Concrete type for repair around ABS drain

toiking

Hello everyone!

I have no experience with cement so I come you for your expert advice. Any suggestions would be greatly appreciated. In order to install/anchor a new toilet flange correctly I plan to pour cement to fill in the space between the drain and the existing concrete. This will also prevent toilet leak from entering below the tile and slab.

1. Since the ABS turn is visible and only partially under concrete, can I pour concrete directly on to of the ABS? Or should I put a layer of gravel in between?

2. For concrete, which type should I use?
Hydraulic water-stop cement? Anchoring cement? Quick-drying?
Anchoring cement would seem to be the best choice to hold down the flange bolts but I'm concerned about its expansion properties putting pressure on the ABS turn.

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