There are numerous cements types available in market, everyone has its own purpose. In earlier days only one cement was available, Ordinary Portland cement. Now we have 33, 43, 53 and 53S (special grade cement for manufacture of sleepers for Indian Railways) grade cements and several types like Sulphate resistant cement, low heat cement, rapid hardening cement, high early strength cement, hydrophobic cement and high aluminium cement. The choice of the cement depends upon the nature of work, local environment, method of construction.
There is another type is pozzolana cement and slag cement. It is like Ordinary Portland cement, but the fly ash in thermal power stations and slag in steel plants are added in this cement to conserve energy in the production of cement and to utilise the available by-products. Earlier in India 76 percent of cement was pozzolana cement or slag cement. Now the percentage came down to 30 percent.
In the three grades of cement 33 grade has eliminated by market. And now mostly preferred grade in market is 53 grade. In fact, all cements are one and the same but for minor changes in the characteristics. By and large grade 33 cement would meet the structural requirements of ordinary and small scale consumers. 43 grade cement may be utilised for precast concrete production besides the sleeper manufacturers and other building components producers, 53 grade cement may be utilised by builders of heavy infrastructure such as bridges, fly overs, large span structures and high rise structures. The common man’s perception that 53 grade. 53 grade cement is the best cement is not only due to the aggressive marketing strategies of the cement manufacturers but also on the presumption that the heat generated during hardening of concrete is an index of its quality. When 53 grade, cement is used the heat generated is very high. Hence, consumers believe that it is a better cement when actually it is not so.
Concrete is mainly produced in ready mixed plants the world over. Most of the producers of the concrete use OPC and a substantial quantity of cementitious materials like pozzolana or granulated blast furnace slag. In India ready mixed concrete has just had the beginning. The blending of cementitious materials at site is not advisable now. Hence, blended cement production is very essential. At present production of blended cement is around 30 percent. This has naturally to increase to at least 50 per cent. The blended cement do not gain strength immediately and also do not generate heat like OPC. Hence, the common belief has been that blended cement is an inferior cement. Actually it is not so. No doubt, the rate of gaining strength is slow in the case of blended cement. The advantages in blended cement are when water is added, pozzolana material combines with lime and alkalies that are present in cement and forms a compound which contributes to strength. Additionally, it makes the concrete less impermeable and sulphate resistant. It also contributes to the workability, reduces bleeding and controls destructive expansion from alkali aggregate reaction. Leaching of free lime is also reduced. It also reduces heat of hydration, thereby controlling temperature differentials and minimizes the incidence of cracks.
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