Standards Organisation of Nigeria and Portland Cement

Chat212 - Mail News... Report

►We have in the two previous weeks looked at Portland cement, its pricing and its proper application in construction.
►The manufacture of Portland cement clinker requires large amounts of energy. Considerations of the environmental impact of the manufacturing process have brought about the situation in which for the cement sold in the market, there has been a reduction   in the amount of clinker whilst other materials such as limestone and pulverised fly ash are added to make up for the clinker that has been removed.

The reduction in the amount of clinker has been done whilst maintaining the high performance of concrete made with the resulting composite cements. Portland limestone cement is the composite cement available from local manufacturers.

The limestone content could be between 6% and 35% whilst the clinker content could be between 65% and 94%. The lower energy costs in making Portland limestone cement should be passed on to the consumer in lower prices of the 50kg bag of cement in the market place.

The Standards Organisation of Nigeria, SON, placed a newspaper advertisement indicating the uses to which cement strength classes 32.5, 42.5 and 52.5 could be put. We have indicated previously that the strength of a cement is determined from the 28-day strength of concrete made with the cement.

Thus cement strength class 32.5 can give 28-day compressive strength of concrete between 32.5N/mm2 and 52.5N/mm2, cement strength class 42.5 can give 28-day compressive strength of concrete between 42.5N/mm2 and 62.5N/mm2, while cement strength class 52.5 can give 28-day compressive strength of concrete above 52.5N/mm2.

Thus, it is apparent that cement strength class 32.5 can serve the same purpose as cement strength class 42.5 where strength requirements do not exceed 52.5N/mm2.

We should at this juncture take a look at the place of cement in the Nigerian economy. My estimates are that in each year we currently use three no 50kg bags of cement per person.

Thus, total quantity of cement used is 480million bags and at N1800 per bag, this would give a total expenditure of N864 billion per year. The revenues of the cement manufacturers are currently about N500 billion per year and if we allow a differential in price between the manufacturers and the retailers, we would require cement imports to meet our needs.

Also, if our various governments are providing building and civil infrastructure at an appropriate level, we should be using about 6 bags per year per person. Thus the cement manufacturers continue to announce expansion plans because there is a demand to be met.

Engineering standards with respect to a subject are established on the basis of the current state of scientific knowledge and engineering practice of the subject. The announcement by the SON of the use to which the various cement strength classes could be put throws up several puzzles.

For example, what is the state of scientific knowledge about Portland limestone cement available in Nigeria and designated as 42.5 strength class and how should it be applied to make concrete with 28-day compressive strengths of 20N/mm2 and 25N/mm2 which are the strengths in use for building construction?

What is the role of the limestone in the Portland limestone cement? Also, Portland limestone cement is available in strength class 32.5. Why can we not use this cement to make concrete with 28-day compressive strengths of 20N/mm2 and 25N/mm2?

The SON should withdraw its newspaper advertisement and bring out a more comprehensive statement such that the process of establishing standards would not be brought into disrepute.