The ceramic tile floor in the shower has started to loosen and crack after 22 years of use. I have started to remove the loose tiles in an attempt to replace the shower stall floor with new ceramic tile. Under the tile is what appears to be sand about 1 7/8 inches thick. Some areas are loose sand and others appear to have been mortar or cement. This find was a surprise. How should I proceed from this point?

I think it all used to be cement and has deteriorated. The best thing to do is chip out the entire bed of cement and create a new pan. This is no small task so you may want to approach a pro for estimates.

The substrate beneath the tile is what is called in the tile business a "mud" floor. It is portland cement mixed with sand. Over the years, the "mud" floor loses its strength and becomes week. If the floor is coming loose, there is probably only one answer. Underneath the tile and mud there is a shower pan. The pan in your house is most likely a lead pan (As in lead in a pencil) These type of pans typically wear out and begin leaking in about 20 to 25 years after installation. Lead pans are no longer used. These days, we have gone to a pan that resembles the rubber material in a swimming pool. In order to to replace the pan and the floor, you will need to take out the first 2 or 3 rows of tile on the wall as well. Most that I have done work for have decided to replace the entire shower. Mostly because the tile in your floor is most likely a discontinued tile and will be hard to match. You don't have to replace the entire shower, you can the pan, mud floor, and the first 2 or 3 rows of tile around the bottom, and everything will be fine. Hope this helps.