So we've been looking for a turn of the century period / character home as a PPOR that we can renovate. I took this photo at a home open today. There were a few spots (photographed one was by far the biggest one, others were about the size of my hand) where the plaster has come away from the wall and the mortar has turned back into sand. It gives the illusion of sand pouring out of the walls. What causes this (guessing salt damp?) and how is it fixed? Is it possible the fix is needed everywhere as opposed to just the plaster-exposed areas?