Saturday, August 3, 2013

Types of Cement

Cement is a Civil Engineering Material which is worldwide used to form concrete and mortar and other soil stabilization works. It is  possible after the invention of the cement to get the structures with high strength and of various attractive architectural shapes at the same time. The strength of cement depends on its constituents and their relative proportions.
1.Ordinary Portland Cement:
Ordinary Portland Cement(OPC) is used for the normal concrete works. It has cohesive and adhesive properties when mixed with water. After the evaporation of water it forms a solid mass. OPC manufactured by burning the following materials in a definite proportion:
(a) Siliceous Materials 
(b) Argillaceous Material (containing clay)
(c) Calcareous material (Containing Lime) 
The mixture of the above constituents is burnt to a temperature of about 1400 degrees celcius in the Rotary kiln which results in the formation of the clinker.  The clinkers so obtained are cooled and crushed to form the powder form which is known as cement. Cements of different kinds are obtained by mixing the above constituents in different proportions.
Constituents of OPC : 
(i) Lime : 60% to 67%
(ii) Silica: 17% to 25%
(iii) Alumina: 3% to 8%
(iv) Iron Oxide: 0.5% to 6%
(v) Magnesia: 0.1% to 4%
(vi) Soda and Potash : 0.2% to 1%
(vii) Sulphur tri-oxide: 1% to 2.75%
(viii)  Free lime: 1% to 1%
Cement constitutes chemical compounds formed after the burning of the above mentioned ingredients. The main constituents found in cement are as follows:
(1) Tri-Calcium Silicate (C3S)
(2) Di-Calcium Silicate(C2S)
(3) Tri-Calcium Aluminate (C3A)
(4) Tetra calcium Aluminoferrate (C4AF)
About 70% to 80% of cement is contributed by the C3S and C2S which are responsible for the strength of the concrete. C3S or Tri-calcium Silicate has the property to hydrate rapidly so mainly it is responsible for the early strength of the cement and also responsible for the ultimate strength of cement.
Di-Calcium Silicate has the property to hydrate slowly and provides strength after a period of seven days. Tri-calcium Aluminate also helps in gaining early strength due to early hydration but it is said to retard the ultimate strength, and is susceptible to the alkali and salts attacks.
Tetra-calcium alumino ferrate do not provide any strength to the cement and is said to be an unwanted compound in the cement.
2.Rapid Hardening Cement (IS:804)

This is manufactured in the similar manner as we do OPC, however it differs in its constituents, it contains more lime and is burned under a better controlled temperature over a longer time. The clinkers are ground to much finer extent. The quick rate of gain of the early strength is due to the higher content of tri-calcium silicate.  It gains the same strength in 3 days which OPC gains in 7 days for the same water cement ratio, and it gains the strength in 7 days what the OPC gains in 28 days. Advantages of using this type of cement is when the shuttering is to be removed early on to save the time, it is best.
Second it is best for the cold regions, where the frosting conditions occur. In case of roads it can be used to open the road to traffic at lower time period.
3.Low Heat Portland Cement(IS: 269)
In the mass concrete structures like Dams, retaining walls and abutments, if we use the normal cement which generates a large amount of heat(about 80 cal/gm at the end of 3 days), will result in the shrinking of the concrete and may affect the water tightness of the structure. Low heat Portland cement is specifically made for such uses because it generates a heat of about 75 cal/gm at the end of 28 days. Its setting rate is slower than the OPC. It is due to the fact that it contains low percentage of C3S and C3A, which are responsible for the early strength strength of concrete. So its strength is less than OPC in the early days but with the time it gains the same strength as that of the OPC. To ensure the rate of strength code recommends a specific surface requirement of 3200 cm^2/gm.
                                                 4. Portland Blast Furnace Slag Cement(IS: 455)
This cement is manufactured by mixing the blast furnace slag with the OPC clinkers. The properties of the cement are not affected by the blending of the blast furnace cement however it enhances its properties, but the content of the blast furnace slag cement is kept less than 65%. Blast furnace slag has latent hydraulic property which is highly activated when mixed with the cement clinkers. This cement can be used almost everywhere where the OPC is used and in addition to that it can be used in the huge mass concrete structures like dams, retaining walls, foundations and abutments of bridges. This cement has a good sulphate resistance and is very suitable for use in constructions in sea water.
                  5. Portland Pozzolana Cement (IS: 1489)
This cement is made either by blending and grinding the OPC cement clinkers and Pozzolana or by intermixing the OPC and fine pozzolana. Intergrinding must be preferred because it forms a uniform composition while blending forms non uniform composition. The blending should be preferred only when the uniform blending is assured by some mechanical process.  Content of Pozzolana varies from 10 % to 25% by weight of cement. Pozzolana in itself do not have the cementing properties but it forms the cementing material with the lime. When mixed with OPC it reduces the amount of free lime in cement and therefore it reduces the chances of the chemical attacks. It also lowers the heat of hydration and resists the sulphate attacks and was popularly used in the construction of the Bhakra Dam.

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