Question: Water used to mix mortar for a brick masonry wall was pumped from a reflecting pool. The water was clear and we did not believe that it would be a problem. After the wall was completed we learned that a significant amount of copper sulfate had been added to the water to control algae.

Will copper sulfate affect the performance of the mortar?

Answer: Copper sulfate can adversely affect mortar. Copper can retard the set of the mortar but should not affect the performance if the curing time is extended. Sulfate also can affect hardened mortar.

If added in sufficient quantities sulfates can cause the mortar to deteriorate. This deterioration occurs when the sulfates react with calcium hydroxide and calcium aluminate compounds in the mortar. These reactions produce gypsum and ettringite respectively. Both reactions are expansive and cause damage to the mortar and brick.

Copper sulfate used for algae control in a reflection pool, however, would not likely be added in large enough quantities to cause a problem. If it was present in high enough concentrations to cause future deterioration of the mortar due to gypsum or ettringite formation, the mortar would not likely set up during construction and the work would have been stopped.