On Wed, 12 Jan 2011 11:52:28 -0800, bob wrote:
But there are also white hairy mold growing out of the concrete! If I
shine an ultraviolet light on them for a few days they disappear. If I
remove the light they grow back. In some area the mold grow so dense it
looks like coral reef in air.

It's very strange because firstly, I thought the concrete has dried.
Secondly, other than water, what's there for the mold to eat?

You've probably still got moisture trapped in there which is slowly
coming back out and allowing the mold to grow; I've had a similar problem
in our basement with a small patch after rainwater got in via the wood-
chute last summer. The concrete might seem dry at surface-level, but it
doesn't mean that there isn't moisture deeper in which is finding its way
back out via hairline cracks in the concrete.

I turn the uvc light back on. When the white mold disappear, I plan to
clean that concrete area with bleach to see if that would prevent them
from growing back when I remove the light. Is there a better solution?

I did mine with bleach (although at a quite low concentration), and the
mold just came back a few weeks later. I'm going to get some concrete-
sealing paint and try that, I think - it can be applied to concrete which
isn't 100% dry and claims to contain stuff that inhibits mold growth, so
it should kill the mold and prevent moisture which might allow more to
grow.

cheers

Jules