The only way I've had cement go off on me was going hard or lumpy - whatever
wasn't rock
solid would always set. The only problem I've really had was with some
Wickes exterior tile
which is quick setting normally but wouldn't set after 6 months in the open
bag. I wonder
if what you have isn't plain old sand and cement but something more like my
tile cement?

I'd give your test piece a bit more than 24 hours to go off though - give it
another couple of
days before you declare the stuff useless.

Andy.