Is my old bag of cement still usable, and how should I store cement?
The squeeze test that tells you in five seconds whether a bag is still worth using, why cement goes off in a sealed bag, and how to store what is left over.
- Difficulty
- beginner
- Time
- 5 min
- Read
- 3 min
Short answer
Squeeze the bag. If it is uniformly soft and the powder flows freely, use it. If there are hard lumps you can crush between finger and thumb, use it for something unimportant. If there are lumps that will not crush, or the bag has gone solid, it is dead — it has already reacted with airborne moisture and it will never reach proper strength.
Cement does not sit inert in a bag. It reacts with moisture in the air, which the paper sack slows but does not prevent, and once part of the powder has hydrated it cannot hydrate again. A bag that has spent two winters on a garage floor may look identical to a fresh one and produce concrete at a fraction of the strength you think you are getting.
Step by step
- Check the date on the bag.Manufacturers print a production or best-before date. Cement kept in ideal conditions is generally considered good for around six months and is often fine for longer, but the date gives you a starting point.
- Do the squeeze test.Run your hands over the whole bag. Free-flowing powder everywhere means it is fine. Small soft lumps that crumble to powder under thumb pressure mean it has taken up a little moisture and lost some strength — acceptable for a non-critical job.
- Reject anything with hard lumps.A lump that will not crush has already set. Sieving the lumps out does not rescue the bag, because the powder around them has also partly reacted. Do not use it for anything you are relying on.
- Never use doubtful cement structurally.Foundations, a slab that will carry load, a wall, a post that holds a gate. The failure is invisible until it matters. Use fresh cement and put the old bag on something where a poor result costs nothing.
- Store bags off the ground and off the wall.On a pallet or a couple of battens, not directly on a concrete floor, which wicks moisture straight into the paper. Keep them clear of external walls for the same reason.
- Get an opened bag into something airtight.Once opened, tip the remainder into a lidded plastic tub or a heavy-duty sealed bag and squeeze the air out. An opened paper sack folded over lasts weeks, not months.
- Buy what the job needs.Cement is the cheapest part of most jobs and the one most damaged by storage. Buying two spare bags to save a trip usually means throwing two bags away next year.
- Dispose of dead bags properly.Set cement is inert rubble and goes to the tip as hardcore. Loose powder should not go into a household bin loose, into a drain or onto a garden — it is strongly alkaline and it will kill plants and blind whoever opens the bin.
Tips
- Sand and ballast keep indefinitely, but they hold variable amounts of water depending on the weather, which is why the water in a mix is judged by consistency rather than measured.
- Bagged pre-mixed products — mortar mix, concrete mix, postcrete — go off the same way and often faster, because they contain a smaller proportion of cement in a larger bag.
- A bag that has been rained on and dried is not recoverable, however good the outside looks.
Questions people ask
Can I use cement with a few hard lumps if I sieve them out?
Only for something that does not matter. The lumps are the visible part; the powder around them has partly hydrated too, so the strength is unknown and lower than the bag suggests.
How long does cement last unopened?
Around six months is the usual guidance, though a bag kept genuinely dry, off the floor and sealed will often be fine well beyond that. The squeeze test is more reliable than the calendar.
Does postcrete go off in the bag too?
Yes, and it is worth checking before setting a post, because a weak set is not obvious until the post moves months later.