Why Red Brick Turns White: Understanding efflorescence

Proper pre-wetting and then rinsing are imperative. Chemical cleaning often produces stains from the previously mentioned metallic salts. Lime (white) scum can be a by-product of chemical cleaning and is usually caused when the masonry units are not adequately pre-wetted during washing. The strong cleaning acids can dissolve some of the portland cement matrix, which is then absorbed by the dry masonry and runs down the joints and walls to form a thin white film scum (Figure 10).

Chemical cleaners are usually formulated for certain types of salts and substrates. Some formulations may be for common alkali salts (e.g. calcium carbonate, sodium sulfate, and potassium sulfate) producing the usual whitish/grayish appearance. Other cleaners may be formulated for metallic salts from vanadium, manganese, and molybdenum.

Cleaners used for metallic stains can be more aggressive than those for alkali stains, and there is a risk of damaging or discoloring the brick. Product data sheets usually state what type of salt, and what color of masonry units, are compatible—the wrong cleaner can destroy the portland cement matrix of precast stone.

Before chemical cleaners are used, all loose efflorescence should be removed according to the previously mentioned soft brush removal procedure. Chemical removal must be done carefully and the cleaner and procedures should be tested in an inconspicuous area before proceeding. Pre-testing should not be performed on surfaces other than those being cleaned. This is because other surfaces may contain different salts or the substrate may be of another composition and produce different results than when used on the actual substrate to be cleaned.

Some removers may contain chlorine, which is also a salt and may contribute to the efflorescence problem. The cleaning instructions usually state to flush with plenty of fresh water, putting moisture (and possibly some of the partially dissolved salts) back in the wall, thus creating an efflorescence loop. It is best to minimize the amount of cleaner used to better control the amount of fresh flush water.

Mechanical
Mechanical methods, such as brush-blasting, definitely rid the efflorescence, but they are the most severe. These methods can also take off some of the substrate along with the efflorescence, roughen the surface (making it more porous to trap and hold moisture), and remove mortar matrix around the fine aggregate, lowering the mortar’s water repellency and leading to spalling.

Controlling efflorescence
Simply put, cementitious construction experiences efflorescence because it is constructed of natural materials. Ultimately, it is best to execute all available precautions to prevent it from occurring. However, one cannot have all walls facing south or west, or locate all buildings in the desert.

Efflorescence may not be completely eliminated, but there are steps that can be taken during design and construction to reduce the possibility of its occurrence. The suggestions listed in “Best Practices for Design and Construction” include some of the most common and easiest to implement.

Sealers
Generally, sealers are either film-formers or penetrating (pore-plugging). Just as a lid keeps sauce in a jar, the former creates a thin layer over the substrate’s surface to hold in the salts, stopping salt-laden moisture just below the surface. When this happens, moisture usually evaporates and deposits the salts behind the sealer where they crystallize.

During crystallization, the salts expand and can cause brick to spall or deteriorate. On the positive side, film-formers can prevent moisture from entering the substrate from the exterior. Located on the surface, they are exposed to weather, ultraviolet (UV) light, dirt, and abrasion that shorten their life; they may also form a vapor barrier preventing the structure from drying to the exterior.

Some sealers are formulated to penetrate the substrate and plug the pores to stop the salts from ever reaching the surface. They are usually breathable, allowing the structure to breathe without carrying salt deposits to the surface. In cold-weather climates, freeze-thaw cycles can cause moisture trapped below the surface to expand and spall, or otherwise damage the masonry or joint.

Conclusion
As long as there is moisture, there will be efflorescence. Still, there is no definitive answer as to how long efflorescence will last—its lifespan is tied to weather conditions, moisture induced during construction, and design details. To some observers, it just seems to suddenly appear and then disappear; in reality, it can show itself any time from three weeks to more than a half-year after construction is complete.

It has been said efflorescence is controllable—not eliminable—and should not be a problem in modern masonry. Walls are going to get wet unless there is a giant umbrella over the structure. Cementitious materials come in various levels of textures and finishes and usually have some degree of porosity that is a good source of capillary action to transport moisture. This leaves the alkali salts. Considering the various salt sources, it is virtually impossible to ensure buildings remain on a ‘salt-free diet.’ The various salts can be effectively reduced, but not eliminated. Efflorescence, therefore, can be present, but invisible, lying in a dormant state just waiting for the right conditions to grace a building’s face.

Joseph “Cris” Crissinger, CSI, CCS, CCCA, ASQ, has almost 30 years of experience preparing construction specifications. As director of corporate specifications with McMillan Pazdan Smith Architecture (Spartanburg, South Carolina), he is responsible for evaluating new products, maintaining corporate masters, preparing project specifications, assisting in facility assessments, performing field investigations, and coordinating internal training programs. Crissinger is a member of Construction Specifications Institute (CSI), along with the Building Performance Committee of ASTM International, and the Design and Construction Division of the American Society for Quality (ASQ). He can be contacted via e-mail at ccrissinger@mcmillanpazdansmith.com.

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