So there it is, floating concrete, round 1. This is one of the simplest mixtures I've made so far. It's just polystyrene beads in portland cement. The idea is that I could pour this into a mold then coat it in a thin layer of polyester resin, making it waterproof. I'm still not sure that's a good idea because I don't think this would stand up to compression very well, and definitely not to 1000 psi, my eventual test pressure. I will probably add some type of structure to reinforce it. We'll see.