We have the same black mould problem on both patio and stone walls
...........however no amount of pressure washing shifts them ........do not
know what chemical to resort to but sure as hell they need dealing with next
spring ........maybe if i do get them clean i need to seal them all with
Thompsons or something
"David W.E. Roberts" wrote in message
...

"Ben" wrote in message
...
I've asked this question before on this group, but having tried people's
suggestions without success I thought it was worth another go.

I have very unsightly black mould spots on the concrete slabs on my
patio and garden path. An ordinary scrubbing brush has no effect, a lot
of hard brushing with a wire brush has limited effect but has now
started ruining the surface of the concrete (without really bringing off
the mould) so I daren't do it any more. Chemicals I've tried without
success include algae removing patio cleaner, jayes fluid (including
neat), bleach (potassium hydroxide type and sodium hypochlorite type -
including neat), sodium hydroxide, and a sodium dichlorophen fungicide.
Any other ideas I could try?

Anyone who would like to suggest that brushing sand into it would work
where wire-brushing the surface of the concrete off has failed is
welcome to come round and try it.

Pressure washers are good for cleaning concrete.
IMHO they are certainly more effective than wire brushing.
They do tend to lift anything loose, but they bring concrete slabs up like
(almost) new.
Perhaps it isn't mould, but just general filth settling in to the fine
holes
in the surface of the concrete.
I find our slabs go grimy over the year, but a good blast with the
pressure
washer makes them much cleaner.

HTH
Dave R