Replacing shower tiles, just adhesive needed or some kind of sealer?

[nuzuki]

Hello,
I have some tiles that came loose and grout kept cracking in our shower. I've managed to get the tiles off in one piece and have sprayed the area with black mould remover. I currently have a fan on the area drying it out. Do I just refit the tiles with adhesive and then grout or do I need to add some kind of sealer like sbr bond, pva or tanking? It appears to be just plaster behind the tiles. Previously I'd regrouted the tiles maybe 4 or 5 times but want to do the job properly this time round. Any advice is greatly appreciated
Thanks

 

[ree]

You appear to have plasterboard with perhaps a skim finish. But why dont you investigate further?
The horizontal join doesn't appear to have been taped.
There's very little evidence that tile adhesive was applied correctly - which probably means that all your remaining tiles are at risk of becoming loose.

Your call but you are in-between a rock & a hard place: best practice would be to remove all the tiles, & replace the plasterboard (if it is P/B) backing with cement board with taped joins.
The tiles can be rescued by soaking them, and cleaning off all old adhesive residue.
Most important you will need to follow the mixing instructions (its on the powder bags) for fresh adhesive.
Use a 10mm trowel to comb the backing boards and butter the back of each tile or length of mosaic.