Shower floor question

[MARK222]

I am in the process of stripping all the tile from the shower walls and floor to update the shower. The shower is in a Florida home built in the 1980s and sits on a concrete slap (single story). When I removed the floor tile there was an approximate (4” deep) 2” gap from the house concrete slab and the shower floor. I removed tiles pressed up against the shower floor which were placed partially submerged in concrete in this gap. Behind the tiles was another inch and a half of mortar which took up the remaining space behind the tile to the house concrete slab. Is this a common practice? What do I fill this gap with before starting to re-tile walls and floor? I have only tiled one shower 20 years ago and was not familiar with this technique. I do not want to tear out the old floor entirely, I just want to set new tile on the existing shower floor.