What Is Fly Ash?

Fly Ash Tradeworth International

Fly ash is a by-product of burning pulverized coal in an electrical generating station. Specifically, it is the unburned residue that is carried away from the burning zone in the boiler by the flue gases and then collected by either mechanical or electrostatic precipitators.

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Fly ash is used as a supplementary cementitious material (SCM) in the production of portland cement concrete. A supplementary cementitious material when used in conjunction with portland cement contributes to the properties of the hardened concrete through hydraulic or pozzolanic activity or both.

Physical Aspect

Kinds of Fly Ash in Pakistan

The fly ash from the boilers where mechanical collectors are used is coarser than fly ash from electrostatic precipitators.

The color varies from light to dark grey depending upon its carbon contents

The quality of fly ash varies from source to source

Fly ash particles are small, they effectively fill voids

How does Fly Ash produced?

Production of Fly ash Pakistan

Why Fly Ash?

CH Crystal C-S-H

Fly ash has a high amount of silica and alumina in a reactive form. These reactive elements complement hydration chemistry of cement.
When cement reacts with water, we say that hydration of cement has began and produces C-S-H (Calcium-Silicate-Hydrate) Gel.
C-S-H Gel binds the aggregates together and strengthens concrete.

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However, one more compound is produced on hydration that is so different in behaviour. It is non other than the Calcium Hydroxide Ca(OH)2.
In construction industry, it is generally referred to as Free Lime.

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Aggressive environmental agents like water, sulphates, CO2 attack this free lime leading to deterioration of the concrete.

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Fly ash, on itself, can not react with water. It needs free lime, produced on hydration of Portland cement, to trigger off its Pozzolanic effect.
Once it is triggered, it can go on and on!

Concrete Performance

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Specific benchmarks have been set up to evaluate the performance of concrete with respect to durability—mainly Strength and Permeability. This means to produce a durable and long lasting concrete, it must posses: –
High strength And Low permeability

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Fly ash makes concrete denser, and hence less permeable, mainly by : –
Reducing water demand in concrete
Improving microstructure of concrete At the same time, fly ash improves long term strength of concrete due to the continued Pozzolanic reaction as discussed earlier.

Classification of Fly Ash

fly ash supplier in Karachi ASTM C618 Classes of Fly ash
fly ash supplier in Karachi ASTM Specification for Fly ash

Classification of Fly Ash

fly ash brown

Produced from burning harder, older anthracite and bituminous coal.
Contains less than 15% lime.
Requires cementing agent like PC, quick lime, hydrated lime.
Used in high sulfate exposure conditions
Use for structural concrete, high performance concrete, high sulfate exposure concrete.
Useful in high fly ash content concrete mixes.

fly ash in Pakistan

Produced from burning lignite and sub-bituminous coal.
Higher concentration of alkali and sulfate.
Contains more than 15% lime.
Self-cementing properties.
Not to be used in high sulfate conditions.
Primarily residential construction.
Limited to low fly ash content concrete mixes.

Comparison between different classes of Fly Ash & OPC

Pozzolan Type fly ash in Pakistan

Effects of Fly Ash in Fresh Concrete

fly ash Pakistan graph


The use of good quality fly ash with high fineness and low carbon content reduces the water demand of concrete and consequently, the use of fly ash should permit the concrete to be produced at a lower water content when compared to a portland cement concrete of the same workability.

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Generally fly ash will reduce the rate and amount of bleeding primarily due to the reduced water demand.

Flyash Karachi penetration resistance (MPa)

Heat of Hydration

The reduction in the rate of the heat produced and the internal temperature rise of the concrete has long been an incentive for using fly ash in mass concrete construction.

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Generally fly ash will reduce the rate and amount of bleeding primarily due to the reduced water demand.

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Strength Development

By replacing a certain amount of portland cement with the same amount of fly ash and maintaining a constant w/c. As the level of replacement increases the early-age strength decreases. However, long-term strength development is improved when fly ash is used as against portland cement concrete.

Durability of Concrete

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Sulphate Resistance

Studies have demonstrated that the use of sufficient quantities of low-calcium Class F fly ash can increase the resistance of concrete to chemical attack when the concrete is exposed to sulphate-bearing soils or groundwater.

sulfate attack

Alkali-Silica Reaction

It is well established that low-calcium (Class F) fly ash is capable of controlling damaging alkali-silica reaction (ASR) in concrete at moderate levels of replacement (20% to 30%) and the effect has been ascribed to the reduced concentration of alkali hydroxides in the pore solution when fly ash is present.

ASTM C1202 RCPT data

Resistance to the Penetration of Chlorides

Fly ash reduces the permeability of concrete to water and gas provided the concrete is adequately cured (Thomas 2002). This has been attributed to a refinement in the pore structure.

Importance of Curing

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Important for development of strength & durability properties Prevention of moisture loss is important : (a) when W/C ratio is low (b) when cement has a high rate of strength development & (c) when concrete contains mineral admixtures Moisture loss from concrete surface specially for thin elements, relatively high in hot & dry climate

Fly Ash Is GREEN, Not Gray

Fly ash Karachi not Gray it's Green

Green Energy

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Growth of cement usage leads to greater production of CO2 which is one of the major cause of global warming Cement industry contributes 5% of total anthropogenic CO2 emission globally

Global Coal Consumption

World Coal Consumption 2016

Around the world

fly ash used in Roam

It started more than 2,000 years ago… The ash generated from Volcanoes was used extensively in the construction of Roman structures. Colosseum is a classic example of durability achieved by using volcanic ash. This is a building constructed 2,000 years ago and still standing today!

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Ghatghar Dam

But the Ghatghar Roller Compacted Concrete Dam was a unique project in all respects. For the upper dam, 65% fly ash was used with 35% Portland cement. The result was much better than expected.

Burg-Al-Khalifa, Dubai

Fly ash concrete used burj khalifa


Fly ash Pakistan benefits


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