FLY ASH | CONSTITUENTS OF FLY ASH | USES OF FLY ASH | CONSTRUCTION MATERIALS

FLY ASH

Fly ash is a fine-grained residue resulting from the combustion of pulverised coal or lignite in boilers. As it is in ash form it is easily transported by the flue gases and collected by mechanical or electrostatic separators. Mainly it consists of spherical glassy particles ranging from 1 to 150 µm in diameter and the major portion passes through a 4.5 µm sieve.

 

The chemical composition of fly ash varies with the type of fuel burnt, load on boiler and type of separator, etc. High fineness, low carbon content, good reactivity are the necessary qualities of good fly ash.

 

USES OF FLY ASH

Fly ash is most widely used in many profitable applications such as the following:

(i) Used in concrete as an admixture or as a part of replacement of cement.

(ii) Used in making cellular concrete blocks.

(iii) Used in making building blocks.

 

1. Application in Concrete as Admixture

Fly ash is a widely used pozzolanic material all over the world. The addition of fly ash, apart from playing the role of an admixture, imparts the following properties to concrete:

(i) Cement aggregate reaction is reduced by the addition of fly ash.

(ii) Evolution of low heat when fly ash is added to the concrete.

(iii) Water tightness of concrete is greatly improved by the addition of fly ash in concrete.

(iv) Addition of fly ash improves the plasticity and the cohesiveness of concrete mixture and thereby permits the easy placing of concrete and finishing.

(v) Addition of fly ash improves the strength of concrete also.

The importance and use of fly ash in concrete have grown so much that it has become a common ingredient in concrete, particularly for making high-strength and high-performance concrete. The quality of fly ash for use in concrete should satisfy the chemical composition recommended by the Indian Standards (IS: 3812–1981), as given in Table.

Sl. No.	Characteristics	Requirement Mass Percentage
1.	(Fe2O3)	≥ 70.0
2.	(SiO2)	≥ 35.0
3.	(MgO)	≤ 5.0
4.	(SO3)	≤ 2.75
5.	(Na2O)	≤ 1.50
6.	Loss on Ignition	≤ 12.0

Source: IS: 3812–1981.

 

As per the Indian standards (IS: 3812–1981), the physical requirements areas follows:

(i) Minimum specific surface should be 320 m2/kg for use as pozzolana.

(ii) Maximum average compressive strength of lime-fly ash mortar cube should not be less than 4.5 N/mm2 for use as pozzolana.

(iii) On testing a mixture of four parts of OPC and fly ash by weight, the increase in volume should not be more than 0.8%.

(iv) A mixture of OPC and fly ash, the compressive strength should not be less than 80% of the strength of corresponding plain cement mortar cubes.

 

2. Application in Cellular Concrete Blocks

Cellular concrete blocks are produced by autoclaving a set mixture of fine siliceous material such as fly ash and binder in the form of lime.

The cellular concrete blocks have many technical advantages, viz., strength-to-weight ratio, good sound insulation, stability against variations in temperature and humidity, resistance to fire and water seepage, low thermal conductivity, consumes less quantity of mortar in masonry and plaster can be completely avoided as the blocks are machine finished.

 

3. Fly Ash Building Blocks

Fly ash building blocks are manufactured using fly ash, sand and lime which are mixed in the ratio 80:13:7. Bricks are made in a hydraulic press and the semi-dried bricks are cured in a steam chamber at an appropriate temperature and pressure.

Compared to the conventional burnt bricks, the fly ash building blocks are better in shape, technical specifications, compressive strength and impermeability. Furthermore, the fly ash building blocks are about 20% light in weight and about 12% cheap compared to the conventional bricks.

FLY ASH MIX DESIGN AND SPECIFICATION REQUIREMENTS

WHY FLY ASH IS ADDED IN CEMENT?

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