Q.: We have to recommend a suitable coating for concrete tanks. Some will hold solid ammonium nitrate and some solid ammonium sulfate. What will happen if the coating breaks? Will these salts affect the concrete? If the salts do affect the concrete would the effect be diminished or eliminated if the salts were kept dry?
A.: Ammonium nitrate and ammonium sulfate, though they are salts, both behave as acids when moist. Therefore either of these salts in the moist condition would erode concrete. In addition, sulfates react with concrete to cause disruptive expansion, so ammonium sulfate attacks concrete both by erosion and disruptive expansion. If the salts were completely dry their effects would be minimized. However, most concrete, even though dry on the surface, contains a considerable amount of moisture. This moisture could evaporate from the concrete, condense in the salt, and produce enough salt solution to attack the concrete.
What is needed is a coating that, in addition to being immune to these salts, will span any crack that forms after the coating has been applied. Perhaps it should have good abrasion resistance as well. Since you are the coatings expert, and are familiar with the expected service conditions, we leave the specific choice to you.