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, clay, 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 to be sold. Cement mixed with water causes a chemical reaction and forms a paste that sets and hardens to bind individual structures of building materials.
Cement is an integral part of the urban infrastructure. 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 lime aggregate. These are both used to bind rocks, stones, bricks and other building units, fill or seal any gaps, and to make decorative patterns. Cement mixed with water silicates and aluminates, making a water repellant hardened mass that is used for water-proofing.
History of Cement
Cement, though different from the refined product found nowadays, 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. 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. But the real father of Portland cement is considered to be Isaac Charles Johnson, who contributed immensely by publishing the process of developing meso-Portland cement in the kiln.
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, was synthesized by Catskill Aqueduct 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 cement, most of them known as Portland cement or blends, including blast furnace cement, Portland fly-ash cement, Portland pozzolan cement, pozzolan-lime cement, slag-lime cement etc.
Cement is chiefly of two kinds based on the way it is set and hardened: hydraulic cement, which hardens due to the addition of water, and non-hydraulic cement, which is hardened by carbonation with the carbon present in the air, so it cannot be used underwater.
Non-hydraulic cement is produced through the following steps (lime cycle):
- Calcination: Lime is produced from limestone at over 825°C for about 10 hours. (CaCO3 → CaO + CO2)
- Slaking: Calcium oxide is mixed with water to make slaked lime. (CaO + H2O → Ca(OH)2)
- Setting: Water is completely evaporated.
- The cement is exposed to dry air and it hardens after time-consuming reactions. (Ca(OH)2 + CO2 → CaCO3 + H2O)
On the other hand, hydraulic cement is mainly made up of silicates and oxides:
- Belite (2CaO·SiO2);
- Alite (3CaO·SiO2);
- Tricalcium aluminate/ Celite (3CaO·Al2O3)
- Brownmillerite (4CaO·Al2O3·Fe2O3)
The ingredients are processed in the kiln in cement plants. Complete chemistry of the reactions is still a subject of research.
The most commonly used cement nowadays is a hydraulic cement (i.e. hardens when water is added) known as Portland cement or Portland cement blends. These are usually the basic ingredient in making concrete, which is a construction material used as a load-bearing element. Portland cement is suitable for wet climates and can be used underwater. Different types or blends of Portland cement include Portland blast furnace slag cement, Portland fly-ash cement, Portland pozzolan cement, Portland-silica fume cement, masonry cement, expansive cement, white blended cement, colored cement and very finely ground cement.
Composition of Portland cement
85% Portland cement clinker (37-72% of 3CaO.SiO2; 6-47% 2CaO.SiO2; 2-20% 2CaO.Al2O3; 2-19% 4CaO. Al2O3.Fe2O3), 1.5-3.5% gypsum by SO3 content, up to 15% admixtures. For more information read: 8 Main Cement Ingredients & Their Functions
How to make Portland cement
In a cement production plant, limestone and other raw materials such as silicate, bauxite, iron ore etc. are heated so that molecules of carbon dioxide are liberated from the limestone to form quicklime, which combines with the other ingredients, resulting in the formation of calcium silicates and other products. Thus clinker, a rock-hard substance, is made. Gypsum is added to the clinker and then ground into a fine powder, which is the final product known as Portland cement.
Cement Manufacturing Industries in the World
The top three cement producers of 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 LafargeHolcim, Anhui Conch, China National Building Materials, HeidelbergCement, 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 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 plant causes the emission of carbon dioxide, which is one of 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 check the effects of global warming.