Cement Made Simple
The production of portland cement begins with a precise blend of raw materials from among the most abundant resources in the world.
The most common combination used in making cement is limestone, clay and sand. These materials are ground together and then heated in a rotating furnace called a kiln where temperatures reach 1500 ºC (2 732 °F).
This intense heat causes chemical reactions that convert the partially molten raw materials into pellets called clinker. After adding some gypsum and other key materials, the mixture is ground to an extremely fine grey powder called "Portland cement ".
The Manufacturing Process
Two different processes, "dry" and "wet," are used in the manufacture of portland cement.
When 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 15 cm (6 in.). The rock then goes to secondary crushers or hammer mills for reduction to about 7.5 cm (3 in.) 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.
The raw material is heated to more than 1,450 °C (2,700 °F) in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as 3.7 M (12 ft.) in diameter, large enough to accommodate an automobile and often longer 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.
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 with new physical and chemical characteristics. The new substance, called clinker, is formed in pieces 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.
Portland cement, the basic ingredient of concrete, is a closely controlled chemical combination of calcium, silicon, aluminum, iron and small amounts of other ingredients. 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.
The exacting nature of portland cement manufacture requires some 80 separate and continuous operations, the use of a great deal of heavy machinery and equipment, and large amounts of heat and energy. The capital investment per worker in the cement industry is among the highest in all industries.
Each step in the manufacturing process 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.
Sustainable Cement Making Process Flash Animation
Watch the cement making process from the quarry to the shipment of the final product in an interactive Flash animated tour .