There is a thin translucent white film on the face of the brick walls in many locations on a recently completed building.

This film often contains many small cracks and is flaking off the face of the wall.

What causes this film?

A: The translucent white film on the surface of a newly completed wall may have many causes. One possible source is the cleaning process. If the cleaning solution contains an acid and the walls are not thoroughly rinsed following cleaning, salts formed when the acid reacts with the mortar can be deposited in a thin film over the face of the masonry. The film can crack or flake off, depending on its thickness.

The acid also can etch the surface, which increases the water penetration through the face of the joint. This action can lead to leaching of calcium hydroxide by the rinse solution and cause the hazing. When the film is caused by leached calcium hydroxide, it reacts with carbon dioxide in the air to form calcium carbonate, which is insoluble. This chemical reaction is known as carbonation. When this thin film carbonates it is very difficult to remove, except with the use of an acid.

The masonry should be installed as clean as possible to avoid excessive mortar smears on the face that need to be removed with cleaning solutions. I usually recommend that the walls be cleaned using a trisodium phosphate-type detergent. This method does not dissolve the cement paste and is used only if the wall is reasonably clean in the first place. If acidic cleaners are used on the wall, it is important to thoroughly saturate the surface prior to cleaning and thoroughly rinse afterward to avoid this and other problems.

In addition to the possibility of developing a white film on the face of the wall, the use of cleaner solutions containing hydrochloric acid can lead to excessive chloride ions penetrating into mortar. Mortar with high concentrations of chloride ions accelerates corrosion of the embedded steel. Proper use of saturation and rinsing may prevent excessive chloride ion penetration. However, the use of acid solutions can damage building facade materials, and most acids used in cleaners are also harmful to humans and the environment.

The white film may also be an efflorescence problem known as ônew building bloom.ö New building bloom occurs when water within the brick and mortar during construction evaporates and deposits salt or calcium hydroxide or both on the face of the wall. This problem usually occurs within the first few months after the completion of the wall.

If there are no detailing or construction problems that let excessive water penetrate the masonry walls, efflorescence should not return after it is removed. If the efflorescence is composed of soluble salts, it can be removed with water under light pressure or with water and a stiff fiber brush. If the efflorescence is insoluble calcium carbonate, acid is required to remove it following proper saturation and rinsing procedures described above.

If efflorescence returns following several cleanings, there is likely a problem with water penetration or with condensation and moisture migration from humidified interior air. In this case the source or sources of the moisture problems must be identified and eliminated.

The film may be an organic coating applied to reduce water penetration at the surface of the masonry walls. The problem is particularly acute if the coating was applied while the masonry was wet. The white haze may be the coating or moisture behind the coating.

A chemical analysis should be performed on the film to determine what caused it. The method of removal depends on the chemical composition of the film. If the problem is the result of water entering the wall system through poorly designed or constructed details, repairs are required to prevent its return in the future.