The low pH level of water is a result of numerous factors, including alkalinity, chemical composition (especially presence of carbonates, chlorides, sulphates), and temperature.  Calcite (Calcium carbonate, CaCO3) is a natural substance used to increase the alkalinity of the water and thereby alleviating the corrosive nature thereof. Additionally, an increase in the level of alkalinity of the water will result in increased water stability when subjected to changing conditions.

As the water makes contact with the calcite, the calcite slowly dissolves and increases the alkalinity and pH level of the water.