Cement is a binding substance typically used in construction that sets, hardens, and adheres to other materials to bond them together. The material is characterized as hydraulic or non-hydraulic, depending on the ability of the cement to set in the presence of water. Non-hydraulic cement needs dry conditions to set and strengthen, while hydraulic cement, also known as Portland cement, hardens while in contact with water.
Portland cement is the most common type of cement used. It is also the basic ingredient of concrete, which is formed when portland cement creates a paste with water that hardens after binding with sand and rock.
Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients. Raw materials commonly used to manufacture cement include limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore. When heated at high temperatures, these ingredients form a rock-like substance that is ground into the fine powder, otherwise known as cement.
The most common way to manufacture portland cement is through a dry method. The first step is to quarry the principal raw materials. After quarrying, the rock is crushed in several stages to reduce its size. The crushed rock is then combined with other ingredients, such as iron ore or fly ash, and ground, mixed, and fed to a cement kiln.
The cement kiln heats all the ingredients about 2,700 degrees Fahrenheit, known as clinkering temperate, in large cylindrical rotating kilns lined with special firebrick. The heating that occurs in the kiln transforms the raw materials into a new substance called clinker, which comes out as hot grey balls, about the size of marbles. After the clinker is cooled, cement plants grind it and mix it with small amounts of gypsum and limestone, and it is ready for use.
Although the dry process is the most modern and popular way to manufacture cement, some kilns use a wet process. The two processes are essentially alike except in the wet process, the raw materials are ground with water before being fed into the kiln. The cement produced is differentiated by the distribution of the raw materials in its chemical composition and the finishing products, as well as the temperature and duration of heating.
Because cement is one of the world’s most-used resources, the material has an extensive range of standards and specifications to ensure a level of consistency between cement-producing plants. In the United States, portland cements and blended hydraulic cements for concrete conform to the American Society for Testing and Materials (ASTM) C150 (Standard Specification for Portland Cement), C595 (Standard Specification for Blended Hydraulic Cement), or C1157 (Performance Specification for Hydraulic Cements).
This wide range of cement and concrete standards allow laboratories all over the world to test and evaluate concrete mixtures to ensure their strength and safety, and also help to identify the various properties of concrete including strength, elasticity, hardness, and workability. The process for manufacturing cement is relatively similar across the world, however, the reference to cement specifications can be different depending on the jurisdiction. In addition, test methods can vary as well. When ordering concrete for construction projects, it is advisable to work with a local concrete producer to verify that the proper cement is used and meets the appropriate specifications.