I've been meaning to make similar (if coarser) walls out smashed plaster bits. There was a thread on here maybe a year or more ago that had that and a beehive hut made the same way. Can't seem to find the thread though!
What I recall is you get a large baking pan and fill it with a thin layer of plaster, then once the plaster sets, you smash it to bits, and roll the bits in a big to take the edge off a bit (to your preference), then stack the bits in a wall, or whatever other shape, base 'em and paint!