There is a large gap between the toilet closet flange/drain pipe and the concret floor. Water had settled in that gap due to it leaking through the vinyl tiles after floor cleaning, etc. All of the water is now gone. The cast iron closet flange appears intact. I want to seal the gap before re-tiling. What material can I use to seal the gap? I thought of quikrete but might this expand and crack the drain pipe?

Mix thin mixture of Mortar Mix and trowel it in and around the ring. However, since you are retiling your floor, you could ask the tile man to use ThinSet to fill in the gap