Calcium carbonate is neither an acid nor a base, but it is a BASIC SALT. The reason is that Calcium carbonate, CaCO3 is a salt of metal and all salts of metals, when dissolve in water they form basic solution. Most of the people are confused about the basicity or acidity of it because of the presence of Carbonic ion(which is usually acidic in nature). Calcium carbonate solution of 0.00047 M (0.019g/L), under normal atmospheric conditions has a pH of 8.3. It is also often used as an "antacid but it is basic in nature.