gpsman wrote:
On Mar 17, 7:02 pm, Ray K wrote:

Can anyone identify the problem on some of the cinder blocks at the base
of the outside wall of my garage? I'm wondering what the black stuff is.
It doesn't rub off, and it's unaffected by bleach.


If it's as hard as a rock, so hard a steel-bristle brush has little
effect, rub it with a vitamin C tablet. If that has an effect, it's
black algae, and a swimming pool supply will have remover/s.
http://www.intheswim.com/Pool-Chemic...lgae-Treatment
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- gpsman

The black spots are as hard as a rock, but the only effect the vitamin C
tablet had was to whiten the black, like rubbing ordinary chalk on a
sidewalk. I also dampened the vitamin C with water; no effect.

Thanks for the suggestion.

Ray