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 and silicon and aluminium and iron and other ingredients. Many other commonly available natural materials are also used to manufacture cement. Limestones and clay are some of those materials. These ingredients are heated at high temperatures. On heating they form a rock like substance that is grounded into the fine powder that we commonly think of as cement. Bricklayer Joseph Aspdin first made portland cement early in the nineteenth century by burning powdered limestone and clay in his kitchen stove. He laid the foundation for an industry with his crude method. Cement plant laboratories check each step in the manufacture of portland cement by frequent chemical and physical tests. The labs also analyse 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 and clay and other materials are used. After quarrying the rock is crushed. This involves several stages. The first crushing reduces the rock to a maximum size of about six inches. The rock then goes to secondary crushers or hammer mills for reduction to about three inches or smaller. The crushed rock is combined with other ingredients such as iron ore or fly ash. It is then grounded and mixed and fed to a cement kiln. The cement kiln heats all the ingredients to about twenty seven hundred degrees fahrenheit in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as Twelve feet in diameter. They are large enough to accommodate an automobile and longer in many instances than the height of a forty 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. This flame is produced by precisely controlled burning of powdered coal or oil under forced draft. Sometimes alternative fuels or gas is also used to produce this flame. The material moves through the kiln from top to bottom. During this movement 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. The size of these clinkers is 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. This process that saves fuel and increases burning efficiency. The cooled clinker is then grinded by the cement plant. It is then mixed with small amounts of gypsum and limestone. Cement is so fine that one pound of cement contains around one hundred and fifty billion grains. The cement is now ready for transport to ready mix concrete companies. The modern dry process is the most popular and most common way to manufacture cement.