David W.E. Roberts wrote:
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.

It definitely isn't just general filth. On otherwise spotlessly clean
slabs there are black circles about the size of a pea on average that
stand up slightly above the surface as if black paint has been spilled.
On one of the slabs some coloured paint has been spilled at some point
and it gives a very similar effect.

I had resisted using a pressure washer as the patio is just layed
straight onto sand with sand between the slabs and I didn't want to
blast the stuff away, but I guess I'm going to have to give it a try.