News & Events

Masonry Walls – The Why and How of Stains

by Keith Pashina, P.E. and Steve Talafous, P.E. as printed in the CSI-MSP Chapter September 2011 newsletter. Reprinted with permission.

Masonry wall stains can and often do occur. For some buildings, it’s a blemish on the building. For other buildings, it can be a symptom of more extensive or significant problems.

White stains on walls, called efflorescence, are one of the most common staining problems on masonry walls. Efflorescence refers to the deposit of water-soluble compounds that are carried by water through the masonry wall and eventually deposited on the wall surface in crystalline form, which is usually white. Water may come from rain, melting snow, or warm moist indoor air moving through the wall. Masonry materials are permeable to water to some extent, and masonry walls also naturally tend to absorb water through the mortar joints and masonry units.


(above) example of efflorescence


How does efflorescence occur?
The most common explanation is that when water passes through the wall, it encounters water-soluble materials, and the water and dissolved compounds can easily reach the outer wall surface. However, another such form of efflorescence is when the water-carried compounds crystallize below the surface of masonry (cryptoefflorescence). The water-soluble materials are naturally occurring in brick, concrete masonry, mortar, stone, and other secondary sources. As these materials dissolve into the water, they migrate to the outside wall, and form the familiar white patches of dusty powder on the masonry surface. The chemical constituents of efflorescence are usually two or more of the following elements: sodium, potassium, calcium, sulfates, carbonates, bicarbonates, chlorides, and hydroxides. Efflorescence materials are easily removed and are water soluble.
 

(above) excessive water movement is not only causing efflorescence and calcium carbonate stains, but the freezing water is damaging the surface of the brick units

Efflorescence should not be mistaken for one of the most common stains is calcium carbonate (CaCO3), which is essentially the same material that causes hard water stains. The calcium carbonate stains result from calcium hydroxide suspended in water within the masonry, which is then transported to the masonry surface, and forms after the water evaporates and the calcium hydroxide contacts atmospheric carbon dioxide. Unlike efflorescenced, calcium carbonate stains are not water soluble. These stains must be removed by a significantly high concentration of an acidic cleaner that requires the correct product and knowledge to apply properly. Used improperly, the masonry surface can be damaged.

Other stains may be dark brown or rust-colored. These stains can result from a variety of sources, including corrosion of embedded metals, other materials in the masonry wall (such as certain flashings, sealants, or joint materials), or from deposits of dirt, soot, or biologic materials. The deposition of airborne dirt is an excellent growth medium for molds, algae, and other biologic sources.

(above) the brown stains on this brick masonry may be indicating a corrosion problem with the steel shelf angle and lintel above.

Efflorescence is generally more of an aesthetic concern, and usually doesn’t physically harm the masonry. However, it is indicative of unintended water movement which may be detrimental to th e wall. Stains, however, can indicate significant problems. If the stains are resulting from corroding metal, there are potentially serious concealed problems.


(above) the green stains on this limestone wall are from the coper-clad ventilation louver above). Special cleaning materials would be necessary to remove these stians.

The presence of efflorescence or staining is significant as it indicates water movement through the wall. The same water movement may be a symptom of other problems that could include leaking roof parapets, roof, or joints, corrosion of supporting shelf angles and lintels, or building wall air barrier problems. These problems will require further investigation to locate the cause and severity of the problem.

There are also instances where formation of the deposits beneath the surface of the immediate masonry surfaces can deteriorate the masonry, and is known as cryptoeffloresence, which may damage the wall surface in the form of spalls (a spall is the breaking off of the surface in fragments). The formation of crystals within the masonry results in extremely high expansive forces that cause the distress. Many impermeable coatings can trap the compound-carrying water and result in the occurrence of cryptoefflorescence. For this reason, careful selection of wall coatings is suggested.

Stains and efflorescence can be cleaned by good practice cleaning techniques. The general approach is to use the gentlest method first, followed by appropriate chemical or abrasive methods as needed. Careful selection of the methods, materials, and contractor will result in a clean wall. However, in most cases, if the source of water entry is not corrected, it is likely to reappear.

For further information on efflorescence and the methods available to reduce its formation, we suggest referencing ASTM C1400, “Standard Guide for Reduction of Efflorescence Potential in New Walls.”