concrete.


Cost-effective Reduces CO2 Environment safe Good cold weather resistance Non-shrinkable Good workability also takes lower w/c ratio Reduces the normal defects of concrete like cracking, etc Reduces hydration heat


It may affect the color Reduces strength gain Class C fly ash is sensitive for temp, hence when temp is about 200℃ it doesn't give high strength Its need for low w/c ration leads to compulsary use of plasticizers etc for workability

Raul Jimenez
Senior Project Manager

Fly ash is used as cement replacement has following advantageous:

Resists the sulphate attack. Resists corrosion and other aggressive chemicals. Decreases moisture penetration, makes concrete less permeable and increases durability. Extends concrete life Reduces total life cycle cost With slower rate of hydration the use of fly ash increases structural integrity. Reduces shrinkage and creep Reduces C02 emission

The only disadvantageous is slower strength gain.Concrete may not achieve design strength at 28 days. Designers may have to specify 40 or 56 days of strength as acceptance criteria as done in some projects in US.

I have used fly ash ( conforming to ASTM C 618, Class F) as cement replacement ranging from 25% to 40% by weight of OPC.

fly ash content is 15 to 35% for PPC

Fly ash is a pozzolana material used to reduce the effect of Ca(OH)2

Fly ash is not cemented material, but it is a pozzolanic materials that react with the cement Ca(OHaOH)2 and make bonds.

In general, we replace cement to fly ash 20%,

If we add more fly ash, then the strength of the structure becomes less, and durability also reduced, so we need to add optimum fly ash.

Timmothy Ferguson
Assistant Project Manager

Fly Ash is a product or furnace slag received form metal factories and power plants by burning or combustion of coal as fuel.

The maximum percentage of fly ash or 15% by weight of cement shall be replaced by fly ash.

#less percentage does not affect any properties of concrete or cement

#more fly ash reduces the strength of concrete and durability of concrete.

Adequate % of fly ash makes concrete economical and more durable.

Because it reduces the quantity of cement, and slow hydration prevents cracks in concrete.

Fly ash is available in the market cheaper than cement.

Fly Ash concrete mix proportioning Methods

Simple Replacement Method -substitute fly ash for cement. Thb substitution is generally made on a one for one basis either by weight or by volume (water-cement requirements) Modified replacement method -The amount of fly ash put into the mix is greater than the amount of cement removed, the difference being accommodated by a change in the aggregate proportions. Rational Methods - Recommended ACI 211.1


The use of fly ash in portland cement concrete (PCC) has many benefits and improves concrete performance in both the fresh and hardened state. Fly ash use in concrete improves the workability of plastic concrete, and the strength and durability of hardened concrete. Fly ash use is also cost effective. When fly ash is added to concrete, the amount of portland cement may be reduced.

Theodore Ray
Planning and development surveyor

I herd that, the un-burnt carbon present in fly-ash effects concrete adversely. can u explain the same?

Could you provide more explanation regarding your question, please?

Henry Newman
Water resource engineer

Thanks For the answer I would also like to know that how the Quality of Fly ash effects?

Fly as is a by product of power station furnaces fired by pulverized bituminous coal. It replaces portion of concrete and hence improve sustainability of structures. Any replacement percentage can be considered provided that project requirement and specifications are not compromised. For example, concrete strength shall be the same of nearly the same of concrete without replacement materials.

Generally, fly ash reduce heat of hydration, improve sulfate resistance, reduce chloride diffusivity, minimize risk of alkali aggregate reactivity, environmentally friendly, and improve concrete economy.

As the percentage of replacement is increased, these advantages would manifest themselves better. However, higher percentages may decline ultimate concrete strength, and concrete need strict curing regime since early strength development of concrete with fly ash is low.

In contrary, low replacement percentage would provide more certainty regarding strength of concrete, but the replacement would not be effective.

In summary, suitable replacement percentage is not the same for all structures. Each condition shall be investigated and suitable replacement quantity shall be determined.