What do the letters and numbers on a bag of cement mean?

The markings look like a specification you are supposed to decode before buying, and for most domestic work they are not. But the codes do tell you two useful things: whether the bag is a plain cement or a ready-blended product, and whether it is a rapid-setting version that will catch you out on a hot day.

Step by step

1. Read the CEM code first.CEM I is Portland cement with almost nothing else in it. CEM II has a secondary material blended in — limestone, fly ash or slag — which is entirely normal, slightly reduces the carbon footprint, and behaves much the same in the bag. CEM III has a high proportion of blast furnace slag and sets noticeably more slowly.
2. Read the strength class.32,5 or 42,5 with a comma is a European decimal — the megapascals the cement reaches at 28 days in a standard test. Higher is not automatically better for you: the strength of your finished concrete is governed far more by your water content and your ratio than by the class of the bag.
3. Read the N or R after the number.N is normal early strength, R is rapid early strength. R gains strength faster in the first few days, which helps in cold weather and hinders in hot weather because it gives you less working time before the mix goes off.
4. Distinguish plain cement from a blended product.Bags labelled masonry cement, mortar cement or MC already contain a plasticising agent and are mixed with sand only. Do not then add lime or plasticiser to them as well. A bag of plain cement is the one you use with a separate additive if you want one.
5. Know when sulphate-resisting cement matters.Some ground and some old clay bricks contain sulphates that attack ordinary Portland cement over time. Sulphate-resisting cement, and mixes specified for aggressive ground, exist for footings in sulphate-bearing soil and for chimney work. If a ground investigation or a specification calls for it, use it; do not guess it is needed.
6. Check the date on the bag.Cement has a shelf life and absorbs moisture through the paper. The manufacture date is printed on the bag, and a bag that has sat in a damp garage for two winters is not the same material as a fresh one.
7. Ignore the rest for domestic work.The remaining codes on the bag describe conformity testing and manufacturing standards. They matter to a specifier and not to someone laying a shed base.

Tips

• If a bag is described as "general purpose" or "multi-purpose" cement, it is correct for mortar, concrete and render on a house. You do not need to hunt for a particular class.
• Rapid or R-class cement in July gives you a very short window on the spot board. Buy N unless you have a cold-weather reason not to.
• Masonry cement and general cement are not interchangeable in a mix design, because one already contains the air-entraining agent and the other does not.

Common mistakes

• Buying a higher strength class to get stronger concrete — Final strength is dominated by the water-to-cement ratio and the proportion of cement in the mix, not by the class on the bag. Adding extra water to a 42,5 cement gives weaker concrete than a well-mixed 32,5.
• Adding lime or plasticiser to masonry cement — It already contains an air-entraining agent. Adding more entrains too much air and produces a weak, over-soft mortar.

Questions people ask