Is calcium carbonate a base or an acid?

Neither. It's a salt of a strong base Ca(OH)2 and a weak acid H2CO3. CaCO3 (calcium carbonate, a.k.a. chalk) doesn't dissolve in water, so it doesn't behave like a base. read more

CaCO3 (calcium carbonate, a.k.a. chalk) doesn’t dissolve in water, so it doesn’t behave like a base. All acids stronger than H2CO3 (provide their calcium salts dissolve well in water) react with calcium carbonate, yielding CO2. read more

The reaction of an acid with calcium carbonate is an example of an acid-base reaction. In these reactions, an acid will react with a base to form a salt and water. The acid was neutralized by the base. There are really two steps in the reaction of a carbonate (or a bicarbonate) with an acid. read more

Calcium carbonate is alkaline, which is why it's used in many antacid products. Under normal atmospheric conditions, a 0.00047 M (0.019g/L) solution of calcium carbonate has a pH of 8.3 read more