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Fly ash and its classification

Concrete has been the basic building material ever seen the field of construction has came into existence. With the technological advancement the definition of concrete has undergone a few modifications but the prime goal has remained same. The reason behind the modification and evolution is the birth of additive materials like fly ash, silica fume, rice husk etc. which once being a residue has now found one of the important place in the manufacturing of high performance concrete.

Today we are going to discuss about some of the most important aspects of fly ash that evolved it as the most extensive used additive among its group. Fly is a recent invention in the field of concrete technology in terms of strengthening the concrete. Over the years the fly ash has seen much usefulness in the construction industry. Being a residue of coal combustion process it was considered to be waste but the chemical and physical properties of the fly ash as made up for the ground and enable it to be used along with other concrete materials.

Fly ash is generally finely divided residue ash particle resulting from the combustion of coal in the furnaces which blows along with flue gas of the furnace.  Theses ash are collected with the help of electric precipitators and termed as fly ash. Fly ash is the most widely used pozzolanic material all over the world. In UK it is termed as pulverised fuel ash i.e. PFA . Although it is a residue of coal but it contain chemical components like silicon dioxide, aluminium oxide, iron oxide in major quality and apart from these substance reactive silica, magnesium oxide, sodium oxide, calcium oxide, titanium, lead oxide are also found in major quantity which marks fly ash suitable to be used in combination with cement in the production of concrete.

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According to the IS: 3812- Part -1: 2003 the standard chemical requirement of fly ash is existed below

Sl. No.CharacteristicsRequirements
1.Silicon dioxide(Sio2) plus aluminium oxide (Al2O3)plus iron oxide (Fe 2O3) per cent by mass, Min.70
2.Silicon dioxide (SiO2), per cent by mass, Min35
3.Reactive silica in percent by mass, min20
4.Magnesium Oxide (Mgo), percent by Max5
5.Total sulphur as sulphur trioxide, per cent mass, Max3
6.Available alkali, as sodium oxide, per cent by mass, Max1.5
7.Total chloride in present by mass , Max0.05
8.Loss on ignition, per cent by mass, Max5

On the other hand a standard fly ash should satisfy certain physical properties in order to attain high performance concrete mix. According to IS: 3812- part : 2003 these requirements are as follows :

Sl. No.


Requirements grade of fly ash

I                                           II

1.Fineness- specific surface in metre square per kilogram by Blaine’s permeability method, Min320                           250
2.Lime reactivity- average compressive strength in N/ metre square, Min4.5                           3
3.Compressive strength at 28 days inn/ square metre, MinNot less than 80% of the strength of corresponding plane cement mortar cubes
4.Soundness by autoclave test expansion of specimens per cent, Max0.8                           0.8

With the rapid technological advancement the useful properties of fly ash are evolving every day, making it the most extensively used admixture in the concrete production. In actual the usage of fly ash should be increased to a greater extend as the fly ash is nothing but a residue which threatens the stability of environment. The global fly ash production is estimated to be greater than 1.5 billion tons which clearly indicates the extensive emission of carbon dioxide in the atmosphere. India itself produces nearly 75 million tons of fly ash annually and only a mere 5% of the total production is used as a alternative for various constructional materials.

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Classification of fly ash  

Fly ash can be classified into two groups

  1. Class C and
  2. Class F

 Class C

This fly ash is produce by the burning of liginite or sub bituminous coal. It posses both pozzolanic and cementitious properties. More over they do not requires any additive substances to show the pozzolanic properties in them. As we know CaO is an important constituent of fly ash but in some Class C fly ash the quality of CaO is more than 10%.

Class F

Fly ash which is normally produced by burning of anthracite or bituminous coal, are classified in the Class F class. They shows the pozzolanic properties but needs additive for showing the property. They usually has less than 5% CaO content in them.

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