I'm adding 12" to the top of my chimney flue. I'm trying to get it a
little taller then the gas fireplace flue that's right next to it -
the smell of the smoke comes down the gas flue.

So I'm not an expert with mortar. Any tips on mortaring this 13x13x12
pipe? Won't the weight of the flue pipe squeeze a lot the mortar out?

Thanks
JaKe