Infill Material for Shower Box-Out in Slab Question

[Jb9]

Hello,

I am going to have a shower drain in a slab on grade floor and I plan to box out the 2" drain pipe. I have seen different recommendations for how to handle the void around the drain assembly (I will probably use a Schluter Kerdi drain). Some folks use a dry mortar, some folks use sand or gravel. I'm curious to hear what a pro might use to make "the next guy's job" a little easier should the need for maintenance arise down the road.

Thanks in advance.