Cement is a greenish-grey-coloured powder that is made of calcined mixtures of clay and limestone. When mixed with water it becomes a hard and strong building material. Cement is one of the most commonly used construction materials. It is used to construct roads, buildings, bridges, dams, retaining walls, etc. India is the second largest producer and consumer of cement. The Indian cement industry is very large and has shown fast-paced growth in the last two decades.
Did you know? The Three Gorges Dam on China’s Yangtze River is the largest concrete dam that took 17 years to build. It is 185 meters high and 2309 meters long.
What is Cement?
Cement is a fine powdery substance that acts as a binding agent in various building materials. Cement is a key component of concrete and mortar used to construct various buildings, bridges, and other infrastructures. Concrete gets its binding properties from cement, which allows it to harden when mixed with water and become durable.
History of Cement
The use of cement dates back thousands of years to ancient civilizations. The Romans first invented non-hydraulic cement by heating limestone, resulting in calcium oxide and CO2. However, since it was soluble in water, it was not suitable for high-moisture environments as it may collapse the structure. Hydraulic cement was more durable and stable in water and, after that, was used in the construction of various historical sites that still exist today.
But since volcanic ash was rare and the setting time took too long, there was a need for better hydraulic cement. About 1800 years later, England and France tried replacing volcanic ash with clay, shale and slate, but the setting time was still high. Finally, in 1824, Joseph Aspdin of England invented and patented Portland cement, which was further developed into the “modern Portland cement” by his son William Aspdin.
Types of Cement
Cement is primarily classified into two categories: hydraulic cement and non-hydraulic cement.
Hydraulic cement hardens when combined with water. It is produced using limestone, clay and gypsum. These raw materials are burned at a very high temperature to produce hydraulic cement. It is water-resistant.
Non-hydraulic cement hardens when combined with carbon dioxide from the air. It is produced using lime, gypsum plaster and oxychloride. It requires a dry environment to harden.
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Composition of Cement
There are eight key ingredients in cement.
To form silicates and aluminates of calcium
Imparts strength to cement
Gives quick setting property to cement
Provides hardness and colour to cement
Helps in the fusion of raw materials
Acts as a retarder and increases the initial setting time of cement
Provides soundness to cement
Used in the form of soda and potash
Various Stages in the Cement Manufacturing Process
Read all the stages mentioned below to understand how cement is made?
Stage-1: Extraction of Raw Materials
Raw materials consist of limestone, sand, clay, shale and fly ash. They are obtained by blasting the quarry with powerful explosives. The limestone rocks are loaded onto haul trucks and sent to the cement plant. Other ingredients, such as iron ore and gypsum, are obtained from mines.
At the plant, these limestone rocks are dumped into the primary crusher, which reduces the size of the rocks to about 6 inches. The rocks are transported to a secondary crusher, where they are crushed to as small as 3 inches.
Rocks high in calcium carbonate and rocks low in calcium carbonate are crushed separately. These two are mixed in the right proportions depending on the type of cement. This mixture is then conveyed to the raw mill, where it is ground to a fine powder called raw mix. The raw mix is then loaded into a roller mill grinding machine. Essential additives such as silica and iron ore are added to it. The ingredients are mixed and ground uniformly, producing a dry rock powder called a raw meal.
The raw meal powder is now dumped into a pre-heater, making it 10 times hotter within a few seconds. The pre-heater is equipped with a flash calciner which removes 95% of the carbon dioxide in the powder so that the lime is isolated. This process binds the minerals together so that they will harden when hydrated with water.
Stage-5: Kiln Phase
Now the powder is conveyed to a rotary kiln, a huge cylindrical furnace. It is set at a specified angle so that when the raw mix reaches the lower part of the kiln, a component called clinker is produced in marble-sized nodules. Clinker is produced from the raw mix through a series of chemical reactions, which can be summarized as follows:
- Evaporation of free water
- Dehydration and the formation of oxides
- CO2 is evolved, and calcium oxide is produced through calcination
- Cement clinker is formed
Stage-6: Cooling and Final Grinding
After exiting the kiln, the clinker is immediately cooled down by a method called air quenching. The clinker now enters the last stage of grinding, where different additives are added to it. Gypsum is added to delay the setting time and give compressive strength to the cement. The clinker mixed with additives is now transferred into rotating cement mills containing metal balls that grind them into a fine powder, cement. Finally, cement is stacked in large silos.
Stage-7: Storage of Cement
Storing cement properly is very critical to maintaining its quality. It should be stored in a moisture-free environment. Cement that has been stored for a long time may react with moisture and become coarse, reducing its strength. This may even render the cement unusable. Thus cement must be kept in an airtight place and tested for moisture content before use. More than 5% moisture renders it useless.
Uses of Cement
Cement is the most popular binding agent in building materials. Following are the various areas in which cement is used.
- It acts as a binding agent in mortar and concrete.
- It is used to build heat-resistant and fireproof buildings due to its high alumina content.
- It is used in the construction of hydrographic structures.
- It is used to build chemical-proof structures.
- It is used in grouting, which involves filling cracks, joints and openings in various structures to improve their strength.
- It also helps construct stronger and more durable roads due to its rigidity.
- Colored cement is used to build aesthetic structures.
Environmental Effects of Cement Manufacturing
Though cement is the most important building material, it is also the third largest industrial source of pollution. The kiln process of cement manufacturing involves the emission of large amounts of carbon dioxide, which leads to global warming. Leading cement manufacturers are involved in extensive research to reduce or eliminate these emissions.
The demand for cement is constantly on the rise due to the enormous growth of the real estate sector. Thus, the future of the cement sector looks bright and highly profitable, provided companies formulate proper strategies for product development. At the same time, environmental safety must also be kept in mind.
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