Why we use Fly Ash as a concrete admixture

What is Fly Ash:

• Fly ash is finely divided residue resulting from the combustion of powdered coal and transported by the flue gases and collected by electrostatic precipitator.
• Fly ash is the most widely used pozzolanic material all over the world.
• The importance and use of fly ash in concrete has grown so much that it has almost become a common ingredient in concrete, particularly for making high strength and high performance concrete.
• The utilisation of fly ash as a supplementary cementitious material. High volume fly ash concrete is a subject of current interest all over the world.

Fly Ash as a concrete admixture:

• The use of fly ash as concrete admixture not only extends technical advantages to the properties of concrete but also contributes to the environmental pollution control.
• There are two ways that the fly ash can be used: one way is to intergrind certain percentage of fly ash with cement clinker at the factory to produce Portland pozzolana cement (PPC) and the second way is to use the fly ash as an admixture at the time of making concrete at the site of work.
• One of the important characteristics of fly ash is the spherical form of the particles. This shape of particle improves the flowability and reduces the water demand. The suitability of fly ash could be decided by finding the dry density of fully compacted sample.

Class of Fly Ash:

• Class F: Fly ash normally produced by burning anthracite or bituminous coal, usually has less than 5% CaO. Class F fly ash has pozzolanic properties only.
• Class C: Fly Ash normally produced by burning lignite or sub-bituminous coal. Some class C fly ash may have CaO content in excess of 10%. In addition to pozzolanic properties, class C fly ash also possesses cementitious properties.

Effect of Fly Ash on Fresh Concrete:

• Use of right quality fly ash, results in reduction of water demand for desired slump.
• With the reduction of unit water content, bleeding and drying shrinkage will also be reduced.
• Since fly ash is not highly reactive, the heat of hydration can be reduced through replacement of part of the cement with fly ash.The reduction of temperature rise for 30% substitution of fly ash.

Effects of Fly Ash on Hardened Concrete:

• Fly ash, when used in concrete, contributes to the strength of concrete due to its pozzolanic reactivity. However, since the pozzolanic reaction proceeds slowly, the initial strength of fly ash concrete tends to be lower than that of concrete without fly ash.
• Due to continued pozzolanic reactivity concrete develops greater strength at later age, which may exceed that of the concrete without fly ash.
• Fly ash concrete should be cured for longer period. In this sense, fly ash concrete used in under water structures such as dams will derive full benefits of attaining improved long term strength and water-tightness.

Effect of Fly Ash on durability of Concrete:

• Sufficiently cured concrete containing good quality fly ash shows dense structure which offers high resistivity to the infiltration of deleterious substances.
• It is also recognised that the addition of fly ash contributes to the reduction of the expansion due to alkali-aggregate reaction.

High Volume Fly Ash Concrete (HVFA):

• High volume fly ash concrete is a concrete where in 50 to 60% fly ash is incorporated. It was first developed for mass concrete application where low heat of hydration was of primary consideration.
• Subsequent work has demonstrated that this type of concrete showed excellent mechanical and durability properties required for structural applications and pavement constructions.
• Due to very low water content of high volume fly ash concrete, the use of superplasticizer becomes necessary for obtaining workable concrete. Use of air-entraining admixtures is also concurrently used.