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Portland cement was first made by a British
bricklayer, Joseph Aspdin in 1824. The cement that he made using the
processes of mixing, grinding, burning and regrinding limestone and
clay produced a superior quality of cement. The first portland
cement plant was set up in 1871, by David Saylor.
When the manufacturing of portland cement
first began on a commercial scale, there were ninety-one different
formulas that the manufacturers used. However, the National Bureau
of Standards set up in 1917 and the American Society for Testing
Materials established a standard formula for the manufacture of
cement. (The National Bureau of Standards is now known as the
National Bureau of Standards and Technology.)
What does portland cement contain?
Portland cement contains about sixty per cent of lime, twenty
five per cent silica and five per cent alumina. The other ten per
cent consists of iron oxide and gypsum. It is the gypsum that
regulates the hardening time or setting of the cement. Lime for
making the cement is obtained from materials such as limestone,
oyster shells, chalk and marl, which is a type of clay. The silica
and alumina are obtained from shale, clay, silica sand, slate and
blast furnace slag, while iron oxide comes from iron ore, pyrite and
other such materials.
How is cement produced?
Portland cement is produced using a chemical procedure that involves
three stages.
In the first stage, quarried limestone is
pushed into primary crushers, which crush the huge rocks into
smaller and softer rocks. Hammer mills or secondary crushers then
break the smaller pieces that are about nineteen millimeters wide.
Then the crushed rock and the other ingredients are mixed in the
right proportions and ground in rotating ball mills and tube mills,
wherein the mixture is ground into fine particles. The mixture is
ground wet or dry. This stage is known as the crushing and grinding
stage.
In the second stage, the ground mixture is
fed into cement kilns. Cement kilns are huge cylindrical furnaces
made of steel and lined with firebricks. A cement kiln is the
biggest piece of moving machinery (measuring twenty-five feet in
diameter and seven hundred and fifty feet in length) and it rotates
at a speed of one turn every minute. The fine mixture that is fed at
the top takes about four hours to travel through the kiln. The
source of heat for the kiln is usually oil, gas or powdered coal and
the mixture is burnt at a temperature of more than 1500 degrees
centigrade. The heat changes the mixture to a substance called
clinker. It comes out in marble sized pieces.
The third stage is known as the finishing
grinding stage. Here the marbles that are taken out of the kiln are
cooled by large fans. The clinker is either stored as it is or it is
mixed with gypsum and reground in tube mills or ball mills. The end
product is the fine grey powder that we call portland cement.
America produces seven per cent of the
world’s cement. Other countries that produce cement are China,
France, Italy, Japan, Russia and Germany. |
