# Mortar for firebrick



## 54roadmasters (Oct 1, 2005)

Hey guys,

I am building a fire pit out of granite cobblestone. It is about 20" high, 60" OD. I will fill all but the top 8" with gravel for drainage. Question: Should I use fire brick on the inside for say the last 12 inches or will the cobblestone be OK to leave exposed. Either way, do I need to use a special mortar, mix standard mortar differently or what for the joints of the cobblestone and firebrick?


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## K2 (Jul 8, 2005)

54roadmasters said:


> Hey guys,
> 
> I am building a fire pit out of granite cobblestone. It is about 20" high, 60" OD. I will fill all but the top 8" with gravel for drainage. Question: Should I use fire brick on the inside for say the last 12 inches or will the cobblestone be OK to leave exposed. Either way, do I need to use a special mortar, mix standard mortar differently or what for the joints of the cobblestone and firebrick?


We used to do fireplaces with fire brick fire boxes 20 years back. We just made the regular motar mix richer with portland cement. Not quite a goo but close. If i was going to use a premixed mortar I would add portland for sure.


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## FireballBrick (Oct 14, 2005)

Using the cobblestones may be ok, but I wouldn't put my life on it. Regular brick and some rock have air pockets in them that will expand and literally pop when heated. My dad built a half-assed grill out of concrete pavers, and they have done well, and they are directly lining the firebox. I don't think I would use it as a firebox on a professional job though. Like previously posted, you should not use regular mortar for anything involved with the heat. It simply crumbles when it is exposed to extreme heat. Have you seen the movie Ladder 49? Remember when he digs throught the brick wall with his bare hands? I said no way! My buddy who brought the movie over is a firefighter and told me that it really could happen, that brick walls cave in during fires all the time. Anyhow, back to your question, at your brickyard or supply, they should sell a mortar specifically for high heat areas. Call your brick supplier, they will tell you where to find it in your area.


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## 6stringmason (May 20, 2005)

Granite is pretty dense and solid but still not as good as a firebrick. For mortar I would think fireclay would be your best bet.


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## JustaFramer (Jan 21, 2005)

I wonce used a 3 hr fire rated mortar to plug a hole in a fire wall. Got it from McMaster-Carr. 


There was a few ways to fix this wall one was to pull out the walk in Freezer :no: The other one was to build a bump out :no: So we settled on plugging with the mortar and fire rated caulk. Big hole and multiple conduit lines run. 

All I know is the building inspector passed it. :thumbup:


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## jvcstone (Apr 4, 2005)

Fire brick = fire clay. Either the dry clay you mix yourself, or that "sairset material in the gallon cans. The brick and clay have essentially the same coefficient of expansion so there is no separation. Do not use a portland mix as the joints will begin to crumble out. 

A lot of granite carvers use a propane torch to pop the surface off. The same thing will happen in your firepit if you don't use the fb liner. Make sure you leave a space between the disimilar materials because they too will expand at different rates. resulting in cracked work. ( but what the heck, it's probably going to crack anyway )

JVC


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