I posted late at night and may not have been very clear about my question. There's a brand of oat crackers that I like, however, on the list of ingredients I see calcium carbonate. Hv has enlighted us to dangers of calcium hydroxide and the quality difference between calcium carbonate vs. calcium malate or citrate. I don't eat these cracker for the calcium value, I just like them. But I wasn't sure if calcium carbonate is just non-effective (not very bioavalable) or if it has a negative nutritional value like binding with other nutrients and depleting the body. If it's just not a good source for calcium then I'll continue to enoy them because the rest of the ingredients are decent and I like the taste. But if they put me in the negative I'll give them up. Plus I occasionally see calcium carbonate on the label of other products and want to be informed for future reference.

The carbonate is safer than the hydroxide, which is actually caustic. Both the hydroxide and the carbonate will neutralize stomach acid, but the carbonate does not burn the tissues the way calcium hydroxide can.

 We have a similar problem here. Our soil is loaded with limestone and a cement hard clay called caliche, which is comprised of calcium carbonate and magnesium carbonate.

These carbonate based waters are not going to be good for the stomach acid, but you can squeeze a little lemon juice or add a little citric or malic acid to the water to remove the carbonate.

To balance out the excess calcium you can increase your intake of green leafy vegetables, which will provide more magnesium.