Generally the commonest cement on our construction market being Portland cement is manufactured using two basic raw materials, calcareous and siliceous (generally argillaceous) material.
These two materials are mixed together and heated to a high temperature within a revolving kiln. The heated mixture produced is called cement clinker which is then grinded into powder form.
In its raw state after production cement is a green-grey powder that sets hard within a few hours after the addition of water (hydration).
It therefore acquires strength with time. As indicated above, there are many types of cement available on the market, the most common type is known as ordinary Portland cement (OPC).
Cement usually comes in paper bags containing 50 kg of cement each. Cement can also be supplied in bulk into silos.
The function of cement is to bind or bond aggregates together, fill voids in aggregates and to form a compact mass.
Among the number of properties of cement, I am much concerned about its strength as a property. Depending on the fineness of the cement powder which also determines the strength, cement comes in classes such as 32.5R, 42.5R, 52.5R etc.
Grades mentioned in cement as 32.5R, 42.5R, 52.5R in fact indicate the strength of the cement. 52.5R class cement simply means that when a compressive strength is conducted on a 28 day cured concrete cube made from that cement under a standard condition will be 52.5MPa or 53MPa.
The same test when conducted on 32.5 will yield you 32.5MPa or 33MPa.
According to a number of cement manufacturers and research institutions,
Grade 32.5 is applicable where a low initial strength is required
Grade 42.5 is usually used for compressive strength of concrete at 28 days exceeding 30N/mm2 (ie. C30 according to The Belgian Building Research Institute)
Grade 52.5 applicable in situations where even higher initial strength is required.
According to some of the manufacturers and importers of cement, whiles the class 32.5 is meant for plastering/rendering, screeding, block laying and similar works the 42.5 is meant for structural works such as columns, beams, reinforced concrete walls etc. Some manufactures go on further to say that the 42.5 grade cement produces concrete with longer life and extra strength more that the class 32.5.
A simple survey conducted on venders and builders (artisans) within some parts of Tema and Aburi indicated that most of the people do not know about these classes of cement and where and when to use which grade of cement.
A minority of the respondents are also aware but do not bother when to use which kind of cement grade for reason of affordability and availability.
Until recently we did not have these varied classes of cement on our markets and had no confusion of which cement to use for what kind of work. I believe for reasons of affordability, design specification and competition we have these grades on our markets.
In order to avoid any confusion that is putting a number of people in suspense and which may also bring some disaster, manufacturers should produce their minimum grade to have the required strength to carter for negligence and ignorance on the part of users. Also possibly this minimum class of cement should be banned from our markets. Finally in the interim, the authorities (MMDAs) should ensure that builders are using the right class of cement for the right work.
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