The message
from "Mike Lyle" contains these words:

Actually - builder's sharp sand is fine. You can wash it if you
like before using it, but it's not critical. What is NOT useful
is the sort of sand you use in mortar.

Live and learn. I didn't know there were two kinds for building. That's
probably why my cathedral fell down.

--
Mike.

If I recall correctly, Sharp sand is from former river beds and glacial
deposition, whereas 'builders'' or 'Mortar' sand is from blown dunes,
has uniform grain size and usually contains salt.

--
Compo