"Nick Maclaren" wrote in message
...
In article ,
Phil L wrote:

Me too, and on concrete which are both alkaline based, nevertheless, wet
sand/cement seems to remove it, provided of course that it is brushed and
washed away.

Alkalinity is not on or off. Calcium carbonate IS alkaline, but
is a very weak alkali; calcium hydroxide and sodium carbonate
are MUCH stronger alkalis.



Concrete is mildly alkaline, but the pH at the surface is lowered by
exposure to acidic precipitation (the process is, confusingly, called
'carbonisation', but isn't the same sort of 'carbonisation' used in the
production of coke or steel). That's why steel reinforcement in, for
example, line posts, starts to rust and expand, spalling off the concrete -
it happens when the 'carbonisation' penetrates down to the metal. If the
nominally alkaline surfaces on which algae have been observed have been
significantly neutralised by acidic pollution, perhaps the Spp concerned
don't have to be as lime tolerant as might be expected, after all?