What is That White Powdery Stuff on My Brick Wall?

Efflorescence is mostly cosmetic but can lead to structural problems if not treated properly.

In chemistry, efflorescence (which means “to flower out” in French) is the loss of water (or a solvent) of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air. Efflorescences can occur in natural and built environments. On porous construction materials it may present a cosmetic problem only (primary efflorescence), but can sometimes indicate serious structural weakness (secondary efflorescence). – Wikipedia 

We know that efflorescence is a fine, white, powdery deposit (see photos above) of water-soluble salts left on the surface of masonry as the water evaporates. These efflorescent salt deposits tend to appear at the worst times, usually about a month after the building is constructed, and sometimes as long as a year after completion. Three conditions must exist before efflorescence will occur. If one is left out, efflorescence cannot form.

Three Things Must Be Present (or it won’t form)

• There must be water-soluble salt in the wall.
• There must be enough moisture in the wall to dissolve the salt.
• There must be a path for the salts to migrate through.

HOW TO PREVENT

• Using a low alkali Portland cement 
• Use clean sand when making mortar and grout. Sands high alkali sulfates increase the likelihood of efflorescence. Use clean, washed sand.
• Use potable water. Clean, potable, salt free water must be used at all times.
• Smart building design keeps water OUT – overhanging eaves, copings and flashings, and careful attention to landscaping and sprinklers reduce the chances of water entering the wall.
• Use a “tooled” compacted mortar joints (concave or “V” type) will also reduce the potential for water intrusion.

HOW TO REMOVE

• Dry brush and sandblasting should be used as a last resort. Both can damage the brick finish and open the brick and mortar to more water.
• Use a chemical wash. Muriatic acid in a mild solution, usually one part muriatic acid (hydrochloric acid, HC1) to 12 parts water. Pre-soak the wall with clean water and flush/wash the wall good afterwords. NOTE: Some brick finishes could be damaged by acid wash. Call your brick manufacturer.
• Sealing the brick will help, but it’s best to wait until the condition stabilizes.

More about cleaning…

Care must be taken not to trap the salts below the surface of the masonry. This condition is known as crytoflorescence. If the salts are stopped just below the surface, for instance by a silicon water repellent, the water will still evaporate, depositing the salts behind the surface, which then crystallize. The expanding salt crystals can exceed the tensile strength of the brick causing spalling or disintegration of the brick.

South and West facing walls are normally less prone to efflorescence since the sun exposure moves the point of evaporation further into the wall. The point of evaporation is where efflorescence occurs. On the other hand, North and East facing walls are normally cooler and the point of evaporation remains on the surface of the wall where the efflorescence occurs.

It is not a cure-all to simply seal a wall when efflorescence already exists. The presence of efflorescence shows that the salts are already in the wall, have sufficient water to be made soluble, and that migratory paths exist for the salt solution to travel through to the surface. It would be better, if possible, to wait until the efflorescence problem has been reduced to a minimum before sealing the wall.

Efflorescence is a controllable condition that should not be a problem in modern masonry. Breaking the chain of conditions necessary for efflorescence can be done with good details, the correct materials and quality construction. – Masonry Institute

Soure: http://www.masonryinstitute.org/pdf/612.pdf