In this article Learn about : Brick earth composition,{ clay(alumina), silt, sand}, functions of brick constituent (silica or sand, alumina, lime, magnesia, oxide of iron), harmful ingredients of brick (lime, iron pyrites,Pebbles of stone and gravel etc, Alkalies, Reh or Kallar)
BRICKS EARTH
Composition.
A good brick-earth should be such that when prepared with water it can be easily moulded, dried and burnt with out cracking or warping. It should contain a small quantity of finely divided lime to help in binding the particles of brick together by melting the particles of sand. A little oxide of iron should also be present which would give the brick its peculiar red colour and act in the same manner as lime.
A good brick earth should preferably conform to the following composition :
Clay (Alumina) 20 to 30 per cent by weight
Silt 20 to 35 per cent by weight
Sand 35 to 50 per cent by weight
The total content of clay and silt should not as far as possible be less than 50% by weight.
functions of brick constituent
(i) Silica or Sand.
It is present either free as sand or in combination as silicate of alumina. Silica is in fusible except at very high temperatures but in the presence of alumina in nearly equal proportions and the oxide of iron to fuses at lower temperatures. Unlike silicate of alumina its presence in clay produces hardness, resistance to heat, durability and prevents shrinkage and warping. Excess of it makes the brick brittle.
(ii) Alumina.
It is a tenacious finely-grained mineral compound present in brick-earth. It is plastic, when wet, and in capable of being molded to any shape. On drying it loses its plasticity and becomes hard, shrinks, warps and cracks. Burning causes the fusion of its constituents thereby making it homogeneous, harder and stronger.
(iii) Lime.
When present in small quantities in finely divided state it reduces shrinkage of bricks and acts as a flux causing silica to melt. It results in binding the particles of brick together resulting in greater strength of brick. Excess of lime causes the brick to melt and lose its shape.
(iv) Magnesia.
In the presence of iron it gives a yellowish tint to the bricks. It should not be present in excess. However, the presence of small quantity of manganese with iron will give the brick darker or even black colour. Total lime and magnesia in case of alluvial soil shall not be more than one per cent and in other cases it will preferably not exceed 15 per cent.
(v) Oxide of iron.
In the presence of silica and alumina, it helps the fusion of brick particle. Also it influences the colour of bricks. It produces a tint varying from light yellow to red depending upon the percentage of iron present in clay. Excess of it makes the colour dark blue. It should not be present in the form of iron pyrites.
Harmful ingredients.
(i) Lime.
If present in excess it melts the brick particles as a result of which the brick loses shape. Lime should also be not present in brick-earth in the form of lime stone or kankar modules. On the burning of bricks, these get converted to quickline which expands on absorption of moisture and causes the cracking and disintegration of bricks.
(ii) Iron pyrites.
Iron pyrites get oxidized in the brick, crystallize and split the brick to pieces. These should be carefully removed from brick-earth.
(iii) Pebbles of stone and gravel etc.
Their presence makes it difficult to mix the brick-earth thoroughly as a result of which the bricks are not homogeneous. It gives weak and porous bricks. Also such bricks cannot be readily cut or worked.
(iv) Alkalies.
Their excessive presence in earth renders it unsuitable for bricks. These act as flux causing the bricks to melt, twist and warp. Presence of common salt in earth taken from seashores or from near salt formations has similar effects to those narrated above and also make the bricks hygroscopic thereby causing efflorescence.
(v) Reh or Kallar.
It is the sulphate of soda mixed with a little carbonate of soda and common salt. Its presence in brick earth prevents bricks from being properly burnt. After the bricks have been burnt these salts recrystallize and appear as irregular and unsightly white patches on the surface of bricks. They cause the plaster and the surface of bricks to peel-off layer by layer and to ultimately crumble away. Presence of reh or kallar in soil could be easily detected by the presence of efflorescence on the sides of fresh excavation, if the soil is moist.