If you want to test whether it's alkaline you could add some vinegar to a small amount. If it fizzes and the released gas doesn't have a smell (you are sensible enough to do this outside & not take a lungful!) - then it is probably alkaline & possibly the lime based product others have spoken of.
With the number '3' visible I'm actually wondering if it's 3-in-1 plant food or something similar.
The safest thing to do is obviously to bin it but I have no idea of the regulations concerning safe disposal of unknown chemicals in France. I would have to pay to have it safely disposed of here - in the same way I have to pay to have old tins of paint etc treated & dealt with.