Nick Maclaren muttered:

Don't use builder's sand! But do use sharp sand from a builder's
merchant. No, it's not treated with chemicals.

We were quite lucky with the tons of 'topsoil' we had delivered... as
topsoil it was rubbish, they'd mixed in 50:50 with a load of sand to bulk it
out. However it was ideal for the clay and builders' rubble we spread it on
for our lawn, which, 2 years later, looks great.