Cement (5)

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In the most general sense of the word, a cement is a binder, a substance that sets andhardens independently, and can bind other materials together. The word "cement" traces tothe Romans, who used the term opus caementicium to describe masonry resembling modern

concretethat was made from crushed rock with burnt lime as binder. The volcanic ash and

pulverized brick additives that were added to the burnt lime to obtain a hydraulic binder werelater referred to as cementum, cimentum, cment and cement.Cement used in construction is characterized as hydraulicornon-hydraulic. Hydraulic cements

(e.g.,Portland cement) harden because of hydration, chemical reactions that occurindependently of the mixture's water content; they can harden even underwater or when

constantly exposed to wet weather. The chemical reaction that results when the anhydrouscement powder is mixed with water produces hydrates that are not water-soluble. Non-

hydraulic cements (e.g.gypsumplaster) must be kept dry in order to retain their strength.The most important use of cement is the production of mortarand concretethe bonding ofnatural or artificial aggregates to form a strong building material that is durable in the face of

normal environmental effects.Concrete should not be confused with cement, because the term cement refers to the

material used to bind the aggregate materials of concrete. Concrete is a combination of acement and aggregate.

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Types of cements

# Ordinary Portland Cement # Rapid Hardening Cement

# Extra Rapid Hardening Cement # Sulphate Resisting Cement

# Quick Setting Cement # Super Sulphated Cement

# Low Heat Cement

# Portland Pozzolana Cement # Air-Entraining Cement # Coloured Cement

# Hydrophobic cement # Masonry Cement

# Expansive Cement # IRS-T 40 Special Grade Cement

# Oil-Well Cement # Rediset Cement

# High Alumina Cement # High Early Strength CementVarieties of Cement in India

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There are some varieties in cement that always find good demand in the market. To knowtheir characteristics and in which area they are most required, it will be better to take a lookat some of the details given below.

* Portland Blast Furnace slag cement (PBFSC): The rate of hydration heat is found lowerin this cement type in comparison to PPC. It is most useful in massive constructionprojects, for example - dams.

* Sulphate Resisting Portland Cement: This cement is beneficial in the areas whereconcrete has an exposure to seacoast or sea water or soil or ground water. Under any suchinstances, the concrete is vulnerable to sulphates attack in large amounts and can causedamage to the structure. Hence, by using this cement one can reduce the impact of damageto the structure. This cement has high demand in India.

* Rapid Hardening Portland Cement: The texture of this cement type is quite similar tothat of OPC. But, it is bit more fine than OPC and possesses immense compressiblestrength, which makes casting work easy.

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* Ordinary Portland Cement (OPC): Also referred to as grey cement or OPC, it is of much

use in ordinary concrete construction. In the production of this type of cement in India,Iron (Fe2O3), Magnesium (MgO), Silica (SiO2), Alumina (AL2O3), and Sulphur trioxide(SO3) components are used.

* Portland Pozolona Cement (PPC): As it prevents cracks, it is useful in the casting workof huge volumes of concrete. The rate of hydration heat is lower in this cement type. Fly

ash, coal waste or burnt clay is used in the production of this category of cement. It can beavailed at low cost in comparison to OPC.

* Oil Well Cement: Made of iron, coke, limestone and iron scrap, Oil Well Cement isused in constructing or fixing oil wells. This is applied on both the off-shore and on-shoreof the wells.

* Clinker Cement: Produced at the temperature of about 1400 to1450 degree Celsius,clinker cement is needed in the construction work of complexes, houses and bridges. Theingredients for this cement comprise iron, quartz, clay, limestone and bauxite.

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* White cement: It is a kind of Ordinary PortlandCement. The ingredients of this cement are inclusiveof clinker, fuel oil and iron oxide. The content of iron

oxide is maintained below 0.4% to secure whiteness.White cement is largely used to increase the aestheticvalue of a construction. It is preferred for tiles andflooring works. This cement costs more than greycement.

Apart from these, some of the other types of cementthat are available in India can be classified as:

* Low heat cement* High early strength cement* Hydrophobic cement* High aluminium cement

* Masonry cement Ordinary cement

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It is uncertain where it was first discovered that a combination of hydrated non-hydraulic lime and a pozzolan produces a hydraulic mixture (see also: Pozzolanicreaction), but concrete made from such mixtures was first used by the Ancient

Macedonians[1][2] and three centuries later on a large scale by Roman engineers.[3]They used both natural pozzolans (trass or pumice) and artificial pozzolans(ground brick or pottery) in these concretes. Many excellent examples of structuresmade from these concretes are still standing, notably the huge monolithic dome ofthe Pantheon in Rome and the massive Baths of Caracalla.[4] The vast system ofRoman aqueducts also made extensive use of hydraulic cement.[5]

Although any preservation of this knowledge in literary sources from the MiddleAges is unknown, medieval masons and some military engineers maintained anactive tradition of using hydraulic cement in structures such as canals, fortresses,harbors, and shipbuilding facilities.[6][7] The technical knowledge of makinghydraulic cement was later formalized by French and British engineers in the 18thcentury.[6]Types of modern cement

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Cement is made by heating limestone (calcium carbonate) with

small quantities of other materials (such as clay) to 1450 C in akiln, in a process known as calcination, whereby a molecule ofcarbon dioxide is liberated from the calcium carbonate to formcalcium oxide, or quicklime, which is then blended with the othermaterials that have been included in the mix. The resulting hardsubstance, called 'clinker', is then ground with a small amount ofgypsum into a powder to make 'Ordinary Portland Cement', themost commonly used type of cement (often referred to as OPC).

Portland cement is a basic ingredient of concrete, mortar and mostnon-specialty grout. The most common use for Portland cement is

in the production of concrete. Concrete is a composite materialconsisting of aggregate (gravel and sand), cement, and water. As aconstruction material, concrete can be cast in almost any shapedesired, and once hardened, can become a structural (load bearing)element. Portland cement may be grey or white.

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You may often wonder why concrete behaves the way it does: why

does it give off heat when setting, why does concrete gain strengthdifferently with different cements, why does it shrink, why aresome concretes more resistant to deterioration? Most of theproperties of fresh and hardened concrete are affected to some

extent by the cement properties. Therefore, an understanding ofcement characteristics can provide insight to the many material-related questions that arise in concrete construction.

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In the manufacture of cement, limestone, sand, clay, and iron ore areblended, ground, and heated to 1400C

1550C (2550F - 2825F) in a

rotating kiln. The resulting material, called clinker, is cooled, pulverized,and mixed with gypsum to create what is known as portland cement, a

hydraulic material primarily made up of calcium silicates.Cements differ from plant to plant due to changes in raw material

properties, kiln temperatures, and fineness upon grinding. These changescan significantly affect concrete properties when different cements are

used in concrete.Besides the main constituents used in the manufacture of portlandcement, minor components are naturally present. Important minorcomponents include the alkalies, sodium and potassium. Although

present only in small amounts, the alkalies can affect the setting time,strength development, reactivity with fly ash or slag, and the durability ofconcrete. Other minor components oxides other than the main oxidescomprising the four clinker phases C3S, C2S, C3A, and C4AF are derived

from raw materials, fuel, refractory material and wear parts frommanufacture equipment, and generally do not affect the portland cement.