It is interesting how there ended up being bubbled areas. What caused that? I was going to guess that it looked a bit like cardboard puckering, but that doesn't seem to be the case as the building looks like it is foam? Perhaps the grout mix wasn't blended well enough? Honestly, I never have played with the stuff but I'm curious to hear what your experience has shown.
For cement textures I prefer to use texture pastes found in art stores. Not as heavy, applies mostly like paint, and has been very easy to work with.
Graffiti, posters, and a few other greeblies will really make this building shine. Maybe sponge paint on some lighter/darker spots or even some green for moss or lichen? Breaking up the field of grey will help, however you set about it. Maybe throw up some boarded up windows? No need to make an actual window, you can just slap a bunch of boards up to give the appearance of a window being covered? That would add some browns higher up on the wall. Maybe add some sand or fine gravel around sections of the base (or roof) where you don't anticipate placing scatter terrain against the walls? You could also add some grates or access plates here and there. Even down near the ground like windows looking into a basement level, perhaps. Lots of room to tinker!
Great work!