I am repairing flood damage in a friend’s home in Houston and upon removing the clay tile from the floor of a toilet room off of the master bath, I found it lacked the concrete slab underneath which the rest of the house has. The rest of the bathroom had been carpeted. It doesn’t look like the concrete had been removed, rather, it was never poured. I’ve not seen anything like this, does anyone know why? The plan is to tile the complete master bath area but it seems like we will have to pour concrete in this particular area.
The floor was probably installed over the main/original slab using a "dry pack" method, wherein a sand and cement mixture is mixed to a stiff (but not wet) packable and slopeable consistency. The flood likely compromised the integrity of the dry-pack.
After careful tile removal, you may be able to use a wet mortar mixture to resurface the original floor. I would, however, worry about moisture underneath.