How to seal around metal gas pipe penetration through poured concrete above ground?
I would use &quot;duct seal&quot;. It's a putty-like material that's a bit sticky. You'll find it in the electrical department. It's much more robust than foam.
Caulk would be difficult to work with on a gap that big and would be more difficult to remove if you had to do any modification later. You could probably get good results with silicone, though. I'd go with clear or light gray. Don't use latex as it shrinks badly.