What kind of cement/sealant for repairing seal between liner and stove at collar?

[indymoose]

Hello:

I’m installing a block off plate above my stove, which sits inside an old masonry fireplace. I removed the “beauty plate”, which is just a piece of 8” stove pipe that covers the stainless steel liner connected directly to the stove. I noticed that some of the cement/mortar/sealant at the collar joint is gone. See attached.

Is this furnace cement? Something else? What should I use to patch this?

Thanks in advance.