Review of ordinary portland cement standards and proposed changes

Anand Jain of UltraTech is highlighting the changes proposed by BIS in the OPC codes.

The most important issue now emerging before construction industry is sustainability. Can our construction materials be specified in a way that does not have negative impact on our finite mineral resources? The major mineral resource used in production of OPC is limestone. The cement grade limestone reserves are limited and with present pace of cement production, may last only for next few decades. The cement demand in India is likely to grow with a CAGR of 6% to 8% for next 2 to 3 decades and it will further put great pressure on limited reserves of limestone. It is therefore important that clinker factor in cement is bought down and replaced by other industrial by-products which have either pozzolanic or hydraulic properties without impacting performance of cement as a binder. It is very positive development that during the last one and half decade, the use of blended cements like PPC and PSC has been accepted in the market and viewed as value addition by the end consumers. The share of these cements has increased to about 70% of total cement production in India, and has greatly helped in conservation of limestone and extending the useful life of mines in cement plants.

PPC and PSC can have fly ash and ggbs in their composition upto 35% and 70% respectively as part replacement of clinker. However in OPC, additional minor constituents (fly ash, ggbs, limestone, silica fume, metakaoline, rice husk ash) up to 5% were permitted by BIS in 2003 on the lines of European Standard EN 197-1. Since then, based on experience gained in India and abroad, some more industrial by-products are found as potential replacement of clinker in OPC. These and some more changes that are being proposed by BIS and further needed in the sixth revision of IS 269 as per draft code circulated in September? 2014 are discussed in this paper.

Historical Background:
The standard for ordinary Portland Cement was first published in 1951 and subsequently revised in 1958, 1967, 1976, 1989 and 2013. On introduction of grades in OPC on the basis of compressive strength of cement, the standard of 53 grade was introduced under IS 12269: 1987. The construction industry realized the need of high strength OPC and in 1976 a new standard "Specification for High Strength Ordinary Portland Cement" under IS 8112: 1976 was published. This standard was subsequently revised in 1989 and rechristened as "Specification for 43 grade Ordinary Portland Cement". The Indian Railways through RDSO Lucknow, issued specification for cement to be used for concrete sleepers under IRS T-40-1985. This standard was kept outside the purview of BIS up to June? 2000. The Ministry of Consumer Affairs, Govt. of India directed that as cement is falling under the category of compulsory certification mark, the specification of cement for concrete sleeper should also be brought under BIS. In accordance of Govt. directive, the standards of 43 and 53 grade OPC were amended in June? 2000 to include the specification of cement for concrete sleeper as 43-s and 53-s respectively. The standard IRS T-40 was subsequently withdrawn by the Indian Railways.

Proposed Changes:
BIS has now proposed few major changes in the OPC codes as per draft circulated vide their letter in September? 2014 for public comments. The important changes are;

a)Unified OPC Standard:
In the revised standard, the existing Indian Standards pertaining to 33, 43 and 53 grade OPC will be combined which were previously covered separately in the Indian Standards IS 269: 2013, IS 8112: 2013 and IS 12269: 2013 respectively. The IS 8112 and IS 12269 will be withdrawn subsequently by BIS and unified standard of OPC will be issued under IS 269. The new standard will include the specification of OPC 33, OPC43, OPC53, OPC43s and OPC53s.

Justification:
This change is being made with a view that design, quality control and field engineers will no more be required to refer to various OPC standards and full information on different classifications of OPC will be available in one standard. It will be helpful to the construction professionals connected with the use of cement and concrete.

b)Inclusion of other by-products as Performance Improvers:
In 2003, first time BIS permitted use of fly ash granulated slag, silica fume, limestone, rice husk ash and metakaolin individually or in combination as part replacement of clinker upto 5% in all classes of OPC. Other industrial by-products which also have either pozzolanic or hydraulic properties were tested, evaluated and discussed for use as PI in OPC. After prolonged discussions and performance evaluation by reputed National Laboratories, the by-products with specification as given in Table 1 are being permitted as PI in OPC.

Justification:
Large quantity of metallic slags other than granulated blast furnace slag are available in India. Availabi?lity of these slags including yearly production is given in table- 2 The glass content or reactive silica in these slags vary from 32% to 42% which is almost same as of fly ash available from different thermal power plants. The grindability (Bond Index) (KWh/t) varies from 16 to 28, highest for LD (Linz-Donawitz Convertor) slag produced by major steel producers like SAIL, Tata Steel, RINL, etc. The Bond Incex of other slags like Lead-Zinc and Copper varies from 16 to 18. The copper and Zinc slag have potential to replace clinker up to 30% to 40% respectively while LD slag can replace upto 10% as per the test reports prepared by major National Laboratories. The higher percentage of clinker replacement in OPC can be made after gaining more knowledge and experience on the subject.

c)Upper limit on compressive strength:
In order to fulfill the uniformity criteria, upper limit on 28d compressive strength of OPC 33 and 43 grade has been made. The maximum 28day compressive strength for 33 and 43 grade will be 48 and 58 MPa respectively, except 43s. It is in line with European standard EN 197-1 where upper limit for categories of 32.5 and 42.5 are pegged at 52.5 and 62.5 respectively. The limits in Indian Standard will be more stringent with margin of 15 MPa in comparison to margin of 20 MPa in EN Standard. No upper limit is prescribed for 53 grade OPC and 53s as these cements are mainly used for higher grades of concrete, where their full potential can be utilized in concrete mix proportioning. In EN 197-1 also there is no upper limit on 28day compressive strength for 52.5 category cement.

