Light colored brick are often susceptible to staining from vanadium found within the masonry units. Vanadium oxide and sulfates can create a yellow and green colored stain that can appear on the surface of brick units and mortar. Some tan, gray, brown, or dark colored units contain manganese. Acids liberate vanadium or manganese within the brick units and cause staining. Therefore, acid-based cleaners should generally not be used with masonry made with units susceptible to such staining.
If care is taken during construction to avoid excessive mortar smears, it is possible to clean the face of the wall using tri-sodium phosphate and detergents. This cleaning solution is primarily effective in removing dirt from the surface of the wall.
However, if performed early, some thin mortar can be removed using this detergent with a stiff fiber brush. Generally, such a mild cleaning technique needs to be done within the first few days following installation of the masonry. Cleaning prior to seven days, however, increases the chance of removing mortar from joints since the material may not be sufficiently cured by this time. For this reason, Brick Industry Association (BIA) Technical Note 20 recommends waiting seven days, although it cautions that waiting prolonged time periods makes mortar stains very difficult to remove.
After a week or more, however, my experience is that mortar smears will be very difficult to remove from the face of the brick units without using some dilute acid solution. There are some proprietary cleaning solutions made with mild organic acids, which do not contain hydrochloric acid, that are designed to remove mortar. Such products may be effective in these cases.
These cleaners are used with the “bucket and brush hand method” discussed in BIA Technical Note 20, or by light pressure washing. Thoroughly saturate the surface of the masonry with clean water prior to applying any product to prevent absorption of the solution by the masonry. Enough water should be applied to the surface so that additional moisture sheets off the face of the wall rather than soaking into the surface after several seconds.
Cleaning should begin at the top of the wall and move downward. Windows, concrete, stone, and other surfaces that can be damaged by the cleaning solutions must be protected. Following a cleaning, the wall must be thoroughly rinsed from top to bottom.
Since vanadium and manganese stains can develop with even the use of mild organic acids, it is prudent to clean an inconspicuous test area prior to initiating work on large areas of the wall. The test patch should be allowed to dry for about a week before the area is evaluated.
In projects where the contractor has left excessive mortar smears or has delayed cleaning for several months, mild organic acids may not be effective for use on the wall. BIA Technical Note 20 mentions the use of dry sandblasting to eliminate staining when brick contain vanadium salts or manganese. They caution, however, that with “improper execution, the face of brick units and mortar joints may be scarred.” I do not recommend sand blasting for cleaning masonry walls because of the risk of permanent damage.
The book, Masonry Design & Construction by Harry C. Plummer, stated that “Acid should not be used on light colored brick except in extreme cases.” For these “extreme cases,” the author recommended using not more than 1 part chemically pure muriatic to 15 parts water. This solution is significantly more dilute than the 1 part muriatic acid to 9 parts water dilution recommended for other masonry units.
He went on to state, however, that “Acid should never be used on brick subject to green staining.” As with any cleaning solution, presoaking and thoroughly rinsing of the wall are essential.
I typically do not recommend using hydrochloric acid based cleaners, except as a last resort when other methods have failed. Even if presoaking and rinsing procedures are carefully followed, acidic cleaners can result in staining, acid etching, or long-term problems of corrosion due to the introduction of chloride ions into the mortar.