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