The Natural Philosopher wrote:

If its too wet it slumps.

It can almost be as dry as you like frankly. It will in time set no
matter how little water there is in it.

that makes things easier then, I presume the moisture from the sand will
be more than adequate for "setting" purposes then.

Though they do suggest leaving the heating on for 2 weeks before laying
chipboard on top for it to set.

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