2. CEMENT
Cement is a binder, a substance that sets and hardens and can bind other materials together. Cements used in construction can be characterized as being either hydraulic or non-hydraulic, depending upon the ability of the cement to be used in the presence of water.Non-hydraulic cement will not set in wet conditions or underwater, rather it sets as it dries and reacts with carbon dioxide in the air. It can be attacked by some aggressive chemicals after setting.Hydraulic cement is made by replacing some of the cement in a mix with activated aluminium silicates, pozzolanas, such as fly ash. The chemical reaction results in hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This allows setting in wet condition or underwater and further protects the hardened material from chemical attack (e.g., Portland cement).
Use
• Cement mortar for Masonry work, plaster and pointing etc.
• Concrete for laying floors, roofs and constructing lintels,beams,weather-
shed,stairs,pillars etc.
• Construction for important engineering structures such
asbridge,culverts,dams,tunnels,light house,clocks,etc.
• Construction of water,wells, tennis courts,septic tanks, lamp posts, telephone cabins
etc.
• Making joint for joints,pipes,etc.
• Manufacturing of precast pipes,garden seats, artistically designed wens, flower posts,
etc.
• Preparation of foundation, water tight floors, footpaths, etc.
Types of Cements
Many types of cements are available in markets with different compositions and for use in different environmental conditions and specialized applications. A list of some commonly used cement is described in this section:
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Ordinary Portland cement
Ordinary Portland cement is the most common type of cement in general use around the world. This cement 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 is then blended with the other materials that have been included in the mix. The resulting hard substance, called 'clinker', is then ground with a small amount of gypsum into a powder to make 'Ordinary Portland Cement'(often referred to as OPC). Portland cement is a basic ingredient of concrete, mortar and most non-specialty grout. The most common use for Portland cement is in the production of concrete. Concrete is a composite material consisting of aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element. Portland cement may be grey or white.
• This type of cement use in construction when there is no exposure to sulphates in the soil or ground water.
• Lime saturation Factor is limited between i.e. 0.66 to 1.02.
• Free lime-cause the Cement to be unsound.
• Percentage of (AL2O3/Fe2O3) is not less than 0.66.
• Insoluble residue not more than 1.5%.
• Percentage of SO3 limited by 2.5% when C3A < 7% and not more than 3% when C3A >7%.
• Loss of ignition -4%(max)
• Percentage of Mg0-5% (max.)
• Fineness -not less than 2250 cm2/g.
Rapid hardening Portland cement
• It is firmer than Ordinary Portland Cement
• It contains more C3S are less C2S than the ordinary Portland cement.
• Its 3 days strength is same as 7 days strength of ordinary Portland cement.
Low heat Portland cement
• Heat generated in ordinary Portland cement at the end of 3days 80 cal/gm. While in low heat cement it is about 50cal/gm of cement.
• It has low percentage of C3A and relatively more C2S and less C3S than O.P. Cement.
• Reduce and delay the heat of hydration. British standard ( B S. 1370 : 1974 ) limit the heat of hydration of this cement.
Sulphate resisting Portland cement
• Maximum C3A content by 3.5% and minimum fineness by 2500 cm'/g.
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• Firmer than ordinary pot land cement.
• Sulphate forms the sulpha-aluminates which have expensive properties and so causes
disintegration of concrete.
Sulphate resisting Portland cement
• For this cement, the silage as obtained from blast furnace is used
• The clinkers of cement are ground with about 60 to 65 percent of slag.
• Its strength in early days is less and hence it required longer curing period. It proves
to be economical as slag, which is a Waste product, is used in its manufactures.
Pozzolanic cement
• As per Indian standard, the proportions of Pozzolana may be 10 to 25 % by weight. e.2. Burnt clay, shale, Fly ash.
• This Cement has higher resistance to chemical agencies and to sea water because of absence of lime.
• It evolves less heat and initial strength is less but final strength is 28 days onward equal to ordinary Portland cement.
• It possesses less resistance to the erosion and weathering action.
• It imparts higher degree of water tightness and it is cheap.
White Portland cement
• Grey colour of O.P. cement is due to presence of Iron Oxide. Hence in White Cement Fe,,O, is limited to 1 %. Sodium Alumina Ferrite (Crinoline) NavAlF6 is added to act as flux in the absence of Iron-Oxide. •:
• It is quick drying, possesses high strength and has superior aesthetic values and it also cost lee than ordinary Cement because of specific requirements imposed upon the raw materials and the manufacturing process.
• White Cement are used in Swimming pools, for painting garden furniture, moulding sculptures and statues etc.
Coloured Portland
• The Cement of desired colour may be obtained by mixing mineral pigments with ordinary Cement.
• The amount of colouring material may vary from 5 to 10 percent. If this percentage exceeds 10percent, the strength of cements is affected.
• The iron Oxide in different proportions gives brown, red or yellow colour. The coloured Cement are widely used for finishing of floors, window sill slabs, stair treads etc.
Expansive cement
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• This type of cement is produced by adding an expanding medium like sulphoaluminate and a stabilising agent to the ordinary cement.
• The expanding cement is used for the construction of water retaining structures and for repairing the damaged concrete surfaces.
High alumina cement
• This cement is produced by grilling clinkers formed by calcining bauxite and lime. It can stand high temper lures.
• If evolves great heat during setting. It is therefore not affected by frost. Composition of Cement clinker
The various constituents combine in burning and form cement clinker. The compounds formedin the burning process have the properties of setting and hardening in the presence ofwater.They are known as Bogue compounds after the name of Bogue who identified them. These compounds are as follows: Alite (Tricalcium silicate or C3S), Belite (Dicalcium silicate or C2S), Celite (Tricalciumalluminate or C3A) andFelite (Tetracalciumalumino ferrite or C4AF).
Tricalcium silicate
It is supposed to be the best cementing material and is well burnt cement.It is about 25-50% (normally about 40 per cent) of cement. It renders the clinker easier to grind,increases resistance to freezing and thawing, hydrates rapidly generating high heat and developsan early hardness and strength. However, raising of C3S content beyond the specified limitsincreases the heat of hydration and solubility of cement in water. The hydrolysis of C3S is mainly responsible for 7 day strength and hardness. The rate of hydrolysis of C3S and the character of gel developed are the main causes of the hardness and early strength of cement paste. The heat of hydration is 500 J/g.
Dicalcium silicate
It constitutes about 25-40% (normally about 32 per cent) of cement. It hydrates andhardens slowly and takes long time to add to the strength (after a year or more). It impartsresistance to chemical attack. Rising of C2S content renders clinker harder to grind, reducesearly strength, decreases resistance to freezing and thawing at early ages and decreases heat ofhydration. The hydrolysis of C2S proceeds slowly. At early ages, less than a month, C2S has little influence on strength and hardness. While after one year, its contribution to the strength and hardness is proportionately almost equal to C3S. The heat of hydration is 260 J/g.
Tricalciumalluminate
It is about 5-11% (normally about 10.5 per cent) of cement. It rapidlyreacts with water and is responsible for flash set of finely grounded clinker. The rapidity ofaction is regulated by the addition of 2-3% of gypsum at the time of grinding cement. Tricalciumaluminate is responsible for the initial set, high heat of hydration and has greater tendency tovolume changes causing cracking. Raising the C3A content reduces the setting time, weakens
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