That's going to be pretty brittle. what about a wire mesh for reinforcement? Looks at some plaster meshes.

i've had success doing this by essentially replicating tilt-up methods. instead of foam-core i used a 1/4" mdf slab as a casting bed that was re-usable and i used ripped mdf strips (finished edge facing the casting) that were sacrificial formwork. i used just enough spray-mount on the thin strips to get it to stick to the mdf slab and then ran a bead of sealant around the outside to anchor the strips down and make a dam. if your cutting craft is good, you shouldn't have a bad leaking problem.

i sprayed oil onto the surfaces as a release agent. i used plastic insect screen material as a reinforcing mesh, sandwiched between two pours done right after the other to avoid a cold joint. do not try to push the mesh down into the rockite.

after curing, gently remove the edge strips and if the cast didn't pop right off, you can very gently warp the mdf to break the bond.

expect to play a bit with the mix to get the slump right.

Allow lots of time for trial and error.  And probably several duplicates of your formwork.  

I saw too many Rockite nightmares in school, and lived a few myself.  If I had it to do over again, I would use plaster and paint it concrete color.  Or maybe even some type of sheet material.   Especially if working at 1/2 cm thickness.

To quote a great old architect friend when I showed him our Rockite school assignment:  "Son, Rockite is not concrete, it's grout."

That is very thin and not sure at that thickness how you would successfully embedd mesh into it without it surfacing on rather side. Make sure you have the right ratio nailed down, that will help with durability. If you're using foam board, seal the hell out of the joint at the bottom and build out structure to keep the wall from bowing out. That and a few prayers ought to get you there.