What Is Cement Made From?
Cement is made from a mixture of lime, clay, silica, iron and aluminum minerals. Limestone is the most important source for the calcium, while clay and sand are the main sources for the other elements.
Chalk or marl is sometimes used in place of limestone, and shale or slate in place of clay. Blast furnace slags, which contain both calcium and silica, are also used.
Blocks of limestone from a quarry are first fed into giant crushers, from which emerge pieces no larger than 6 inches (15 centimeters) in diameter. Then smaller crushers or hammer mills reduce the stones still more.
Further preparation is done in either of two ways: the wet process or the dry process. In the wet process, water is added to the limestone and other raw materials and they are ground and mixed in proper proportions in the form of a thick fluid called slurry.
The slurry is then fed into a large oven called a kiln, where cement is made. Heat from the kiln changes the materials into marble-sized lumps called “clinker.” The clinker is mixed with a small amount of gypsum (which slows the hardening of cement) and then grounded into a fine cement powder.
The dry process is considered to be more energy-efficient and is gradually becoming more prevalent. In it the materials are blended, ground, and fed to the kiln’s preheater tower in a dry state.
To make concrete, the cement is mixed with crushed rock, sand and water. When the mixture hardens, the materials are bound together in a solid, rock-like mass good for constructing such long-lasting things as concrete sidewalks, highways, bridges, and skyscrapers.