When I replaced a toilet, I discovered that my toilet flange was not actually secured to my concrete subfloor at all and had come out of level, causing a leak between the bowl and the floor on the side that sunk beneath the finished floor. I removed the old flange and discovered that the original builder likely used too much padding around my 4" PVC waste pipe when they poured the slab, leaving approximately 3/4" distance between the waste pipe and the slab. The gap is much wider than that at the top of the pipe where the flange would be secured, but then narrows considerably. Any flange won't be able to rest on surrounding subfloor without pouring new concrete down there.
I plan to wrap the pipe to prevent anything from entering, then pour some sand as backfill down there and ultimately finish it with at least 4-6" of concrete at the top to bring it level with the finished floor (like these videos: https://www.youtube.com/watch?v=Szlhjn1Hfe0, https://www.youtube.com/watch?v=XKxYSOoiGe8, https://www.youtube.com/watch?v=LzPmmN6DAZY)
The problem I see is that the crevice is deeper than in those vids and it becomes so narrow, that if my concrete mix is at its ideal consistency, it's doubtful that I'll be able to work it into the crevice without significant air pockets. Ideally, I'd like something relatively soupy that can just be poured down there, but I realize that a soupy/pourable consistency may compromise the integrity of the mounting surface.
Quikrete makes a product (see vid here) that can be dry poured, which I think may be a good option, but I'm concerned that it could expand while curing and crack my waste pipe.
Whatever I use, I plan to then secure the new flange to the surface using Tapcons.
What's the best way to fill in a gap like this?