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.


Regards,
Nick Maclaren.