Cement is
an important binding material used in construction works. Cement is required
for almost every construction work. Therefore, the composition of cement is of
great interest to engineers.To understand cement structure or composition, one
must know the functionality of cement components.By changing the quantity of a
component during cement production, one can achieve the desired cement quality.
Composition of cement:
There are eight
major components/ingredients of cement. The following image shows the
ingredients of cement:
The general percentage of these materials in cement is given below:
Functions of cement components/ingredients:
The main features
of these cement components are given below along with their functions and
usefulness or harmfulness:
1. Lime: Lime is calcium hydroxide or calcium oxide.
The presence of
sufficient amount of lime requires silicates and aluminates of calcium.
Reduction in lime
reduces the strength of cement property.
Reduction in lime
causes cement to set quickly.
Excessive lime
makes the cement unsound.
Excessive presence
of lime causes cement to disintegrate and expand.
2. Silica: Silicon dioxide is known as silica, the chemical formula
SiO2.
Sufficient amount
of silica should be present in the cement to di-calcium and tri-calcium
silicate.
Silica reinforces
cement.
Silica typically represents a limit of about 30 percent cement.
3. Alumina: Alumina is aluminum oxide. The chemical formula is Al2O3.
Alumina provides
quick setting properties to cement.
The presence of the
expected amount of alumina leads to a lower clinkering temperature.
Excess alumina
weakens cement.
4. Magnesia: Magnesium oxide. The chemical formula is
MgO.
In terms of
percentage,Magnesia should not be present more than 2% in cement.
Additional magnesia
will reduce the strength of cement.
5. Iron oxide: The chemical formula is Fe2O3.
Iron oxide gives
color to cement.
Acting as a flux.
At very high
temperatures, it chemically reacts with calcium and aluminum to form
tri-calcium alumino-ferrite.
Tricalcium
alumino-ferrite provides hardness and strength to the cement.
6. Calcium Sulfate: The chemical formula is CaSO4
It is present in
cement in the form of gypsum (CaSO4.2H2O)
It slows down quickly
which inturn reduces the setting action of cement.
7. Alkaline:
It should not be
more than 1%.
Highly alkaline
substances cause efflorescence.
8. Sulfur trioxide: the chemical formula is SO3
In terms of
percentage,it should not be present for more than 2%.
Cement becomes unsound
due to excess sulfur trioxide.
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