Told you some one would come along soon.!
My expertise,term I use loosely,is with brick.
I used a brand of high temp cement called La Farge,chinese brand is cheaper but not convinced about it. If you are just patching joints to get a seal make your mix what we call down here
FAT.Buy some fire clay mix it with the rest to make it more workable,I was taught not to put plastisers into mortar for extra hot applications but I am not a mason. You want it sticky.
I did build this woodfired oven from scratch so I have a few clues,(contracter friends hooting with laughter at this point)
My expertise,term I use loosely,is with brick.
I used a brand of high temp cement called La Farge,chinese brand is cheaper but not convinced about it. If you are just patching joints to get a seal make your mix what we call down here

I did build this woodfired oven from scratch so I have a few clues,(contracter friends hooting with laughter at this point)