Regrout the chimney

[Bläsa]

The joints of the chimney are starting to get a bit bad in some places (above the roof) so I thought I'd scrape, sand, and chip out some of the joint and make new ones.
What kind of mortar should be used when doing this???

 

[Claes Sörmland]

The chimney's joints are starting to get a bit bad in some places (above the roof), so I thought I'd scrape, sand, and chip out some of the joints and make new ones.
What type of mortar should I use???

The same mortar that was there originally. If it's a house from the 60s, it's probably regular masonry mortar that you can buy in a bag.