The main constituents of good brick earth are alumina, silica, lime, oxide of iron and magnesia.
3.2.1 Functions of Constituents of Brick Earth
1. Alumina
This is the main constituent of every kind of brick earth. This imparts plasticity to the earth so that it can be moulded. If alumina is present in excess, it leads to shrinking and warping during drying and burning. This can be prevented by the addition of sand.
2. Silica
It exists in brick earth as free sand or in a combined form as silicate of alumina. The presence of silica prevents cracking, shrinking and warping of raw bricks. Thus, it imparts uniform shape to bricks. The addition of silica also increases hardness, durability and resistance to heat. Excess of silica removes the cohesion between particles and makes the brick brittle.
3. Lime
It enables the silica to melt during burning and bind the particles together. Lime should be in the form of powder, otherwise lumps of lime are converted into quick lime after burning. Quicklime slakes and expands in the presence of moisture and results in the splitting of bricks into pieces. Excess of lime causes the brick to melt too readily, and the shape will be lost.
4. Iron Oxide
It acts as a flux and helps the grains of sand to melt and bind the particles of clay together. It is responsible for imparting a red colour to the brick. It improves the durability of the brick. Excess of iron oxide makes the bricks dark blue or blackish, and less quantity of iron oxide makes the brick yellowish in colour.
5. Magnesia
A small quantity of magnesia in brick earth imparts a yellow tint and decreases shrinkage. Excess of magnesia causes decay of bricks.
3.2.2 Desirable Chemical Composition of Brick Earth
The desirable chemical composition of brick earth for good quality bricks are given below (IS: 1077, 1976):
- Alumina or clay – 20 to 30% by weight
- Silica or sand – 35 to 50% by weight
- Silt – 20 to 25% by weight
- Other required ingredients are
- Iron oxide
- Magnesia
- Lime
- Sodium potash, etc.
- In the case of alluvial soils, the total lime and magnesia should not be more than 1%; in other soils it should be less than 15%.
- Water-soluble materials in total should be less than 1%.
3.2.3 Harmful Ingredients in Brick Earth
Naturally available soil may contain ingredients other than those required for making quality bricks, which are discussed below.
1. Alkalis
Alkalis in the form of soda and potash lower the fusion point of clay, and cause bricks to fuse, twist and warp during burning. Alkalis present in bricks absorb moisture from the atmosphere and create dampness. Such dampness dries and leaves behind grey or white deposits on the surface of the wall.
2. Pebbles
Although the presence of pebbles causes harm chemically, they are not desirable as they do not permit the clay to be thoroughly mixed uniformly resulting in weak and porous bricks. Regular breaking of bricks during masonry work is not possible because of the presence of pebbles.
3. Iron Pyrites
The presence of iron pyrites in brick earth causes the earth to decompose and oxidize resulting in the splitting of bricks.
4. Vegetation and Organic Matter
Vegetation and organic matter when present in brick earth assists in burning. Incomplete burning of these materials causes the bricks to become porous.
5. Limestone
Limestone and kankar nodules present in brick earth are harmful as that of lumps of lime. On heating limestone is converted into lime, which comes into contact with water, swells and finally results in splitting and crumbling.
3.2.4 Field Testing of Brick Earth
Brick earth in the field has to be tested for consistency, moulding properties and shrinkage.
In order to test for the first property, a handful of soil sample is taken and formed into a ball. The ball is dried in the sun and then examined. Crumbling of the dried ball shows the presence of excess sand. If the ball is hard and shows cracks on the surface, it indicates the presence of less sand. Thus, the soil has to be modified by mixing different proportions of sand or clay such that the above-mentioned defects do not occur.
The soil found or made suitable in the first test is taken and ground well. Then a required quantity of water is added and mixed. The mixed soil is kneaded well to be rolled to form threads of about 3 mm diameter without crumbling. Such a mixture of soil and water is used to make a brick using a mould. The moulded brick should be with clear edges and corners which ensures perfect requirement of water. Otherwise water is added or removed to get a satisfactory brick.
Sample of bricks moulded as above are burnt in a clamp until bricks become red hot. These bricks are examined for shrinkage and deformation after cooling. Bricks which have shrunk evenly and do not show any defects are separated out and others are rejected. The percentage of sorted out bricks should be high, otherwise the soil is modified to get a better percentage of good bricks.
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