What Is Cement Clinker? How Does It Help Make Concrete?

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Cement clinker is a semi-finished product which is obtained by sintering limestone, clay and other additives in the kiln at a temperature of about 1400 °C. Unfinished cement clinker is a gray nodular mass of varying composition.

What Is Cement Clinker? How Does It Help Make Concrete?

Cement clinker is a solid material produced in the manufacture of Portland cement as an intermediary product. Clinker occurs as lumps or nodules, usually 3 millimetres to 25 millimetres in diameter. It is produced by sintering (fusing together without melting to the point of liquefaction) limestone and aluminosilicate materials such as clay during the cement kiln stage.

Cement Clinker Composition:

Clinker is made by heating limestone (calcium carbonate) with small quantities of other materials (such as clay) to 1450°C in a kiln, in a process known as calcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, or quicklime, which then chemically combines with the other materials in the mixture to form calcium silicates and other cementitious compounds. The resulting hard substance, called ‘clinker’, is then ground with a small amount of gypsum into a powder to make ‘ordinary Portland cement’, the most commonly used type of cement (often referred to as OPC). Portland cement is a basic ingredient of concrete, mortar and most non-speciality grout. The most common use for Portland cement is in the production of concrete.

Cement clinker is a by-product of the cement manufacturing process. It is primarily made up of four compounds: iron oxides, aluminium oxide, silicon dioxide and calcium oxide. The chemical formula of cement clinker is CaO Al2O3 SiO2.

The composition of cement clinker is important both for the production of Portland cement and for the evaluation of cement raw materials and clinker products for compliance with specifications such as EN 197-1.

Cement clinker forms when raw materials such as limestone, clay and sand are heated at high temperatures in a rotating kiln to form a grey, hard pellet-like material called clinker. Clinker needs to be ground down into powder to turn it into useable cement powder.

Clinker is made up of four main mineral compounds:

– Tricalcium silicate (3CaO SiO2) – also known as alite

– Dicalcium silicate (2CaO SiO2) – also known as belite

– Tricalcium aluminate (3CaO Al2O3) – also known as brownmillerite

– Tetracalcium aluminoferrite (4CaO Al2Fe2O3) – also known as

Cement clinker is the semi-finished material, in granular form, resulting from the fusion of limestone, clay, bauxite and iron in a rotating kiln at a temperature of about 1400 to 1500 degrees Celsius. When the clinkering process is complete, the material is cooled by air and/or water.

Cement clinker is produced by heating to high temperature a mixture of substances such as limestone and shale. When cement clinker is ground with a specified amount of gypsum, it will produce Portland cement or when ground with specified amounts of gypsum and other pozzolanic materials it will produce blended cements.

The raw material for this purpose are mineral materials containing calcium oxide, silica, alumina and iron oxide. These components are extracted from limestone, chalk, shale or clay. These raw materials are quarried, crushed finely and stored. They are proportioned to produce a mixture with the desired chemical composition and fed into a rotating ball mill together with some added material such as iron scrap or sand. The rotation of the mill causes the charge to spread evenly over its inside surface; after grinding has reached a certain point it is removed through opening at the hollow axis. The grinding takes place around a horizontal axis partly filled with

Cement clinker is made by heating, in a cement kiln, a mixture of raw materials to a calcining temperature of above 600 °C (1,112 °F) and then a fusion temperature, which is about 1,450 °C (2,640 °F) for modern cements. The aluminium oxide and iron oxide are present as a flux and contribute little to the strength. For special cements, such as low heat (LH) and sulfate resistant (SR) types, it is necessary for fluxes to be added in the right amounts to produce the correct chemical reactions during firing.

For an ordinary Portland cement clinker made without additions such as slag or fly ash the major phases are tricalcium silicate (3CaO·SiO2), dicalcium silicate (2CaO·SiO2), tricalcium aluminate (3CaO·Al2O3), tetracalcium aluminoferrite (4CaO·Al2O3·Fe2O3), and gypsum. These components are often generated in situ by heating various clays and limestone.**

The word clinker is derived from the Old English word ‘clinc’ which means a hard stone used in building. Clinker is mainly made up of four mineral compounds that are – Tricalcium Silicate (C3S), Dicalcium Silicate (C2S), Tricalcium Aluminate (C3A), and Tetracalcium Aluminoferrite (C4AF).

These compounds when heated at about 1350 degree Celsius in a rotary kiln, form clinkers. The clinkers then cooled down by cold air from the cooler and mixed with gypsum to form cement powder.

Clinkers are also known as Portland Cement Clinker. The reason for this name is that Portland cement was first produced in the year 1824 by Joseph Aspdin, and he named it so because its texture resembled Portland stone.

The raw feed enters the kiln at the cool end and gradually passes down to the hot end where it is discharged as clinker. The solid clinker that falls to the bottom of the kiln is intermittently collected and conveyed to storage.

The size of a modern rotary kiln may be up to 200 feet in length and 30 feet in diameter. The inside of a rotary kiln is lined with refractory bricks. In addition, there are lifters attached to the rotary kiln’s shell which help to lift and tumble the contents as they move through the rotary kiln.

The fire inside a rotary kiln produces temperatures above 2,600 degrees Fahrenheit (1,430 degrees Celsius). These high temperatures cause chemical reactions between the incoming materials. At first, some water evaporates out of the wet raw materials. As a result, clinkers form in small round pellets after all of the water has evaporated. Finally, these small pellets fall to the bottom of the rotary kiln. However, before they do so, they get very hot for several minutes at around 2600 degrees Fahrenheit (1430 degrees Celsius).

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