INGREDIENT OF GOOD BRICK EARTH

For the preparation of bricks, clay or other suitable earth is moulded to the desired shape after subjecting it to several processes. After drying, it should not shrink and no crack should develop. The clay used for brick making consists mainly of silica and alumina mixed in such a proportion that the clay becomes plastic when water is added to it. It also consists of small proportions of lime, iron, manganese, sulphur, etc. The proportions of various ingredients are as follows:

Silica - 50–60%

Alumina  - 20–30%

Lime -  10%

Magnesia -  < 1%

Ferric oxide -  < 7% } Less than 20%

Alkalis -  < 10%

Carbon dioxide - (Very small percentage)

Sulphur trioxide - (Very small percentage)

Water - (Very small percentage)

Functions of various Ingredients

SILICA : 

It enables the brick to retain its shape and imparts durability, prevents shrinkage and warping. Excess of silica makes the brick brittle and weak on burning. A large percentage of sand or uncombined silica in clay is undesirable. However, it is added to decrease shrinkage in burning and to increase the refractoriness of low alumina clays.

Alumina :

absorbs water and renders the clay plastic. If alumina is present in excess of the specified quantity, it produces cracks in brick on drying. Clays having exceedingly high alumina content are likely to be very refractory.

Lime :

normally constitutes less than 10 per cent of clay. Lime in brick clay has the following effects:

1. Reduces the shrinkage on drying.

2. Causes silica in clay to melt on burning and thus helps to bind it.

3. In carbonated form, lime lowers the fusion point.

4. Excess of lime causes the brick to melt and the brick looses its shape.

5. Red bricks are obtained on burning at considerably high temperature (more than 800°C) and buff-burning bricks are made by increasing the lime content.

Magnesia :

rarely exceeding 1 per cent, affects the colour and makes the brick yellow, in burning; it causes the clay to soften at slower rate than in most case is lime and reduces warping.

Iron :

Iron oxide constituting less than 7 per cent of clay, imparts the following properties:

1. Gives red colour on burning when excess of oxygen is available and dark brown or even

black colour when oxygen available is insufficient, however, excess of ferric oxide makes

the brick dark blue.

2. Improves impermeability and durability.

3. Tends to lower the fusion point of the clay, especially if present as ferrous oxide.

4. Gives strength and hardness