DIY rock

[BCOrchidGuy]

I'm going to be makeing some of my own aragocrete rock, I've actually done some but I keep running into a problem and I'm wondering if anyone else has had the issue.
I'll make the mix up, 1 part white concrete 5 parts aragonite gravel, then I'll add just enough water to make it wet, not sloppy. I'll spoon the mixture out onto a bed of aragonite gravel, then fill any areas I want to keep hollow and then top it with more of the mixture, then cover the works in aragonite gravel. I'll wait 24-36 hours before I remove the peice but it always breaks, it's always still damp. I'm using a foam box as suggested by Garf. Just wondering if anyone has a better idea?
How about curing, anyone try muriatic acid in water for a couple days then fresh water? I know folks have done vinegar.

Doug