The message
from Phil Armstrong contains these words:

Just don't get sharp sand (for making concrete etc etc); you need soft
sand (or whatever builders call it).

Soft sand stains really badly - it's got a lot of fines in it - almost
clay. This is what makes it so stable when firmly compacted.

I've yet to find a good substitue for "real" play sand - apart from
beach sand.

--
Skipweasel
Pay no attention to that man behind the curtain.