Justification:
The provisions being made in Indian Standard are in harmony with EN standard. The upper limit on 28 day compressive strength of 33 and 43 grade OPC which are mainly used for general construction will help the end consumers to get product of better uniformity while sourcing from different cement manufacturers. In absence of such provision, the product of different cement manufacturers may vary widely and the new provision will help in narrowing down this variability.

d)Loss on Ignition (LOI):
In view of different types of performance improvers permitted in OPC, the loss on ignition varies as per the by-product used. In earlier versions of the standard, different limits were provided depending upon the type of PI used in manufacture of cement. To simplify specification, the standard now provides maximum LOI 5% for 33 grade and 43 grade OPC and 4% for 53 grade OPC.

Justification:
The earlier specification was cumbersome and many times, the end consumer was unaware that what type of PI has been used in cement and therefore could not co-relate third party lab results with the test certificate. The anomaly many times created controversy between the supplier and the user. With the new specification, this problem will be resolved and consumers will be fully aware about the product specification and its quality.

e)Limit of SO3:
The maximum limit of the sulphur content calculated as sulphuric anhydride (SO3) percent by mass which was 3% has been revised. The limit is raised to 3.5% for all categories of OPC.

Justification:
In view to promote the use of alternate fuels like petcock, used tyres, etc. from sustainability point in cement kilns, it is very timely and essential provision. The limit of SO3 in other foreign codes is also 3.5 to 4%. This is in harmony with EN and other standards.

Other desirable changes required in OPC Standard:
In addition to the recent changes made by BIS in OPC standard, some more changes are suggested to further rationalize the provisions in harmony with international standards and as per the requirements of construction industry in India. The following changes are suggested;

a)Initial Setting Time (IST):
The initial setting time of all classes of OPC is 30 minute (minimum) except 43s and 53s which is 60 minute (minimum) India is a tropical country where ambient temperature is much higher than European or American countries. The ASTMC-150 for all three classes of OPC (type I, II and III) specifies 45 minute and EN197-1 goes a step further and specifies IST as per strength class of the cement. The IST for 32.5, 42.5 and 52.5 classes are 75, 60 and 45 minute respectively. In view of number of operations to be completed within IST like transporting, placing, compacting and finishing, the initial setting time shall be increased to 60minute (minimum) for 33, 43 and 53 grade OPC. The increase in IST will be in conformity with construction requirements and International Standards.

b)Fineness of Cement:
In present OPC standard for all 3 classes (33, 43and 53), the fineness of cement is specified 225m2/kg (minimum), while for 53s and 43s it is 370m2/kg. ASTMC-150 specifics fineness for type I cement 280m2/kg (min) and EN 197-1 does not specify fineness, for any class of OPC. Fineness of cement has important influence on rate of hydration of cement and its initial strength. It will be more appropriate if the different values of fineness are specified for different grades of OPC in Indian Code. The higher grades of OPC should have more fineness compared to the lower grades of OPC.

c)Permissible Percentage of PI:
In present standard, the percentage of performance improver (PI) for all 3 grades of OPC is 5% (max). As the upper limit on 28 days compressive strength has been provided for 33 and 43 grades there is a need to permit higher percentage of PI in lower grade of OPC. It will have two advantages, first the clinker factor in lower grades of OPC will be reduced and help in conservation of limestone and higher utilization of industrial by-products; secondly, it will help in maintaining the uniformity of the product within the compressive strength range now provided by the standard. It may be worthwhile to consider to increase the limit of PI in 43 grade OPC to 7.5% and in OPC 33 grade to 10%.

The above suggested changes will further rationalize the OPC standard and harmonize with international standards. These changes can be considered in the next revision of the standard by BIS.

Conclusion:
The changes now being made in OPC standard are in harmony with EN and ASTM Standard. These changes will rationalize the specifications and help the construction industry, cement manufacturers and addresses the environment concern. The new provisions will facilitate to achieve better uniformity in product quality, utilization of additional industrial by-products and ease to construction professionals in referring to a unified code. The changes are therefore very progressive and a welcome step taken by BIS in benefitting the cement consumers and the industry.

However, in a dynamic scenario where change is the essence of progress, the revision of standards should also reflect the prevailing field situation. The revision of standards should be timely at frequent intervals and in harmony with international standards and requirements of construction industry. The changes further suggested in this paper in OPC code will help in achieving the above objective.

References:
1.IS 269: 2013 and earlier versions
2.IS 8112: 2013 and earlier version
3.IS 12269: 2013
4.IRST-40-1985
5.Draft OPC Code circulated by BIS for public comments vide their letter no. CED 2:1/T-1, 5,7,47,9 & 66 on dated 30th November, 2014.