I think i saw a guy do something like that with Styrofoam. The same guy bought this other stuff that looks like stone. https://www.youtube.com/watch?v=NJ0a2eaOg5E
the stuff that fake rock is made of is a type of Styrofoam in the video. you can use cement with just sand aggregate or you can use floor tile grout but you will have to seal both. you can make a frame out of Styrofoam or wood both will need a wire mesh on them to hold the cement or grout.
go with black pond and waterfall great stuff foam you can get it in the pond section of most home depots. get a spray bottle and shoot your foam then mist it gently and it will instantly set the skin. I used to make logos for companys for salt water tanks out of the stuff its nontoxic after 24 hours.