Flooring Tile - Replacing tile on shower stall entry ledge

The tiles on the outer side of my shower stall entry ledge were coming loose and seperating from the wall so I removed them to prepare to re-cement them. However behind them I found about 1 inch of what looked like packed loose dirt or mud between the tiles and a solid structure that formed the core of the ledge. Would that be some material that has turned to mold? After scraping it out what can I use to replace it and create a suitable surface to which I can cement the tiles? Please respond urgently because I can't leave it in its current condition.

Hi Shadycharacter;
Welcome to the DIY Forums.
What is the "solid structure" you found under the "mud"?
Is it wood? If so and depending on the age of your home, it could be drywall that has disintigrated over the years due to moisture(leak through the tile).
If this is the case, a concrete backer board will need to be installed to reinstall the tile..
This doesn't sound like the typical characterisitcs of mold. For peace of mind, spray the mud with a 10% bleach solution to wet it.Remove the mud carefully and place in plastic bag. If you come across pieces of shreded paper you will know it is drywall.
Good luck and post back

Hi MajakDragon !

Thank you very much for your speedy reply. The material behind the mud appears to be metal, so I suppose it's the 4" high wall of the lead pan base of the shower stall. The "mud" certainly seems damp so it could be the remains of drywall board even though I see no signs of paper ( but the paper might also have deteriorated completely into mud). The question now appears to be, what should I use to re-build the ledge to its original thickness before cementing the tiles.

I would use concrete backer board to rebuild the ledge. You will have to take into consideration the thickness of the backerboard,thinset and tile. These together will have to equal the total thickness of the void you are filling.
Before rebuilding the ledge be sure to dry out the area so you do not make a home for mold. You may also want to try filling the shower to make sure there is not a leak in the pan that is allowing the water to be absorbed by the ledge material.(just a precaution).

Again,thank you very much, MajakDragon,

Tomorrow I'll go to the hardware store and get the concrete backer board to proceed with the job. BUT how do I secure the backer board to the lead pan?

By the way, I'm pretty sure there's no leak in the pan (which is about 6" high, not 4" as I wrote previously), the moisture that penetrated had to come from gaps in the tile grouting on the inner-top side of the ledge and then spread from there. Do you think the "mud" I found could be the original disintegrated backer board? It does look a lot like compacted dirty sand.

I'm beginning to fear I might have to remove the sliding shower-door track from the top of the ledge and remove all the tiles to replace a more extensive area of backer board deterioration. I certainly hope it doesn't turn out that way.

You have a lot more going on then you think!!!

You have opened a can of worms that needed to be addressed long ago.

You might not want to hear this, but you shower may need to be completely redone. You have more then a repair on your hands.

If you have an exposed pan, you need extensive work! That means the membrane is gone, which could be the cause of your problem.

Thank you for your interest in my problem. I don't actuallly have an "exposed pan". I can see the lead pan only where I have removed the tiles and the rotten tile backing material from the front of the entry ledge on the OUTSIDE of the shower stall, so there's no reason to anticipate a problem with membrane.