There is product information on the back of every cement bag, but do you know what the information means?
For most of us in construction there are two types of cement. The cement that costs more because it comes in a nice waterproof bag and the slightly cheaper stuff that comes in a paper bag which will always get wet no matter how hard you try and keep it dry.
In fact, there are five different types of cement, with each type having several sub types and those sub types each having sub classes and then sub classes of sub classes, still with me? then I’ll try an explain what’s what next.
The main five types are; CEM I, CEM II, CEM III, CEM IV and CEM V.
For general construction uses only CEM I and CEM II are used and we will focus on them in this article.
• CEM I is 100% pure Ordinary Portland Cement known as OPC
• CEM II is diluted with either fly ash, limestone or slag or a combination of the three up to a maximum of 35%
CEM I & II each have three grades based on the concrete strength they produce after 28 days, these are;
• 32.5MPa
• 42.5MPa
• 52.5MPa
What is a MPa? I will cover that below as right now we have to talk about the different classes and I need to concentrate!
Each grade has three classes and each class relates to the time taken for the concrete to gain its strength;
1. Class R means early strength gain
2. Class N means normal strength gain
3. Class L or S means low/slow strength gain
When you think that must be it… it’s not – CEM II has two more sub classes
1. A means the dilution is less than 20%
2. B means the dilution is between 21 – 35%
Still more… the type of dilution or additive is indicated by the letters V – Fly Ash, S – Slag and L – Limestone. If the letter M appear that means there are multiple additives.
Cement Powder from Palmix
At Palmix our cement powder is currently supplied by Hanson or Heidelburg Materials as they are now known and we have CEM I 52,5 – N. From what we have learned about this already, this specification means that its 100% pure with a maximum strength of 52.5MPa and a normal strength gain.
Whilst this product is more expensive than its CEM II brother it makes sense to us to give our customers only the finest powder (combined with prime sand and aggregates it is a recipe for great concrete).
The cost saving between the two types are not to be ignored but for any given mix type the amount of CEM II used would be greater than CEM I. This means that while you think you are making a saving buying the cheaper cement powder you are actually having to put more in to achieve the same strength. For example, our C10/GEN 1 uses 180kgs and our C40 uses 300kgs of CEM I 52.5-N if we chose to use CEM II these quantities would be much higher and therefore cost more.
How do Volumetric Lorries Work?
Incidentally if you were wondering how the volumetric lorries we use know what quantities of each material to discharge and how the computer calculates the volume you have had, further blog articles will be following and they will include the lorry calibration and also our all-important concrete trials. Concrete trials are conducted in a laboratory where our materials are used to make differing concrete mix types which are then cured and crushed. The results achieved tell us how the concrete is performing and whether it complies with British Standards. All very interesting and one I’m sure you won’t want to miss.
So, to return to the original question - what do the details on the back of every cement bag actually mean?
From what we have learned we know that this is a bag of CEM II cement with less than 20% dilution and it has been diluted with limestone dust and will reach a strength of 32.5 Mpa and has early strength gain.
I did mention earlier that I’d explain MPa.
What is an MPa, well Mpa stands for megapascal, a unit of pressure used frequently in Science & Engineering. A megapascal is the equivalent of one million pascals.
To try and explain this in layman’s terms, one million pascals
(1 MPa) is equal to the exerted pressure of a mass of one hundred tons (metric) on an area of one square metre.
Conclusion
• Palmix Concrete use the best available cement powder and not the cheapest
• Each type has grades and each grade gains strength at different periods of time
• In CEM II there can be up to 35% of added impurities
• Cement in a paper bag will always get wet
• 1 MPa is quite a lot
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