Here is a foam and conrete (pump driven waterfall) background that is in my mini terrarium. the pump is hidden behind in a cubby hole cut into bottom of foam.
styrofoam was cut, glued, broken, layered, etc.
coat styrofoam with watery/ thinly miced quickcrete in light layers allowing to dry between coats.
painted.
sealed.
that's it. pretty simple. if the concrete is mixed too thick it will crack while drying which is bad, so be patient and use thin watery layers. do not premix all of your concrete either, just what you need per layer. I used a large old paintbrush to "dab it on" for a more textured look.
