Updated: Jan 30, 2019
Efflorescence is the white chalky powder that you might find on the surface of a concrete or brick wall. It can be a cosmetic issue, or it can be an indication of moisture intrusion that could lead to major structural and indoor air quality issues.
Indications of Moisture
Efflorescence (which means "to flower out" in French) is the dissolved salts deposited on the surface of a porous material (such as concrete or brick) that are visible after the evaporation of the water in which it was transported. The moisture that creates efflorescence often comes from groundwater, but rainwater can also be the source. Efflorescence alone does not pose a major problem, but it can be an indication of moisture intrusion, which may compromise the structural material.
Porous Building Materials
Building materials, such as concrete, wood, brick and stone, are porous materials. Porous materials can absorb or wick water by a process called capillary action. As water moves through the porous material, salts can be drawn with it.
Concrete, wood, brick, stone and mortar are porous materials that contain salts. The ground in which these materials can come into contact also contain salts. Capillary action can literally suck water and transport it through porous building materials.
Porous building materials are capable of wicking water for large distances due to capillary action with a theoretical limit of capillary rise of about 6 miles. That’s 6 miles directly up. Think of a tree and how a tree can transport water from its roots to its leaves. That’s capillary action. And it’s very powerful. When you add salt to that capillary process, it can be destructive.
Salts dissolved by groundwater can be transported by capillary action through porous soil. Building materials in contact with soil will naturally wick the water inward and upward. Take concrete footings -- they are typically poured directly onto soil without any capillary break. Sometimes this is called rising damp. This is the beginning of how water can wick upward into a structure.
When the capillary flow of water reaches the surface of a building material, evaporation occurs. As the water evaporates, salt is left behind. As this evaporation of capillary flow continues, the salt concentration increases, which creates an imbalance, and nature abhors imbalance and always wants to put things back into equilibrium. This is process is called osmosis. To re-establish equilibrium through osmosis, water rushes toward the salt deposit to dilute the concentration. This rush of water creates massive hydrostatic pressures within the porous material, and these pressures are destructive. The pressure from osmosis can create incredibly strong hydrostatic pressure that can exceed the strength of building materials, including concrete.
Here are some examples of how that pressure translates:
diffusion vapor pressure: 0.3 to 0.5 psi
capillary pressure: 300 to 500 psi
osmotic pressure: 3,000 to 5,000 psi
As you can see from the list above, osmosis can create pressure that is greater than the structural strength of concrete, which can be from 2,000 psi to 3,000 psi. The action of water rushing to the surface due to capillary action creates incredible forces that can cause materials to crack, flake and break apart.
When efflorescence leads to strong osmotic pressures—greater than the strength of the building material—and the material literally breaks apart, the resulting damage is called spalling. Hydrostatic pressure can cause spalling, but spalling can also be caused by freeze-thaw cycles in building materials that have a high moisture content. Both efflorescence and spalling can be prevented with capillary breaks, such as by installing a polyethylene sheeting under a concrete slab.
Prevention and Removal of Efflorescence
An impregnating hydrophobic sealant can be applied to a surface to prevent the intrusion of water. It will also prevent water from traveling to the surface from within. In cold climates, this sealant can cause material to break during freeze/thaw cycles.During home construction, bricks left out overnight should be kept on pallets and be covered. Moisture from damp soil and rain can be absorbed into the brick.Install capillary breaks, including polyethelene sheeting between the soil and the building material, such as concrete.
Pressurized water can sometimes be used to remove or dissolve efflorescence.An acid, such as diluted muriatic acid, can be used to dissolve efflorescence. Water should be applied first so that the acid does not discolor the brick. Following application, baking soda can be used to neutralize the acid and prevent any additional damage to the masonry. Muriatic acid is toxic, and contact with skin or eyes should be avoided. A strong brush can be used to simply scrub the efflorescence off.
NOTE: The use of water to remove efflorescence may result in the re-absorption of crystals into the host material, and they may later reappear as more efflorescence. It is advisable that if water is used in the removal process that the masonry is dried off very quickly. In summary, efflorescence is a cosmetic issue, but it indicates a potential moisture problem. Inspectors should know the how capillary forces can cause structural damage to building materials and educate their clients about efflorescence and the potential problems it may cause.
This article originally appeared on nachi.org and is authored by Nick Gromicko and Ben Gromicko. It is used with permission.