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Which cement to use?

Of late, the consumer is confronted with a vide variety of cement in the market andhas to make a choice. The common question asked is which is the best? In the market, wehave ordinary portland cement and the blended cement either as PPC or slag cement.

Even in ordinary portland cement, we have gradations as 33, 43 and 53. In addition,there are various other types of cements such as low heat cement, rapid hardening cement,high early strength cement, sulphated cement, hydrophobic cement, oil well cement, highaluminium cement and masonry cement. Each cement has a specific role to play. We cannotsay a particular cement is the best cement. The choice of the cement depends upon thenature of work, local environment, method of construction etc.

In the earlier days, we had only one cement ordinary portland cement. Subsequently,the availability in abundance of fly ash in thermal power stations and slag in steel plants ledto the production of pozzolana cement and slag cement to conserve energy in theproduction of cement and to utilise the available by-products. India was perhaps one of thefew countries which produced as much as 76 per cent of the total cement produced asblended cement, which is the common name of pozzolanic cement as well as slag cementsas early as in 1982. Then the industry was under the control of controller of cements. After

de-control, the production of ordinary portland cement increased and consequently thepercentage of blended cement came down, currently it is somewhere around 30 per cent.

The introduction of precast / prestressed sleepers for the railways necessitated theproduction of a special grade ordinary portland cement because with the then availablecement, it was not possible to obtain the desired strength as well as the rate of gain of strength for the production of sleepers. Cement industry was permitted to manufacture thespecial grade ordinary portland cement and it was commonly known as sleeper cement.

Subsequently, the Bureau of Indian Standards came forward to grade the ordinaryportland cement into grade 33 cement, grade 43 cement and grade 53 cement. This gradingclosely followed the U.K. grading which was 32.5, 42.5 and 52.5 as well as 62.5, some of Indian companies are also trying to make and persuade the Bureau of Indian Standards tointroduce grade 63.

These higher grades of cements are specifically introduced to cater to the needs of theindustry engaged in the construction of long span bridges, high rise structures and otherstructures of gigantic magnitude requiring high grades of concretes. With the cement thenavailable. It was very difficult to design a mix for obtaining concrete of grade 4.5 and abovecommonly used in such structures. Hence, Bureau of Indian Standards classified ordinaryportland cement into the above 3 grades that is now available in the market.

After total de-control of cement industry in 1989 and abolishing of licensing systemfor setting up of cement plants in 1992, there has been phenomenal growth in theproduction of cement. With the many fold increase in the production of cement, cementmanufacturers adopted and aggressive marketing strategy, each projecting their product asthe best. Manufacturers of 53 grade cement invaded the market and created an impressionamongst the consumers that 53 grade OPC could be used by them as it is the best cement

and grade 33 and grade 43 cements, are inferior.

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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 smallscale consumers. 43 grade cement may be utilised for precast concrete production besidesthe sleeper manufacturers and other building components producers, 53 grade., cementmay be utilised by builders of heavy infrastructure such as bridges, fly overs, large spanstructures and high rise structures. The common man's perception that 53 grade. cement is

the best cement is not only due to the aggressive marketing strategies of the cementmanufacturers 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 isvery high. Hence, consumers believe that it is a better cement when actually it is not so.Each cement has to be chosen for a particular use.

While 53 grade. cement is actually best only for producing concretes of grade 53 andabove, it is not to be used in ordinary structures where concrete grade 20 is normallyadequate.

The total de-control and competition amongst manufacturers and the market forceshave actually eliminated 33 grade in the market. It may be necessary to enforce the partialregulation or self-discipline among the manufacturers of cement to produce at least 1/3rd of the cement in the OPC category as grade 33 cement and 1/3rd another as grade 43 cement

and balance 1/3rd as grade 53 cement and partially regulate the price for these 3 grades sothat, in the market 33 grade cement will become available to the common consumer.

Though originally introduced to save energy in the cement plants and to make use of the available industrial by products, research and experience have shown that blendedcements have more durability characteristics than the OPC of even higher grades.

Cement is primarily used to produce concrete. Concrete is mainly produced in readymixed plants the world over. Most of the producers of the concrete use OPC and asubstantial quantity of cementitious materials like pozzolana or granulated blast furnaceslag. In countries like U.S. and U.K. where 90 percent of the concrete produced is in theready mixed plants and the cementitious materials are blended in the concrete plants ratherthan in the cement manufacturing units. Hence, even now in countries like U.S. and U.K.the production of OPC is very high nearly 90 percent.

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 isvery essential. At present production of blended cement is around 30 percent. This hasnaturally to increase to at least 50 per cent.

The blended cement do not gain strength immediately and also do not generate heatlike 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 blendedcement.

But, ultimately, in the long run it gains as much strength as any OPC. Blendedcements give better workability in fresh concrete and during the hardened stage, give betterdurability. The consumer is to be adequately educated on the advantages of blendedcement. The reluctance on the part of builders to opt for blended cement is to an extentjustifiable because many builders were forced to use PPC when the production of PPC wasas high as 76 per cent during the control days.

They found to their dismay, they have to retain their shuttering for mush longerduration and some times the quality of PPC was of doubtful nature because the fly ashblended with the PPC was of verying quality. However, of late, the selection of pozzolanicmaterials has been more stringent and the quality of blended cement is more consistent.

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The builders can shed their past memories and start using blended cement.Manufacture of blended cement results in substantial savings in energy and product cost.Hence, industry must pass on to the consumers the benefits derived to the users of blendedcement.

In short, every cement has its own role to play and there is no reason to believe onecement is superior to the other cement. For ordinary structure. OPC grade 33 cement orPPC could meet the requirements. For producing concretes of higher grades, mixes have tobe designed using OPC as well as cementitious materials concrete chemicals and sometimes additional fillers and even viscocity agents.

For non structural uses such as masonry and plastering mortar, cement much lower instrength than OPC 33 grade classified as masonry cement is the best suitable material. But,unfortunately, no manufacturer produce this type of cement. More than the choice of thecement, the control exercised in production of concrete in regard to the selection andproportioning of materials, use of the minimum required quantity of water, careful handlingof the mix during transportation and placing, compacting well to get a dense concrete,having an engineered formwork, continuous curing and the like contribute greatly to thequality and durability of concrete.

D. Srinivasan, Indian Concrete Institute, Chennai.