Surface discoloration is the non-uniformity of color or hue on the surface of a single concrete placement. It may take the form of dark blotches or mottled discoloration on flatwork surface, gross color changes in large areas of concrete caused by a change in the concrete mix, or light patches of discoloration caused by efflorescence. In this context, it is not intended to include stains caused by foreign material that comes in contact with the concrete surface after placement and curing, such as storm water runoff, irrigation, corrosion products, oil leaking from automobiles, etc.
Discoloration due to changes in cementitious materials or fine aggregate sources in subsequent batches in a placement sequence could occur, but is generally rare and insignificant. Concrete with a higher water to cementitious materials ratio (w/cm) will generally be lighter in color. Inconsistent use of admixtures, insufficient mixing time, and improper timing of finishing operations can also cause discoloration. A yellowish to greenish hue may appear on concrete containing ground slag as a cementitious material. This will disappear with time, generally within a one year period. Concrete containing slag cement does, however, have a generally lighter color. The discoloration of concrete cast in forms or in slabs on grade is usually the result of a change in either the concrete composition or a concrete construction practice. In most studies, no single factor seemed to cause discoloration.
Factors found to influence discoloration are: the use of calcium chloride, variation in cement alkali content, delayed hydration of the cement paste, admixtures, hard-troweled surfaces, inadequate or inappropriate curing, concreting practices and finishing procedures that cause surface variation of the water-cementitious materials ratio, and changes in the concrete mixture proportions or constituents.
Certain treatments have been found to be successful in removing or decreasing the surface discoloration of concrete flatwork. Discoloration caused by calcium chloride admixtures and some finishing and curing methods can be reduced by repeated washing with hot water and a scrub brush. The slab should be alternately flushed and brushed, and then dried overnight until the discoloration disappears.
If a discoloration persists, a dilute solution (1% concentration) of hydrochloric (muriatic) acid or dilute solutions (3% concentration) of weaker acids like acetic or phosphoric acid may be tried. Prior to using acids, dampen the surface to prevent it from penetrating into the concrete and flush with clean water within 15 minutes of application.
The use of a 20% to 30% water solution of diammonium citrate (2 lbs. in 1 gallon of water) has been found to be a very effective treatment for most discoloration. Apply the solution to a dried surface for 15 minutes. A whitish gel that forms should be diluted with water and agitated by brushing. Subsequently, the gel should be completely washed off with water. More than one treatment may be required.
Some types of discoloration, such as trowel burning, may not respond to any treatment. It may be necessary to paint or use another type of coating to eliminate the discoloration. Some types of discoloration may, however, fade with wear and age.
Chemical methods to remove discoloration may significantly alter the color of concrete surfaces. Inappropriate or improper use of chemicals to remove discoloration may aggravate the situation. A trial treatment on an inconspicuous area is recommended. Acids should be thoroughly flushed from a concrete surface.
The user of chemicals should refer to a Material Safety Data Sheet (MSDS) or manufacturer guidelines to be aware of the toxicity, flammability, and/or health hazards associated with the use of the material. The appropriate safety procedures, such as the use of chemical resistant gloves, goggles, respirators, and chemical resistant clothing may be required in the MSDS.
