The discoloration is likely “efflorescence,” which is an accumulation of minerals and salts on the surface of the block due to repeated bouts of excess water in the material. When concrete block is saturated with water, the minerals that naturally occur in the masonry material are dissolved and, as the water migrates to the surface of the brick and dissipates from evaporation, the mineral deposits are left behind as a thin layer of powder. Repeated saturation and evaporation cycles lead to a buildup of the surface powder. 

    When the block is sealed with paint or another sealer, but moisture has entered the wall anyway, then sometimes the efflorescence will appear along the mortar lines. See our blog post What causes raised white lines of residue on a block wall that are crusty and crumbling? to learn more about this. 

   While the efflorescence is only a cosmetic problem in itself, its appearance on the wall indicates an ongoing water intrusion problem, which can lead to mold growth in adjacent building materials over time. Occasionally, efflorescence is mistaken for mold. Here’s a three ways to tell the difference between the two:

1) When a sample of efflorescence is pinched between the fingers, it will crumble into a powder. Mold will not.

2) Efflorescence grows on inorganic masonry materials, while mold does not; with the exception that a dirt/dust buildup on the surface of a moist masonry wall will sometimes grow mold.

3) Efflorescence will dissolve in water, while mold will not.   One common cause of minor efflorescence is a sprinkler head that sprays on the wall, like in the photo below of efflorescence on a brick wall  A pressure washer and/or a diluted acid solution is typically used for removal of efflorescence, with the surface promptly dried afterwards to prevent reabsorption of the water. 

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Here’s links to a collection of our other blog posts about CONCRETE AND CONCRETE BLOCK:

• Do stucco walls mean a house is concrete block?

• What causes stair-step cracks in a block or brick wall?

• Why is the concrete window sill cracking?

• What causes a horizontal crack in a block or brick wall? 

• What are the pros and cons of concrete block versus wood frame construction?

• What does CBS mean in house construction?

• What is "Ocala" block?

• There's cracks running along the home's concrete tie beam. What's wrong? 

• What would cause long horizontal lines of brick mortar to fall out?

• How do I recognize structural problems in a retaining wall? 

• Should I be suspicious about a concrete block house covered with siding? 

• What is fiber reinforced concrete?  

• How can I tell if cracks in the garage floor are a problem or not? 

• How do I recognize serious structural problems in a house?

• Why did so many concrete block homes collapse in Mexico Beach during Hurricane Michael? 

• How can I tell if the concrete block walls of my house have vertical steel and concrete reinforcement?

• How can I tell if the exterior walls of a house are concrete block (CBS) or wood or brick?

• What is concrete spalling? 

• What is a "slab on grade" house?  

• What causes a vertical crack in an exterior concrete block or brick wall? 

• Why is there a "WARNING! POST-TENSION SLAB" sticker in my house?

• How can I identify a home as ICF (Insulated Concrete Form) construction?

• How can I tell if cracks in the wall or floor are getting worse or staying the same?

• Why do concrete blocks have holes in them? 

        Visit our EXTERIOR WALLS AND STRUCTURE page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.