Uses of Fly Ash

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

1. Used in concrete as an admixture or as a part of replacement of cement.
2. Used in making cellular concrete blocks.
3. Used in making building blocks.

1. Application in Concrete as Admixture

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

1. Cement aggregate reaction is reduced by the addition of fly ash.
2. Evolution of low heat when fly ash is added to the concrete.
3. Water tightness of concrete is greatly improved by the addition of fly ash in concrete.
4. Addition of fly ash improves the plasticity and cohesiveness of concrete mixture and thereby permits easy placing of concrete and finishing.
5. Addition of fly ash improves the strength of concrete also.

The importance and use of fly ash in concrete has 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 Indian Standards (IS: 3812–1981), as given in Table 13.1.

Table 13.1 Chemical composition requirement

Source: IS: 3812–1981.

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

1. Minimum specific surface should be 320 m²/kg for use as pozzolana.
2. Maximum average compressive strength of lime-fly ash mortar cube should not be less than 4.5 N/mm² for use as pozzolana.
3. 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%.
4. 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 silicous 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 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.