what is the thick coating on chimney bricks in our basement?

The part of the chimney that is in the basement has a thick layer of some kind of paste on it. It's not masonry paint, it's much thicker. It's not plaster. It may be some kind of sealing or insulating compound.
I'm trying to fix a damaged section, and if I could use some of the same gunk, the fixed part would better match the rest of the masonry.
All suggestions appreciated.

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