I will use it, just make sure the calcium and alk. is still in the right range.
The moisture in the air get trapped by the salt mix, and it will turn very hard. Still dissolve like it used to, but you need to do more work. There will be some white powder that will not disssove due to the irreversible reaction (harmless) that get the carbonate (along with calcium) to come out of the salt, and you won't be able to put it back into solution easily.
The salt will has less value due to the carbonate and calcium is not in there anymore, but you can compensate for it after the salt is mixed with calcium and alk. additive, but all in all, it is still safe to use.