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Monday, 29 August 2016

how cement is made in Ghana

How Cement is Made
Portland
cement
is the
basic
ingredient of concrete. Concrete is formed when portland
cement creates a paste with water that binds with sand
and rock to harden.
Cement is manufactured through a closely controlled
chemical combination of calcium, silicon, aluminum, iron
and other ingredients.
See an overview of the cement manufacturing process in
this 7-minute animation.
Common materials used to manufacture cement include
limestone, shells, and chalk or marl combined with
shale, clay, slate, blast furnace slag, silica sand, and
iron ore. These ingredients, when heated at high
temperatures form a rock-like substance that is ground
into the fine powder that we commonly think of as
cement.
Bricklayer Joseph Aspdin of Leeds, England first made
portland cement early in the 19th century by burning
powdered limestone and clay in his kitchen stove. With
this crude method, he laid the foundation for an industry
that annually processes literally mountains of limestone,
clay, cement rock, and other materials into a powder so
fine it will pass through a sieve capable of holding
water.
Cement plant laboratories check each step in the
manufacture of portland cement by frequent chemical
and physical tests. The labs also analyze and test the
finished product to ensure that it complies with all
industry specifications.
The most common way to manufacture portland cement
is through a dry method. The first step is to quarry the
principal raw materials, mainly limestone, clay, and
other materials. After quarrying the rock is crushed. This
involves several stages. The first crushing reduces the
rock to a maximum size of about 6 inches. The rock then
goes to secondary crushers or hammer mills for
reduction to about 3 inches or smaller.
The crushed rock is combined with other ingredients
such as iron ore or fly ash and ground, mixed, and fed
to a cement kiln.
The cement kiln heats all the ingredients to about 2,700
degrees Fahrenheit in huge cylindrical steel rotary kilns
lined with special firebrick. Kilns are frequently as much
as 12 feet in diameter—large enough to accommodate an
automobile and longer in many instances than the
height of a 40-story building. The large kilns are
mounted with the axis inclined slightly from the
horizontal.
The finely ground raw material or the slurry is fed into
the higher end. At the lower end is a roaring blast of
flame, produced by precisely controlled burning of
powdered coal, oil, alternative fuels, or gas under forced
draft.
As the material moves through the kiln, certain elements
are driven off in the form of gases. The remaining
elements unite to form a new substance called clinker.
Clinker comes out of the kiln as grey balls, about the
size of marbles.
Clinker is discharged red-hot from the lower end of the
kiln and generally is brought down to handling
temperature in various types of coolers. The heated air
from the coolers is returned to the kilns, a process that
saves fuel and increases burning efficiency.
After the clinker is cooled, cement plants grind it and
mix it with small amounts of gypsum and limestone.
Cement is so fine that 1 pound of cement contains 150
billion grains. The cement is now ready for transport to
ready-mix concrete companies to be used in a variety of
construction projects.
Although the dry process is the most modern and
popular way to manufacture cement, some kilns in the
United States use a wet process. The two processes are
essentially alike except in the wet process, the raw
materials are ground with water before being fed into
the kiln.

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