Cement is one of the most important building materials. Cement is a binder used to bond building blocks, bricks, concrete slabs, etc.
Cement is a finely dispersed powdery substance mainly composed of limestone, sand and clay, bauxite and iron ore. Also, Portland cement may contain some inclusions: shells of molluscs, chalk, marl, clay, blast furnace slag, shale. Raw ingredients are processed in a cement plant, where the raw materials are heated and formed into a hard stone called clinker, which is then ground into a fine powder for sale. Cement, mixed with water, causes a chemical reaction and forms a solution that hardens and hardens to bond the individual structures of building materials.
Cement is an integral part of urban infrastructure. Concrete is made up of cement, water, sand and gravel mixed in specific proportions, while mortar is made up of cement, water and lime aggregate. Both are used to bond natural stone, bricks and other building blocks, fill or seal any gaps, and make decorative items. Cement is often mixed with hydrous silicates and aluminates to form a water-repellent, hardened mass that is used to waterproof swimming pools and canals.
History of cement
Cement, although different from the refined product currently produced, has been used in many forms since the dawn of human civilization. From volcanic ash, crushed pottery, calcined gypsum and hydrated lime to the first hydraulic cement used by the Romans in the Middle Ages, the development of cement continued until the 18th century, when the first Roman cement was patented, which gained popularity but was replaced by Portland cement in the 1850s. years.
In the 19th century, the Frenchman Louis Vicat laid the foundations for the chemical composition of Portland cement, and in Russia Yegor Cheliev published the methods of making cement, methods of using cement and its advantages. Joseph Aspdin brought Portland cement to the English market, and his son William Aspdin developed the “modern” Portland cement, which soon proved to be in demand. But the real father of Portland cement is considered Isaac Charles Johnson, who made a huge contribution by publishing the process of developing Mesoportland cement in a kiln.
In the 19th century, Rosendale cement appeared in New York. Although the stiffness of this cement made it quite popular, market demand soon declined due to the long cure time and Portland cement became a favorite again. However, Rosendale’s new blend, which is both very strong and requires less curing time, has been patented and is now often used for bridge and highway construction.
The cement used today has undergone experiments, trials and significant improvements to meet the needs of the modern world. Today’s cement is the basis for durable concrete for roads and highways, hydraulic solutions that withstand sea water, and renders for humid climates. Various types of modern cement, most of which are known as Portland cement, including blast furnace cement, ash Portland cement, Portland pozzolan cement, pozzolan lime cement, slag lime cement, and others also find their application in civil engineering.
Chemistry of cement
There are basically two types of cement based on how it hardens: hydraulic cement, which hardens due to the addition of water, and non-hydraulic cement, which hardens by carbonation with the carbon present in the air.
Non-hydraulic cement is produced in the following stages (lime cycle):
Calcination: lime is obtained from limestone at temperatures above 825 ° C for about 10 hours. (CaCO3 CaO + CO2)
Slaking: Calcium oxide is mixed with water to make slaked lime. (CaO + H 2 O Ca (OH) 2), while the water completely evaporates.
The cement is exposed to dry air and hardens after prolonged reactions. (Ca (OH) 2 + CO 2 CaCO 3 + H 2 O)
On the other hand, hydraulic cement is mainly composed of silicates and oxides:
Wollastonite (2CaO • SiO 2);
Alite (3CaO • SiO 2);
Tricalcium aluminate / celite (3CaO • Al 2 O 3)
Brownmillerite (4CaO * Al 2 O3 * Fe 2 O 3) or tetra calcium alumoferrite
The ingredients are processed in kilns in cement plants. The complete chemistry of these reactions is still the subject of research.
The most commonly used cement today is hydraulic cement (i.e., hardened by the addition of water) known as Portland cement or Portland cement mixture. This type of cement is usually the main ingredient in the manufacture of concrete, which is a building material used in its hardening as a supporting element of a building structure. Portland cement is suitable for humid climates and can be used underwater. Various types or mixtures of Portland cement include Portland cement made from blast furnace slag, Portland cement made from ash, Portland pozzolan cement, Portland silica cement, Expansive cement, white mixed cement, pigmented cement.
Portland cement composition
85% Portland cement clinker (37-72% of 3CaO.SiO 2; 6-47% 2CaO.SiO2; 2-20% 2CaO.Al2O3; 2-19% 4CaO. Al 2 O 3 .Fe 2 O 3), 1, 5-3.5% gypsum by SO 3 content, up to 15% impurities.
How to make Portland cement?
In a cement plant, limestone and other raw materials such as silicates, bauxite, iron ore, etc. are heated so that carbon dioxide molecules are released from the limestone to form quicklime, which combines with other ingredients to form calcium silicates and other products. Thus, a clinker, a hard stone, is obtained. Gypsum is added to the clinker and then ground into a fine powder, which is the final product known as Portland cement.
Cement industry in the world
The three leading cement producers in the world are the United States, China and India. Among these countries, only China produces about 45% of the total world cement production.
Global cement consumption continues to grow as it is a non-recyclable product and therefore every new construction or renovation needs new cement. Especially in the economies of Asia and Eastern Europe, cement production is an important element of progress.
There are about 2,273 cement plants in operation 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 world consumption of cement, as shown by some statistics, is up to 23 million metric tons.
Among the developed capitalist countries, the leading cement producers are the USA, France, Italy and Germany. Iran, currently the leading producer in the Middle East, is the third largest cement producer in the world. Asian and African countries are also making progress in cement production.
The baking process in a cement plant causes the release of carbon dioxide, which is one of the main greenhouse gases responsible for global warming. In order to reduce and even eliminate the harmful environmental consequences of using cement, leading industries are currently trying to introduce innovative technologies that allow the use of recycled materials and renewable energy sources. Green Cement is a completely new sustainable building material that is the result of extensive research to address the effects of global warming.
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