Cement

Cement is an integral part of the urban infrastructure. Cement, one of the most important building materials, is a binding agent that sets and hardens to adhere to building units such as stones, bricks, tiles, etc. Cement generally refers to a very fine powdery substance chiefly made up of limestone (calcium), sand or clay (silicon), bauxite (aluminum), and iron ore, and may include shells, chalk, marl, shale, blast furnace slag, slate. The raw ingredients are processed in cement manufacturing plants and heated to form a rock-hard substance, which is then ground into a fine powder. Cement mixed with water causes a chemical reaction and forms a paste that sets and hardens to bind individual structures of building materials. Cement hardens both in the air and under water and remains in its hardened state once reached.

It is used to make concrete as well as mortar, and to secure the infrastructure by binding the building blocks. Concrete is made of cement, water, sand, and gravel mixed in definite proportions, whereas mortar consists of cement, water, and fine aggregate. These are both used to bind rocks, stones, bricks, and other building units, fill or seal any gaps, and make decorative patterns. Cement mixed with water silicates and aluminates makes a water-repellant hardened mass that is used for water-proofing.

Cements are classified according to their early and final strength as well as their composition. Depending on the desired application, different types of cement – each with a specific composition are necessary. Cement characteristics can also be modified through the use of additives.

History of Cement

Cement has been used in many forms since the advent of human civilization. From volcanic ashes, crushed pottery, burnt gypsum and hydrated lime to the first hydraulic cement used by the Romans in the middle ages, the development of cement continued to the 18th century, when James Parker patented Roman cement, which gained popularity but was replaced by Portland cement in the 1850s.

In the 19th century, Frenchman Louis Vicat laid the foundation for the chemical composition of Portland cement and in Russia, Egor Cheliev published the methods of making cement, uses of cement and advantages. Bricklayer Joseph Aspdin of Leeds, England first made portland cement early in the 19th century by burning powdered limestone and clay in his kitchen stove. With this crude method, he laid the foundation for an industry that annually processes literally mountains of limestone, clay, cement rock, and other materials into a powder so fine it will pass through a sieve capable of holding water. Joseph Aspdin brought Portland cement to the market in England and his son, William Aspdin, developed the modern Portland cement, which was soon in quite high demand.

In the 19th century, Rosendale cement was discovered in New York. Though its rigidity made it quite popular at first, the market demand soon declined because of its long curing time and Portland cement was again the favorite. However, a new blend of Rosendale-Portland cement, which is both highly durable and needs less curing time and is now often used for highway or bridge construction.

The cement used today has undergone experimentation, testing and significant improvements to meet the needs of the present world, such as developing strong concretes for roads and highways, hydraulic mortars that endure sea water, and stucco for wet climates. Different kinds of modern cements are Ordinary Portland cement (OPC), Portland Slag cement, Portland Pozzolana cement, White cement, Sulphate resisting cement, Low heat Portland cement, Rapid hardening cement, Quick setting cement, Blast Furnace Slag cement, High Alumina cement, Coloured cement, Air Entraining cement, Expansive cement, Hydrographic cement etc.

Cement Manufacturing Industries in the World

The top three cement producers in the world as recorded in 2010 are the USA, China and India. Among these countries, China alone manufactures about 45% of the total worldwide production of cement. Global consumption of cement continues to rise since it is a non-recyclable product and so every new construction or repair needs new cement. Especially in the economies of Asia and Eastern Europe, cement production is an important element of progress.

According to the global cement directory, there are about 2273 active cement production plants in the world. Some of the leading cement manufacturers are Lafarge Holcim, Anhui Conch, China National Building Materials, Heidelberg Cement, Cemex, Italcementi, China Resources Cement, Taiwan Cement, Eurocement and Votorantim. The total global consumption of cement, as indicated by statistics in 2015, measures up to 18 million metric tons, most of which is attributed to the rising national economy of North America.

Among the developed capitalist countries, the leading producers of cement are the USA, France, Italy and Germany. Iran, now the top producer in the Middle East, occupies the third position in the world for cement manufacture. Asian and African countries are also progressive in the production of cement.

The kiln process in cement plants causes the emission of carbon dioxide, which is one of the major greenhouse gases responsible for global warming. With a view to reducing, even eliminating, the harmful environmental impacts of cement usage, leading industries are now trying to implement technologies that utilize recycled materials and renewable energy sources. “Green cement” is such a sustainable construction material that is the result of extensive research related to the adverse effects of global warming.

Manufacturing of cement

Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients. Common materials used to manufacture cement include limestone, shells and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand and iron ore. These ingredients, when heated at high temperatures form a rock-like substance that is ground into the fine powder, i.e. cement.

The most common way to manufacture cement is through a dry method. Although the dry process is the most modern and popular way to manufacture cement, some kilns in the United States 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.

In dry process, the first step is to quarry the principal raw materials, mainly limestone, clay, and other materials. After quarrying the rock is crushed. This involves several stages. The first crushing reduces the rock to a maximum size of about 6 inches. The rock then goes to secondary crushers or hammer mills for reduction to about 3 inches or smaller.

The crushed rock is 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 to about 2,700 degrees Fahrenheit in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as 12 feet in diameter and longer in many instances than the height of a 40-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, produced by precisely controlled burning of powdered coal, oil, alternative fuels 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 called clinker. Clinker comes out of the kiln as grey balls, 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. After the clinker is cooled, cement plants grind it and mix it with small amounts of gypsum and limestone. The cement is now ready for transport to various construction sites.

Cement plant laboratories check each step in the manufacture of cement by frequent chemical and physical tests. The labs also analyze and test the finished product to ensure that it complies with all industry specifications.