How to backfill plumbing project in above grade concrete slab?

[NYCpro]

Additional plumbing (all cast iron) was installed in above grade concrete slab. The trench is about 5" deep and 12"x14" (see pic). The slab itself is about 8" thick reinforced concrete. Can I just re-pour concrete over it or should we backfill the space around the pipes with something first (like crashed stone or sand)? When we were opening the previous plumbing, the original [smaller] cut out for the closet bend was filled with mortar mix.