Which mortar for the chimney?

[The Ulf]

Hello hello!

I need to plaster the chimney in the attic of our summer house. It leaked in a couple of places during the pressure test, but the chimney sweep said it's enough to give it a round of plastering. It is apparently important to use the right mortar. How do you know what the right mortar is? The house was built in 1975.
Grateful for an answer!

 

[Gochg]

Common plaster mortar e.g. Finja. Lime mortar with a slight addition of cement. Is okay for houses built in the 70s.