Kalk Powder
 

Anyone know if it has a shelf life? Can it become unusable?

I know that if it clumps, or starts to get "gritty" its no longer good. As far as I'm aware, and have heard (http://www.talkingreef.com/forums/po...de-44-a-2.html) its okay as long as it doesn't get wet, or absorb moisture.