Followings are the constituents of good brick earth
1.
ALUMINA (Al2O3)-It is the
chief constituent of good brick earth. 20%-30% alumina is necessary to form the
brick earth of good quality. It imparts plasticity to 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 wrap during dying,
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 contains about 50-60% of silica.
The presence of silica prevent the
shrinkage, cracking and warping of raw bricks. Thus it imparts uniform shape to
the bricks and durability of bricks depends upon silica. Excess silica destroy
the cohesion between particles and the brick become brittle.
3.
LIME ( CaCO3)-a small quantity of
lime not more than 5% is desirable in good brick earth. It causes silica in
clay to melt on burning and hence help to bind it.
The excess lime cause brick to melt and
hence its shape is lost.
4.
IRON OXIDE (Fe2O3)-Iron
oxide performs two functions, first it helps in fusing the sand like lime and
second it provides red colour to the brick. It kept below 5-6% because excess
of it may result in the dark blue or black colour of bricks.
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