Greetings and decaying gypsum board question

[brownriver]

Hello all,
I'm from New York City, live in an interwar apartment building, and have done a lot of gutting and rebuilding in it. It's quirky in its own way; my subfloors were set on sleepers resting in cinders as an early form of fireproofing...
Right now I'm dealing with a bathtub surround. The entire bathroom was redone in the 1980s, and it was a poor job. I'm not in a position to redo the entire thing, so we're just trying to make it a bit nicer for now, until we can redo it entirely. We will be reglazing the bathtub, but first I wanted to regrout the tiles because the old grout was cracked and falling out in areas. While digging out grout, some of the tiles came loose from the wall. One length of wall is cement board underneath, but the wall spanning the width of the tub turns out to be sheetrock (gypsum board). It's not a good backing in a bathroom, but most of the tiles are still solidly attached after 50 years. I would like to reattach the two tiles that fell off the plasterboard. But when they came off, the paper that usually bonds to the gypsum itself came off with it, exposing the gypsum, which is by now fairly crumbly.
Clearly I should remove the whole thing, but I can't right now. So I'm looking for ideas on what I might apply to the gypsum that would stabilize it enough that I could then apply a thinset for the tiles. A friend suggested that hydraulic lime might react with the gypsum and create a sort of slurry that would then harden together and bind; that might work, but I don't know if that's what would actually happen. I also thought I could try applying a deep penetrating primer that would stabilize the gypsum enough.

Thank you for any thoughts or suggestions you might have!

 

[fernicarry]

Something like Sikabond might be enough to consolidate the plasterboard to refix the tiles onto with a thicker bed of tile adhesive. Depends how much strength is left in it.

For a similar case of failed grouting and mushy plasterboard I cut away a section and fixed a piece of plywood in place (cement board or tile backer board would have been better) with copious amounts of expanding foam to seal down behind the bath, then Sikabond to seal the surface, then re-attached the tiles. Quite the bodge but its held up for over 3 years without problem.

Another suggestion, if its an internal partition wall, is to cut away the plasterboard on the other side to get access and stabilise it from behind.

 

[paulc]

Something like Sikabond might be enough to consolidate the plasterboard to refix the tiles onto with a thicker bed of tile adhesive. Depends how much strength is left in it.

I would also suggest SBR, or failing that, a thinset primer. Mix a dash of SBR in with the adhesive (thinset) and embed some mesh scrim tape in the mix when applying it.