I have unearthed some old sealed bags of ready mixed concrete (Blue Circle brand) in my garage. They are well past their use-by date but haven't gone hard.
Will the cement have gone off even though it hasn't hardened? I can think of ways to use it (laying slabs, shoring up around a path, etc.) that don't require it to be particularly strong, is it worth mixing and trying for this sort of application?
Yes
Is that yes to "will it have gone off?' or is it yes to "is it worth trying.....?". :-)
The latter. You don't have a lot to lose, anyway. You probably wouldn't want to use it in any application requiring high strength. Just mix up a small amount and see what it's like once it is set. If it crumbles easily it may not be any good but if it stays together it should be ok.
Make sure that you keep the unmixed portion dry though. Many years ago I tried to save time by dry mixing some sand and cement for use on the next day, just by adding water. BIG MISTAKE! I didn't realise it at the time but the cement had absorbed some moisture and the resulting mortar was very week. I had to knock down what I'd built and start again!
OK, thanks for clarifying. :-)
Unused cement draws CO2 from the atmosphere. If theses are sealed impermeable bags then there is a fighting chance it should be ok.
Best check a small amount. Don't forget full strength is obtained after a few days from absorbing CO2.
I thought that only applied to (slaked) lime mortar, not Portland cement.
No. It applies to all mortars and cements Does lime mortar even have any strngth? :=-)
It must have some - Georgian architecture used lime mortar, and where the buildings survive they are at least 250 years old. I believe it takes a couple of weeks to set really solid though, judging from what I read in a Listed Building planning application for repairs to one.
I think one of its advantages is that it never sets solid so can accommodate some movement.
It could take centuries for the CO2 to percolate into a thick column or wall to convert the Ca(OH)2 into CaCO3.
And allows water vapour through . . .
It could, butt it doesnt
weak mortar does, too
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