There are two types of builders sand
soft for use in mortar no good for gardening the edges of the sand
particles are rounded off giving poor drainage.

Sharp sand for use in fine Concrete floor screeds etc.
This is ideal for potting composts and general gardening use it is
normally washed before delivery to removes salts etc. that could
comprimises its use in Concrete.

Sharp sand has sharp edges providing good drainage I have just dug five
tons into a bed I am preparing for some Lilies.

jw 111 wrote:
Is there really any reason why builders sand should not be used rather than
the more expensive horticultural sand, in making up a soil mixture for pots?