Which cement to use?

Of late, the consumer is confronted with a vide variety of cement in the market and has to make a choice. The common  question asked is which is the best? In the market, we have 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,  high aluminium cement and masonry cement. Each cement has a specific role to play. We cannot say a particular cement is the best cement. The choice of the cement depends upon the nature 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 led to the production of pozzolana cement and slag cement to conserve energy in the production of cement and to utilise the available by-products. India  was perhaps one of the few countries which produced as much as 76 per cent of the total cement produced as blended cement, which is the common name of pozzolanic cement as well as slag cements as 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 the percentage of blended cement came down, currently it is somewhere around 30 per cent.

The introduction of precast / prestressed sleepers for the railways necessitated the production of a special grade ordinary portland cement because with the then available cement, 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 the special grade ordinary portland cement and it was commonly known as sleeper cement.

Subsequently, the Bureau of Indian Standards came forward to grade the ordinary portland cement into grade 33 cement, grade 43 cement and grade 53 cement. This grading closely 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 to introduce grade 63.

These higher grades of cements are specifically introduced to cater to the needs of the industry engaged in the construction of long span bridges, high rise structures and other structures of gigantic magnitude requiring high grades of concretes. With the cement then available. It was very difficult to design a mix for obtaining concrete of grade 4.5 and above commonly used in such structures. Hence, Bureau of Indian Standards classified ordinary portland 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 system for setting up of cement plants in 1992, there has been phenomenal growth in the production of cement. With the many fold increase in the production of cement, cement manufacturers adopted and aggressive marketing strategy, each projecting their product as the best. Manufacturers of 53 grade cement invaded the market and created an impression amongst 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.

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. 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. Each cement has to be chosen for a particular use.

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

The total de-control and competition amongst manufacturers and the market forces have actually eliminated 33 grade in the market. It may be necessary to enforce the partial regulation 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 so that, 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 blended cements have more durability characteristics than the OPC of even higher grades.

Cement is primarily used to produce concrete. 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 countries like U.S. and U.K. where 90 percent of the concrete produced is in the ready mixed plants and the cementitious materials are blended in the concrete plants rather than 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 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.

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

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

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 blended cement.

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

For non structural uses such as masonry and plastering mortar, cement much lower in strength 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 the cement, the control exercised in production of concrete in regard to the selection and proportioning of materials, use of the minimum required quantity of water, careful handling of 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 the quality and durability of concrete.

D. Srinivasan, Indian Concrete Institute, Chennai.