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. By this crude method he laid
the foundation for an industry which 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 is so fine that one pound of cement contains 150 billion
cement, the basic ingredient of concrete,
is a closely controlled chemical combination of calcium, silicon,
aluminum, iron and small amounts of other ingredients to which gypsum
is added in the final grinding process to regulate the setting time
of the concrete. Lime and silica make up about 85% of the mass.
Common among the materials used in its manufacture are limestone,
shells, and chalk or marl combined with shale, clay, slate or blast
furnace slag, silica sand, and iron ore.
Each step in manufacture of portland cement
is checked by frequent chemical and physical tests in plant laboratories.
The finished product is also analyzed and tested to ensure that
it complies with all specifications.
Two different processes, "dry" and "wet," are used in the
manufacture of portland cement.
rock is the principal raw material, the first step after quarrying
in both processes is the primary crushing. Mountains of rock are
fed through crushers capable of handling pieces as large as an oil
drum. 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.
In the wet process, the raw materials, properly proportioned,
are then ground with water, thoroughly mixed and fed into the kiln
in the form of a "slurry" (containing enough water to make
it fluid). In the dry process, raw materials are ground, mixed,
and fed to the kiln in a dry state. In other respects, the two processes
are essentially alike.
raw material is heated to about 2,700 degrees F 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. 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 or gas under forced draft.
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 with new physical and chemical characteristics. The
new substance, called clinker, is formed in pieces about
the size of marbles.
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.
Virtual Tour of
on Cement Basics.