The first major reason would be that the basement wall has gotten into direct contact with a high concentration of moisture. Water build up, rain, flooding, or just the depth of your basement relative to your water table can cause this darkening of your basement wall. This typically doesn’t go away, but it will change with seasons (sometimes).
Second reason this could be happening is that you have Mold growth on your foundation walls. The combination of factors that could be adding to this would be a plaster coating on your foundation wall (skim coating) and moisture contact with that surface. Mold won’t grow unless there are spores present to germinate on the organic material and water to drink. Mold tends to also enjoy darker areas, so this might happen more behind solid objects like a bookshelf or appear in the furthest corners away from major light sources.
Third reason this could happen would be Mildew. Like mold it will need moisture and organic material found in the skim coating covering your foundation walls. It is slick and if forming on your basement floor can be extremely hazardous.
Forth reason is a staining caused by an oil-based product. Heating oil, lubrication oil, paint thinner or gasoline are all things that can cause staining on concrete. These stains are extremely hard, if not impossible, to remove from the surface of concrete. Other things can color your basement walls like paint or wood stains. Whenever working with materials such as this make sure to use a drop cloth to protect from spills.
If you see that the stain is rust colored this could be a sign that your rod ties are leaking/seeping water.
If there are white efflorescent patterns (normally look like ocean waves) on your wall, this is a tell tail sign that water has, in fact, evaporated from that surface. This also means that water is finding its way through your foundation wall, however slowly it might be.
Photo thanks to Mayang.com
Related Reading and Links
Read about the Top 6 Basement Mistakes that contractors and homeowners make.
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