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In my basement, there is a powdery, bright white substance between some of the bricks. It looks like it was something applied on top of the mortar joint. I scraped a couple places where it was falling out, and about 1/8 inch below is hard mortar.

mortar closeup

If I touch it lightly, it falls apart and indeed has been falling off the wall and onto the floor by itself. Basically it's expanding slowly and pushing the coat of paint off the wall.

enter image description here

This wall is below grade. I had a friend stop by who happens to be a mason, and he said it wasn't anything to worry about. But what is the white stuff?

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up vote 11 down vote accepted

The white powdery substance is called efflorescence. It's bascially mineral salts leaching out of the masonry.

The efflorescence itself may not be anything more than a cosmetic problem, however the appearance of efflorescence indicates that there's moisture present in the masonry. This is not entirely surprising--you're talking about an old brick wall below grade.

It may be prudent to monitor the structural integrity of the mortar joints over time to make sure they are not degrading due to the moisture.

The Masonry Institute of America published a nice informative paper on efflorescence, how to avoid it in new construction, and how to remove it from the surface.

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