MANUFACTURE OF PORTLAND CEMENT
The raw materials required for manufacture of Portland cement are calcareous materials,
such as limestone or chalk, and argillaceous material such as shale or clay. Cement factories
are established where these raw materials are available in plenty. Cement factories have come
up in many regions in India, eliminating the inconvenience of long distance transportation of
raw and finished materials.
The process of manufacture of cement consists of grinding the raw materials, mixing
them intimately in certain proportions depending upon their purity and composition and
burning them in a kiln at a temperature of about 1300 to 1500°C, at which temperature, the
material sinters and partially fuses to form nodular shaped clinker. The clinker is cooled and
ground to fine powder with addition of about 3 to 5% of gypsum. The product formed by
using this procedure is Portland cement.
There are two processes known as “wet” and “dry” processes depending upon whether
the mixing and grinding of raw materials is done in wet or dry conditions. With a little change
in the above process we have the semi-dry process also where the raw materials are ground
dry and then mixed with about 10-14 per cent of water and further burnt to clinkering
For many years the wet process remained popular because of the possibility of more
accurate control in the mixing of raw materials. The techniques of intimate mixing of raw
materials in powder form was not available then. Later, the dry process gained momentum
with the modern development of the technique of dry mixing of powdered materials using
compressed air. The dry process requires much less fuel as the materials are already in a dry
state, whereas in the wet process the slurry contains about 35 to 50 per cent water. To dry
the slurry we thus require more fuel. In India most of the cement factories used the wet
process. Recently a number of factories have been commissioned to employ the dry process
method. Within next few years most of the cement factories will adopt dry process system.
In the wet process, the limestone brought from the quarries is first crushed to smaller
fragments. Then it is taken to a ball or tube mill where it is mixed with clay or shale as the case
may be and ground to a fine consistency of slurry with the addition of water. The slurry is a
liquid of creamy consistency with water content of about 35 to 50 per cent, wherein particles,
crushed to the fineness of Indian Standard Sieve number 9, are held in suspension. The slurry
is pumped to slurry tanks or basins where it is kept in an agitated condition by means of
rotating arms with chains or blowing compressed air from the bottom to prevent settling of
limestone and clay particles. The composition of the slurry is tested to give the required
chemical composition and corrected periodically in the tube mill and also in the slurry tank
by blending slurry from different storage tanks. Finally, the corrected slurry is stored in the final
storage tanks and kept in a homogeneous condition by the agitation of slurry.
The corrected slurry is sprayed on to the upper end of a rotary kiln against hot heavy
hanging chains. The rotary kiln is an important component of a cement factory. It is a thick
steel cylinder of diameter anything from 3 metres to 8 metres, lined with refractory materials,
mounted on roller bearings and capable of rotating about its own axis at a specified speed.
The length of the rotary kiln may vary anything from 30 metres to 200 metres. The slurry on
being sprayed against a hot surface of flexible chain loses moisture and becomes flakes. These
flakes peel off and fall on the floor. The rotation of the rotary kiln causes the flakes to move
from the upper end towards the lower end of the kiln subjecting itself to higher and higher
temperature. The kiln is fired from the lower end. The fuel is either powered coal, oil or natural
gass. By the time the material rolls down to the lower end of the rotary kiln, the dry material undergoes a series of chemical reactions until finally, in the hottest part of the kiln, where the
temperature is in the order of 1500°C, about 20 to 30 per cent of the materials get fused.
Lime, silica and alumina get recombined. The fused mass turns into nodular form of size 3 mm
to 20 mm known as clinker. The clinker drops into a rotary cooler where it is cooled under
controlled conditions The clinker is stored in silos or bins. The clinker weighs about 1100 to
1300 gms per litre. The litre weight of clinker indicates the quality of clinker.
The cooled clinker is then ground in a ball mill with the addition of 3 to 5 per cent of
gypsum in order to prevent flash-setting of the cement. A ball mill consists of several
compartments charged with progressively smaller hardened steel balls. The particles crushed
to the required fineness are separated by currents of air and taken to storage silos from where
the cement is bagged or filled into barrels for bulk supply to dams or other large work sites.
In the modern process of grinding, the particle size distribution of cement particles are
maintained in such a way as to give desirable grading pattern. Just as the good grading of
aggregates is essential for making good concrete, it is now recognised that good grading
pattern of the cement particles is also important.
