Hi folks! I'm fairly new to the renovating game. I've done virtually our entire apartment (previously untouched since the 50s) and am now taking on the challenging job of renovating the bathroom. I've encountered an issue with the walls adjacent to the doorway and to a lesser extent one wall. The tiles were attached to a cement plaster backing, but removing them revealed "bricks" of crumbling purple pumice-like material, very light and full of air bubbles, about 1.5-2 inches deep. Much of it has crumbled into pieces and come completely away from the dry wall behind it, leaving extremely large holes 6"x7" in size. A couple of questions (1) What is this purple stuff? (2) Recommendations on repairing? The local building supply store has suggested I might be able to fill it all and repair with plaster, but since last talking to them even more has come away as I prep it, so even bigger holes. I'm wondering if perhaps setting up some bathroom-approved drywall or similar might be a better option? One other wall is made of the same stuff but appears attached to a concrete backing and has held together fine with only a few relatively minor holes. I think it should be fine plastering to smooth it out - however I need to add half an inch thinkness, so perhaps covering it with drywall might be better there too? Advice appreciated!