Calcium carbonate is probably the most common type of scale found in cooling water systems; it forms an extremely tenacious heat insulating deposit.

Calcium and bicarbonate alkalinity are found in almost all cooling waters. The addition of heat, or a sharp rise in pH, will cause calcium bicarbonate to decompose and form carbon dioxide and calcium carbonate.

Although the bicarbonate salt is moderately soluble, the carbonate salt has very low solubility. The figure below shows the relationship of temperature to the solubility of calcium carbonate.

Note that calcium carbonate is extremely low in solubility at any temperature. It is logical that the deposition of calcium carbonate is found along heat transfer surfaces where cooling water is subjected to the highest temperatures.

Due to problems associated with calcium carbonate scale formation, it is important that the cooling water control parameters be such that the potential for CaCO3 formation is minimized. Probably the best way to qualitatively predict the formation of CaCO3 scale is through the use of the Langelier Saturation Index or the Ryznar Stability index, both of which are discussed in more detail in link.