Introduction to Fly ash and Classification of Fly ash and Properties of Fly ash

Introduction to Fly ash and the Durability of Fly ash ?
       

Pulverized fuel ash commonly known as Fly ash, is comprised of the non-combustible mineral portion of coal. When coal is consumed in a power plant, it is first ground to the fineness of powder. Blown into the power plant’s boiler, the carbon is consumed — leaving molten particles rich in silica, alumina and calcium. These particles solidify as microscopic, glassy spheres that are collected from the power plant’s exhaust before they can “fly” away — hence the product’s name: Fly Ash.

Fly ash is a pozzolanic When mixed with lime (calcium hydroxide), pozzolanics combine to form cementitious compounds. Concrete containing fly ash becomes stronger, more durable, and more resistant to chemical attack.

Classification of Fly ash:

  ASTM C618 classifies the fly ash into two classes:

·          Class F

·          Class C

·         Class F Fly ash:

Fly ash normally produced from burning anthracite or bituminous coal falls in this category. This class of fly ash exhibits pozzolanic property but rarely if any, self-hardening property. In Class F fly ash, total calcium typically ranges from 1 to 12 percent, mostly in the form of calcium hydroxide, calcium sulphate, and glassy components in combination with silica and alumina.

·         Class C Fly ash:

Fly ash normally produced from lignite or sub- bituminous coal is the only material included in this category. This class of fly ash has both pozzolanic and varying degree of self cementitious properties. Most class C fly ashes contain more than 15 % CaO. But some class C fly ashes may contain as little as 10 % CaO.

NOTE: The chief difference between Class F and Class C fly ash is in the amount of calcium and the silica, alumina, and iron content in the ash.The chemical properties of the fly ash are largely influenced by the chemical content of the coal burned (i.e. bituminous, sub-bituminous, lignite). 

Quality of Fly ash:

The quality of Fly ash is governed by IS 3812-2003-Part1&2.In this Standard Pulverized Fuel Ash (PFA i.e., fly ash, bottom ash, pond ash or mound ash) has been divided into two categories:

·      Siliceous PFA normally produced by burning anthracite or bituminous coal and normally contains reactive CaO less than 10% by mass.

·         Calcareous PFA normally produced by burning lignite or sub-bituminous coal and reactive CaO not less than 10% by mass.

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