hydraulic cement around pvc waste pipe


i pulled my toilet (on the 2nd floor) and want to seal around where the pvc waste pipe comes out of the floor. there are large gaps between the plywood and the pipe and the thinset for the tile begins a few inches from that spot. i thought i'd simply use a backer to create support and then use some hydraulic cement to form a tight seal around the pipe.
question. do i need to worry about the hydraulic cement expanding and crushing the pipe? there will be expansion room vertically and possibly some laterally outward, but i want to make sure because if it does crack the pipe, it becomes a major repair project instead of a simple minor project. thanks
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I've used this to seal where the waste pipe exits through my block wall foundation. No worry, it won't crack the pipe. It doesn't expand much. But I'm not sure it will stick to the wood well enough. I'd be inclined to use expanding foam instead. Spray it in the gap, let it harden, and trim away any excess.
HTH,
Paul
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Why do you want to fill this gap? As long as the toilet is covering the hole, just leave it. Your other option is to use some of that "Great Stuff" expandable foam, which would be easier to do. But almost all toilet flanges have a hole next to them, so why waste your time doing this?
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