I think the algae or mold (aspergillus niger?) has penetrated into the
porosity of the slabs. Standard algae killers-cationic surfactants will
inhibit and remove surface material . I have found that this plus a stiff
scrubbing brush works with the algae in our location.
Many years ago I was investigating degraration of sand resin composites and
could see the penetration of black mould into the surface porosity. A small
amount of mould gives a lot of stain.
Regards
David

" Emrys Davies" wrote in message
...
Try brushing one of the spots with neat Jeyes Fluid and I think you will
have some success.

Regards,
Emrys Davies.


"Tumbleweed" wrote in
message ...
In particular, its a pernicious black type that grows in 'spots' that
resist
all but the most determined powerwashing, in fact the concrete goes at
the
same time. Power washer, bleach,and jeyes fluid are all proving no
match.
Any suggestions? Are there algae cleaners you can buy that are any
better?

--
Tumbleweed

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