Fly Ash – Definition, Characteristics, Advantages

Fly ash or Pulverized Fuel Ash (PFA) is the residue from the combustion of pulverized coal collected by mechanical or electrostatic separators from the fuel gases or power plants. It constitutes about 75% of the total ash produced.

  • Fly ash was first used in large scale in the construction of Hungry Horse dam in America in the approximate amount of 30% by weight of cement. Later on it was used in Canyon and Ferry dams etc.
  • In India, Fly ash was used in Rihand dam construction replacing cement up to about 15%.

Definition – Fly ash is a Pozzolana material. Pozzolanas are siliceous or aluminous material which do not have any cementitious properties in itself, but when water is added to pozzolanas materials, they form compounds of low solubility having cementitious properties. Pozzolanas may be natural or artificial, fly ash being the best known artificial pozzolanas material.

Before the advent of cements, Pozzolana materials were used with lime to make concrete. Currently its principle use is to replace a proportion in cement when making concrete.

The advantage of adding pozzolana materials like fly ash on concrete produced is:

  • Economical,
  • Improved workability of concrete mix with reduction of bleeding and segregation
  • It increase imperviousness, to freezing and thawing and to attack by sulphates and natural water.
  • More eco-friendly as the fly ash used is the waste material of power plants.
  • Reduction in Heat of Hydration
  • Reduction in disruptive effects of alkali-aggregate reaction.
  • Lower susceptibility to dissolution and leaching.

Extraction of Fly Ash

Fly ash may be extracted from flue gases of ground or pulverized or crushed coal or lignite fired boilers or by any suitable process: such as, by cyclone separation or electrostatic precipitation. Bottom ash from the boilers shall not be added to the fly ash. Fly ash collected at later stages of electrostatic precipitator are finer than the fly ash collected at initial stages of electrostatic precipitator.

Specifications of Fly ash

Fly ash consists of spherical glassy particles ranging from 1 to 150 µm, most of which passes through a 45 µm sieve.

More than 40% of the particles, which are under 10 microns contribute to early age strength of concrete (7 days and 28 days).

Particles of sizes 10 to 45 microns reacts slowly and are responsible for strength gain from 28 days to 1 year.

Chemical and Physical Requirements of Fly Ash

The quality of fly ash is governed by IS 3812 – Part I – 2013. The BIS specification limit for chemical and physical requirements of fly ash are given in table below.

chemical requirements of fly ash
Chemical requirements of fly ash
physical requirements of fly ash
Physical requirements of fly ash

Low carbon content, high fineness, good reactivity are the essence of good fly ash. Since fly ash is produced by rapid cooling and solidification of molten ash, a large portion of components comprising fly ash particles are in amorphous state, due to it, fly ash greatly contribute to the pozzolanic reaction between cement and fly ash. One of the important characteristics of fly ash is the spherical form of the particles. This spherical shape of particles improves the flowability and reduces water demand.

Effects of Fly Ash on Concrete Properties

Water Requirement – The use of fly ash in limited amounts as a replacement for cement or as an addition to cement requires more water for the same slump value because of fineness of the fly ash. It is generally agreed that the use of fly ash as an admixture rather than as a replacement of cement, reduces, bleeding and segregation. If the sand is coarse, the addition of fly ash produces beneficial results and for fine sand, its addition may increase the water requirement for a given workability.

Compressive Strength – Since the pozzolanic action is very slow, an addition of fly ash up to 30% may result in lower strength at 7 and 28 days, but may be about equal at 3 months and may further increase at ages greater than 3 months provided curing is continued.

Modulus of Elasticity – It is lower at early ages and higher at later ages.

Curing Conditions – It is similar to Portland Cement Concrete.

Shrinkage of Concrete – Coarser fly ashes and those having a high carbon content are more liable to increase drying shrinkage than finer fly ashes and those having a low carbon content.

Permeability – On addition of fly ash to concrete, the permeability of concrete decreases. 28 days pulverized fly ash concrete may be three times as permeable as ordinary concrete but after 6 months, it may be less than one quarter permeable.

Resistance to Chemical Attack – Fly ash slightly improves the resistance of concrete to sulphate attack by decreasing the quantity of calcium hydroxide.

Heat of Hydration – Fly ash reduces the heat of hydration. A replacement of 30 % of cement by fly ash may result in a reduction 50 to 60% heat of hydration.

Setting time – A 30% substitution of fly ash may result in an increase of initial setting time up to 2 hours.

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