Most people have seen "efflorescence" even if they didn't know what it was or that it even had a name. The whitish build-up visible on masonry structures is caused by moisture moving through the material. As the moisture evaporates from the surface, the salts carried in the moisture builds up to create what we call "efflorescence." It is very common on foundations below grade that have not been adequately sealed on the exterior or that are for some other reason being impacted by sub-surface water. Moisture can also wick up through the foundation footing and then evaporate out through the wall. While mostly cosmetic (in some cases it can lead to spalling and weakening of the concrete or brick structures), the main thing that is indicated by efflorescence is: "there is moisture in the material." The picture at left shows typical efflorescence on a basement wall. The picture to the right shows where efflorescence was created in areas that were not covered by the plastic ground cover (the plastic ground cover has been removed for picture taking purposes).
This next picture shows a "close-up" of the delicate structures that are sometimes created. The picture on the right is severe efflorescence on the face of a fireplace.
Efflorescence is also very common on the faces of brick chimneys. This is usually the result of failing chimney mortar caps, missing flue hats, failing mortar joints and/or cracks that can also allow moisture into the chimney structure. The moisture can then travel through the chimney structure and come out in other parts of the chimney where efflorescence will build up (it might only be visible in the basement or crawl space). The chimney in this next picture almost looks like it had been painted white and now most of the paint is gone. But this is also efflorescence---due primarily to a mortar cap in poor condition and no hat on the flue.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board