I recently started renovating one of our bathrooms. After removing a few tiles there was a second layer of tile beneath. Below the second set was a concrete/mortar base that is ~1.5 inches thick and has broken in a few places to reveal sand inside.
According to my research this is an older style of base for laying tiles. The house is from the early 60s so that lines up. However I have not found much info beyond that. I am wondering whether I can put tile back down on top of it and how.
Additionally there are a few cracks where the top came off while removing tile and revealing the sand beneath. Most are small but one is larger. Would this require a complete demo of the mud base? Can I patch it with concrete or something similar? The big hole is by my kids step stool in bottom right, it is about 5-6 inches at it's longest.