Best product to trademark furnace duct? Whats the best product to seal the round ducting from the boiler,...

Whats the best product to seal the round ducting from the boiler, where on earth it inters the smoke stack/chimney flue in the crypt. Everytime I see one of these they look like hoary plaster that is partly falling off and broken to bits. I saw one once that looked similar to mortar or concrete that seemed to be strong and contained by good shape. What do you recommend?
Answers:    There is a product call furnace cement. Rectorseal offers it surrounded by their product line; I believe it is branded as Pyroseal. It will come surrounded by a 3# tub, which is about a quart or so. It comes premixed, so it is a no-brainer. This should be plenty to trademark your flue where it enter your masonry chimney. If you can not find it at a home center, I would encourage you to stop by your local plumbing/heating supply house. You can look it up at www.rectorseal.com. It will give you a full description as in good health as applications and how to use it.
we often will pack insulation around the pipe w/o the rag of couse, making sure its not sticking out. then bring back some motar mix and apply around it and that should do the trick