Ive done this. Works well. I used rock salt but I think it can make it too porous and frail.
Pretty easy just make a good thick mix (not to runny). Create shapes in sand pour in the mix let it cure. You get more interesting results if you make a shape in the sand pour in the mix cover it with sand leaving some still exposed and add more mix. Slowly building it in all three dimensions.
For the mix I would use some rocksalt but mainly on the outside becuase it will leave u with real cool texture, Argonite, best thing if you can get it are plastic shavings, live rock rubble.
Feel free to experiment with what you put in the mix and how you create your shapes thats part of the fun. It takes a few days to cure but when you your rock out of the sand and you do not like just toss it. There cheap and easy to make.
After a couple weeks you will have a collection of pieces that are really interesting I went for long branching tonga like structures.
The crappy part is they take months of soaking in water to leach off the cement chemicals.
Good luck take pictures and share.
