Insulation. But you'd have to also tackle the issue of moisture (from cleaning the floor, etc). Last thing you want to think about doing is packing something like mastic or refractory cement into the void between the pipe and tile. It'll be a losing battle. The ideal thing to do is properly insulate it and then enclose it in a chase, or reroute it in an existing chase (with insulation). If I couldn't do either, I'd wrap the pipe with fiberglass insulation tape a bunch of times, coat it with mastic, and paint it with a moisture proof coating. But I'd be prepared to maintain it on a regular basis.
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