what to use, grout or concrete mortor or ???

Just moved in to a new house and fixing various problems. The master bath has a shower tiled in rock. Looks kind-of like a cave, but we like it. The shower valves were leaking so i had to take them apart and rebuild them. They work great now, no leaks.
The eshcusions were a mess, there were big globs of silicone all around them. I cleaned all that out, but now I need to fill the space so they will sit flush against something. The grout between the stones looks very rough like cement mortar that you use to build a cinder block wall.
I'm wondering what I should use so it doesn't fall apart. I was thinking regular grout would not work because the area to fill is pretty big and it has to be thick.
What should I use that will not break apart and will stick good?

Not your every day shower. I was thinking thin set. Not sure how it will stand up but the color may be close to your joints. I'm sure others with more tile experience will be along.

John

Might as well use the same grout as they have between the "rocks".

You may get some more ideas at "Tile Your World" website www.johnbridge.com. Not sure what I'd use. How big are the typical gaps? A sanded grout can be used quite thick, but typically works best between the edges - it might stick to the surface, but that isn't its primary purpose. Thinset is somewhat porous and normally can't be applied more than 1/4" thick. A medium bed mortar can be much deeper, and would stick. It would all depend on how finished you want it to look and how thick the material needs to be.

I don't know what they used.
The average gap between the rocks is about 1/2"
The area by the valve needs to be around 1/2 to 3/4 inch thick.
I'm really thinking they did not use grout, it is very rough, really looks like cement motor
like you would see in a cinder block wall or a rock wall.
I did find out the guy who built the house was a mason.