Cement, mortar and concrete are not the same thing
This is the most common confusion, and it is worth clearing up first.
- **Cement** is the grey powder in the bag. On its own it is used for almost nothing: it is the glue.
- **Mortar** is cement + sand + water. It is for building walls, rendering and bedding.
- **Concrete** is cement + sand + gravel + water. This is what has structural strength: slabs, footings, columns.
If somebody tells you they have laid a slab “in cement”, they almost certainly laid concrete.
Reading the bag
The markings follow the European standard and always say the same things, in this order.
**The letter: what it is made of.**
- **CEM I** — almost pure Portland cement. The strongest and the fastest to gain strength. For structural concrete and precast work.
- **CEM II** — Portland with additions (limestone, slag, pozzolana). This is the general-purpose one and it covers almost all ordinary jobs.
- **CEM III** — with a lot of blast-furnace slag. Slower setting, gives off less heat and copes better with aggressive environments such as coastal air.
- **CEM IV and V** — pozzolanic and composite, for more specific uses.
**The number: how strong.** 32.5, 42.5 or 52.5 megapascals at 28 days. The higher the number, the greater the strength and the higher the price.
**The last letter: how fast.** **N** is normal and **R** is rapid: it gains strength sooner, which is welcome in winter and a nuisance in summer, because it leaves you less time to work with it.
What to use for each job
- **General work, rendering, building partitions:** CEM II 32.5 N. It is the biggest seller and covers most jobs.
- **A slab, footings, anything that has to carry load:** CEM II 42.5 or CEM I.
- **Near the sea or in contact with the ground:** a sulfate-resisting cement (SR) or a CEM III. In Menorca this matters more than people think.
- **In a hurry, or working in the cold:** a 42.5 R.
- **Exposed joints, decorative pieces, laying pale marble:** white cement, which also will not stain light-coloured stone.
How much to buy
As a rough guide, ordinary bedding mortar uses around one 25 kg bag for every two or three square metres of one-centimetre render. For concrete, a cubic metre swallows about 300 kg of cement.
These are back-of-the-envelope numbers: tell us about the job and we will work it out properly.
Storing it without ruining it
Cement does not go off by date, it goes off with damp. A bag that has taken up moisture forms hard lumps and loses strength even when it looks like it still works.
- On pallets, never straight onto the floor.
- Away from walls.
- Under cover and sheeted.
- First in, first out: it should be used within three or four months.
If you squeeze a handful and the lumps do not break up between your fingers, that bag is no longer any use for anything that has to carry load.
Ask us before you buy
Over-specifying costs money too: a 52.5 to render a wall is throwing euros away. Tell us what you are doing and we will tell you exactly which bag you need and how many.