These acids react with calcium components of the cement, forming calcium sulfate, a voluminous material that is soft and weak. The surface of unprotected concrete disintegrates rapidly. Many other products and compounds can destroy concrete as well. The coating system must protect the concrete from microbiological attack. The coating system must protect the concrete from spalling and cracking caused by moisture penetration.
Moisture enters the concrete and is drawn inside by capillary action.
In cold temperatures the moisture freezes in the pores, swells, and causes spalling. It is also important to note that concrete can vary greatly in its chemical and physical properties from pour to pour. Even pours made on the same day by the same crews using the same materials can vary in porosity and other properties. The preparation of concrete surfaces to be coated is as important as the surface preparation of steel. Laitenance is the most common surface imperfection in concrete.