Or have a wander round any local 'garden centre' or equivalent and see if you can find a similar bag with a similar red/yellow label
Sounds most like lime to me, but I'd chuck it. I've bought it like that for growing brassicas, then found soil here is alkaline.
Could it be Epsom salt?
Could be a French attempt at making a cheese as good as Cheddar !!
Could be rat poison
That's it, it's going in the bin!
How big is this bag of white powder. It could be really valuable in certain quarters!!
Put a small amount of it in water - the reaction might tell you something (either positive or negative).
Ok, will give that a go - what 'reaction' am I looking for?
I haven't a clue what it is but enjoyed reading the replies, very funny you guys.
i would bin it safely .