A better way would be to silicone foam together and carve it into whatever shape you want. Then paint concrete over it. It's the same way I did my 3d background that's still currently a work in progress when I get around to it. It takes a while to cure before it's aquarium safe, but the possibilities are nearly endless.
It would look a little more natural and less... painted.. but I got impatient and stopped paying attention to what I was doing.