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Composition of a good brick earth:
Following are the constituents of a good brick earth:
(1) Alumina (Al2O3):
It is the chief constituent of a good brick earth. A content of about 20% to 30% is necessary to form the brick earth of a good quality. It imparts plasticity to the earth so it helps in the moulding of the brick earth.
If alumina is present in excess with inadequate quantity of sand then the raw bricks shrink and warp during drying, on burning they become too hard. So it is important to have an optimum content of alumina.
(2) Silica (SiO2):
It exists in the brick earth either free or combined. As free sand it is mechanically mixed with clay and in combined form it exists in chemical composition with alumina. A good brick earth should contain about 50% to 60% of silica.
The presence of this constituent prevents the shrinkage, cracking and warping of raw bricks. It thus imparts uniform shape to the bricks. The durability of bricks depends upon proper composition of silica in brick earth. The excess of silica destroys the cohesion b/w particles and brick become brittle.
A small quantity of lime not more than 5% is desirable in good brick earth. It should be present in very fine state, because even small particles of size of a pin-head can result in the flaking of the brick.
The lime prevents shrinkage of the raw bricks, sand alone is infusible, but it slightly fuses at kiln temperature in presence of lime. Fused sand acts as a hard cementing material for brick particles.
The excess of lime causes brick to melt and therefore its shape is lost. The lumps of lime turns into quick lime (CaO) after burning and this free lime can later react with water to form slaked lime. This process is called slaking it may result in splitting of the brick into pieces.
(4) Oxide of Iron (Fe2O3):
Iron oxide performs two functions, first it helps in fusing of the sand like lime and second it provides the red color to the bricks. It is kept below 5 to 6% because excess of it may result in the dark blue or black color of brick.
It is used to provide a yellow tint to the bricks. Its content is only about 1% or less.
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