White blotches on basement walls can be a problem in and of themselves but can also indicate a deeper problem: basement moisture. Basement moisture is not something that can be "lived with." It is not enough to dry it up when you see it because the long-term consequences are mold damage and possibly foundation damage. The white blotches themselves are either white mold or basement efflorescence. Either could indicate a need to seal basement walls.
White Mold
White mold is a fungus that feeds off a wide range of organic materials which include many construction materials such as wood. It is a potential health hazard when present in large quantities, especially among people with allergies and respiratory problems. Although mold cannot feed off concrete directly, the dust and dirt that collect on its rough
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Water leaking through the concrete can carry dissolved salts that get deposited on the wall surface after the water has evaporated. Unlike white mold, the salt itself is harmless and is readily cleaned off.
However, simply removing efflorescence does not solve the underlying problem which is the water leakage through the wall. The efflorescence will simply return over time as more leakage occurs, and as mentioned previously, the leakage can cause other serious problems. One exception is efflorescence resulting from the initial concrete placement. If the efflorescence returns after its removal, you have a water leakage problem.
How to Tell the Difference Between White Mold and Efflorescence
Place a tiny amount of the material into some water, if it dissolves, it is not mold. In addition, efflorescence are small crystals that you can crush into a fine powder. Don 't rule out the possibility that you could have both white mold and efflorescence in your basement.
The Sources of Basement Moisture
The moisture source could be condensation from humid air, leaking plumbing, or leakage from your basement