Bathroom wall repairs.

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!

Googling around it looks like it might be an old form of aerated concrete, which was apparently invented here in sweden and popular in the 50s when this apartment was built and a common brand was called "blue concrete" which might explain the now purple tinge.

It gave you insulation and waterproof backing for tile and lasted fifty years. Sounds like it was a good product. This is at the end of its life and should probably be change out. If you remove the whole mess you could replace it with foam and concrete backer board.
I would be suspect of the other wall also, doing alot of work and finding a problem later would be awful.

Thanks for the reply!

I'm on a deadline here, want to sell as fast as possible.

The other wall is pretty solid, I'm ok with it. Would be a hell of a job to take it out.

The ones around the doors, give it wriggle and a lot of it just falls off.

What type of foam would you recommend? A spray or standard insulation foam? We're talking a small area here, 5-20 sq ft split in 2 thin upright areas.

his is my first time working inside a wall. But I need to learn as we're buying an old house

I would think a 2" board would be easy to work with.