How about the stick of chalk I use on my blackboard?
I don't know how pure that would be. I bet if you go to the pharmacy you can find some antacid tablets that are 100% CaCO3.There is no reason to carbonate/add chalk/baking soda to your brewing liquor.
There is if your water has no akalinity, like when you're starting with RO.
Only if you are brewing a dark beer like a RIS would you need to add alkalinity and in that case I would just use tap water rather than RO water. All you need is the CaCl or CaSO4. Gordon only adds CaCl or CaSO4 to most of his beers per his book and I think he has had one or two beers turn out ok. I think?