It might not have to be fire resistant. Depending on the type of venting and the approval of the boiler. It looks like a coaxial venting system, meaning that it exhaust is sealed in the middle of the intake which is evident because of the visible screws. If the pipe is a soft plastic it's almost certainly zero clearance to combustibles but it's still safest to check the installation manual. If it is zero clearance you can fill the gap with whatever your heart desires, provided your heart doesn't desire something that dissolves pvc.
Here's an example of a coaxial venting system I have sitting in my garage for some reason
I'd stuff it with some batt insulation pink or green and cut some wire mesh to give morter a binding surface. I find non-shrink grout tools well and obviously doesn't really shrink or crack but it's hard as rock if you ever need to remove it. Dryish mixed morter works well.