Calcium carbonate is also used to neutralize large amounts of stomach acids. Calcium carbonate is a good antacid because it is alkaline. Thus, it neutralizes highly acidic stomach juices and is used to treat heartburn, acid indigestion and upset stomach.
Why calcium carbonate rather than calcium hydroxide for neutralizing stomach acid? – http://wiki.answers.com/Q/Why_calcium_carbonate_rather_than_calcium_hydroxide_for_neutralizing_stomach_acid