Types of Portland Cement
Portland cement can be further sub divided into the following different types. The difference in the properties of the various types of cement are basically on account of variance in the proportions of its ingredients and degree of fineness to which the clinkers are ground.
1. Ordinary Portland Cement:
This is the basic type of cement which is used on large scale in all general type of construction work. It is also used for making water supply tank, reinforced concrete work etc.
2. Portland Pozzolana Cement:
This can be defined as a general purpose cement which can be used in all situations where ordinary Portland cement is considered suitable. Portland Pozzolana cement is manufactured either by grinding together, Portland cement clinkers and Pozzolana with addition of gypsum or calcium sulphate or by intimately and uniformly blending Portland cement and fine Pozzolana.
3. Rapid Hardening Portland Cement:
The main difference between the ordinary Portland cement and this type of cement is that the latter attains greater strength at an early stage. The property of high early strength is achieved almost entirely due to higher degree of fineness in grinding burning at higher temperature and increased lime content in the composition of cement. The strength development of this cement at the age of 3 days and 7 days is almost the same as the respective 7 days and 28 days strength of ordinary Portland cement with the same water cement ratio. The main advantage of using rapid hardening cement is that the form-work can be removed earlier and reused in other areas thereby effecting considerable saving in cost of form-work. It also permits faster completion of the work and thereby the structure can be put to use early.
4. Low Heat Cement:
Considerable quantity of heat is evolved during the chemical reaction that takes place during the setting and hardening of cement. In case of ordinary structures, it gets lost by inspiration from the surface of the member and as such produces no harmful effect. However, in case of mass concrete work such as dams, massive retaining walls, bridge abutments etc., heat of hydration generated (by use of ordinary Portland cement) proves dangerous. In such cases, the rate of loss of heat from the surface is much lower than the build up of heat inside the concrete mass which results in setting up tensile stresses during curing and finally leads to cracking of concrete mass. It is seen that the heat of hydration can be reduced by making suitable alteration in the chemical composition of cement. In low heat Portland cement tri-calcium aluminate and tri-calcium silicate which have the maximum contribution towards heat generation are kept to the minimum and the amount of di-calcium silicate increased. Low heat Portland cement has not only lesser and slower heat generation but also greater resistance to cracking less rapid strength development with equal strength at advanced ages and greater sulphate resistance due to decreased tri-calcium aluminate. Such a cement is, however, not suitable for structures of ordinary nature.
5. White or Coloured Portland Cement:
White cement is a type of ordinary Portland cement which is pure white in colour, and hence very useful for cast stone, terrazzo flooring or face plaster to walls and other ornamental works. It is also used for traffic curbs, bridge rails and aerodrome markings. It is available in the market under various trade names as “snow crete”, “Atlas” “Medusa” and “Silvicate” etc. The strength of white cement is slightly less than that of ordinary cement and it is 4 to 5 times costlier than the latter.
In this case special care is taken to exclude iron oxides which impart grey colour to the cement during manufacture. Raw materials used are china clay and pure limestone. Only oil is used as fuel for burning and the rotary kiln is lined with special fire clay bricks so that the mixture does not come in contact with iron or steel during manufacture.
Coloured cement are produced by adding 5 to 10% of a suitable mineral pigment to ordinary Portland cement or white cement before it is ground. The pigments use should be stable under the action of sunlight and weather, be free from soluble salts, must be in finely divided state and should not effect the quality of cement nor get effected by it.
Various metallic oxides are generally used as pigments. Coloured cements are sold in the market under the trade names “colour crete” “Rainbow” and “Snowcem” etc., and they are used for decorative finish in swimming pools, fountains, floors, tennis courts, etc.
6. Sulphate Resisting Portland Cement
This type of cement is used in the construction of foundation in soil where sub-soil contains very high proportion of sulphates. The constituents of sulphate resisting Portland cement are so adjusted that the tri-calcium aluminate (C3A) and tetra calcium aluminate-ferrite (C4AF) are kept very small. Cements with such composition have excellent resistance to sulphate attack.
7. Water Repellent Portland Cement
This type of cement is chiefly used in water tight concrete and water tight renderings to check moisture penetration is basements etc., and for coloured renderings and stucco. It contains a small percentage of waterproofing material uniformly mixed with the cement and is manufactured under the name “Aquacrete”. This cement is prepared by using ordinary or rapid hardening cement and white cement. Considerable care is needed to avoid reduction in strength if such a cement is used in concrete.
8. Water-proof Portland Cement
Water-proof cements are prepared by mixing with ordinary or rapid hardening cements, a small percentage of some metal stearates (Ca, Al etc.) at the time of grinding. Concrete made with such a cement is more resistant to penetration by water and some oils than that made from ordinary Portland cement and as such it is used for the construction of water retaining structure like tanks, reservoirs, retaining walls, swimming pools, dams, bridges, piers etc. It is also more resistant to the corrosive action of acids and alkalies or other harmful salts usually present in industrial waters.
9. Portland Blast Furnace Cement
In this case, the normal cement clinkers are mixed with up-to 65% of blast furnace slag for the final grinding. The resulting cement is quite cheap and has almost the same properties as Portland cement. Its early strength is less than of normal cement but becomes the same after 28 days. In addition, it is more resistant to dilute acids and sulphate soils. It has a lower heat evolution but the speed the hardening falls off very rapidly when the temperature decreases. Also, it requires an extended curing time owing to its slower rate of strength development. In view of its having low heat of hydration and better resistance to attack by sulphates and alkalies, concrete made with this type of cement can be used with advantage in mass concrete work such as dams, foundations and abutments of bridges, retaining walls etc., and for construction in sea water.