Was trying to skim the ceiling last night but it seems I've had the
bag of plaster much longer than I thought I had because it was several
months past it's use by date.

I made up one batch last night and by the time I'd cleaned the bucket
out, it had already started going off and was completely un-usable.
I haven't done any plastering for a while so thought that maybe I'd
got the mix wrong. I tried again with a much thinner mix, but again
it only lasted a few minutes before starting to go off. It was at
this point that I noticed the use-by date on the bag.

I'm just wondering what it is in plaster that causes it to go off like
that and if there's anything I could have done to keep it for longer.
The bag had never been opened, it was just one of those things where I
bought it then kept putting the job off while I did other things
instead. Obviously I put it off for too long.