Concrete around Tub/Shower Drain

Hey all!

This is a great forum and most of my questions have been answered by searching, but I couldn't seem to find the answer to this.

I had to cut up the floor to install a tub/shower and a shower in the basement. I want to know if I have to put concrete back in the empty areas or can i simply fill it with rock?

Here's an example of the tub/shower:

Rock is fine.