Floating Concrete!

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.