Mortar and clay chimney flue question


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
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12" may not do it, you are back drafting, you are also pulling down Co, a poison.
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wrote:

12" may not do it, you are back drafting, you are also pulling down Co, a poison.
Also exhaust fans may be creating negative pressure and sucking smoke down the flue.
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snipped-for-privacy@yahoo.com wrote:

That's why it's mixed stiff.
But, I'd ask what's the rest of the chimney--a flue liner block doesn't a chimney make; it needs lateral protection/support.
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The chimney is made with clay flue liners all the way down.
JaKe
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snipped-for-privacy@yahoo.com wrote: ...

Standing alone???
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