Portland cement can be made by following two different processes – a dry one and a wet one. The manufacturing process has moved on significantly since bricklayer Joseph Aspdin first made portland cement in his kitchen stove in England in the 19th century.
The basic ingredients of both the dry and wet processes are the same. By mass, lime and silica make up approximately 85% of portland cement. The materials that are commonly used are limestone, shells, chalk, shale, clay, slate, silica sand, and iron ore.
Since limestone is the main component, often cement plants are located near limestone quarries. The first step in both manufacturing processes after quarrying is primary crushing. Crushing reduces the size of the rock to three inches or smaller. Next, the raw materials are combined in the correct amounts and fed into the kiln system. In the dry process, the materials are grounded, mixed, and introduced into the kiln system in a dry state. In the wet process, the raw materials follow all of the steps with water added and are introduced into the kiln system in a slurry state.
In the kiln system, the first process is pre-heating. The combined materials are fed through a series of vertical cyclones. As the material moves through these cyclones, it comes into contact with the hot kiln exhaust gases. The exhaust gases pre-heat the material before it enters the main part of the kiln. The pre-heat process allows the chemical reactions that take place in the kiln to happen quicker and more efficiently.
In the main kiln, the raw materials are then heated to approximately 2,700 degrees F. In the kiln, the initial raw ingredients combine to from clinker. Clinker is mainly made up of tri and di-calcium-silicates which are the main chemicals that bond together when water is added to cement. Unwanted gases, including carbon dioxide, are also emitted from the process. In the next state of the process, clinker is cooled in coolers. The hot air from the coolers is returned to the pre-heater in order to save fuel in the overall process.
The clinker is then ground to produce portland cement. Gypsum is added during the grinding process to control the set rate of the cement. Slag and fly ash can also be added to control other properties of the final product.
Both the dry and wet processes are very energy intensive. The wet process, however, uses more energy than the dry process due to the amount of water that must be evaporated before clinker can be produced. The cement industry is constantly looking for ways to make the manufacturing process more efficient. For example, alternative fuel sources are now being used extensively throughout the industry to heat the kilns to reduce the amount of natural resources used in the process.
