Discoloration of Concrete Surfaces

Reasons for Discoloration

Causes of discoloration include changes in the cement composition, inadequate mixing, hand-troweled areas, improper curing technique, hydration, calcium chloride use and changes in the alkali content of the cement mixture. Any of these potential issues may cause blotting and inconsistent coloring in a concrete surface.

Slag cement, also called ground granulated blast furnace slag or ground slag, is a white powder formed by the reduction of iron and steel. When ground slag is added to cements, the resulting surface may add yellowish and green hues in the concrete. These hues will eventually fade over time and should not be a major concern.

To prevent discoloration, avoid or limit cements with high alkali and calcium chloride contents. Cements with calcium chloride or other chloride mixtures are the most common reason cements will discolor. Avoid changing cement types during the process to ensure the same mixture and properties of the cement stay the same. Lower the amount of hand-troweling. Particles from the trowel can discolor the cement. Cure the concrete as uniformly as possible using curing sprays and keeping areas exposed to the wind and sun into consideration.

Discoloration can be removed in some cases. A mixture of 1 percent hydrochloric acid or 3 percent solutions of weaker acids may be added to the discolored areas of the concrete. The concrete should be flushed with water before adding acidic solutions and flushed again after the solution has been applied for 15 minutes. Removing discoloration may take multiple attempts. Some discolorations will fade with time and wear. Other discolorations, such as hand-trowel burns, will not fade or be removed by acid.