High Heat Mortar Primer

[james]

Hello all,

The topic of mortar comes up often, so we are making this a sticky posting that will always be at the top of the "Getting Started" forum for reference.

Here is some good background information on the high heat mortar you should use to assemble a Forno Bravo Casa or Premio pre-cast oven, or to build a brick Pompeii Oven.

The best solution is to use Refrax, or another pre-mixed true refractory mortar. We stock Refrax and highly recommend it. It is pre-mixed (just add water), sets hard, is easy to work with, fully cured quickly, and is heat resistent to 1700F. Basically, it's made specifically for pizza ovens and fireplaces.

If you don't want to worry about shipping Refrax, or want to save some money, you can make a fire clay/Portland cement mortar. Fire clay is a heat resistent clay made up of aluminate and silica. When you mix fire clay with Portland cement, sand and lime, you get a mortar that is more heat resistant than concrete (Portland cement with a sand and gravel aggretate), though less resiliant or thermally conductive than a true aluminate mortar.

Here in Sonoma county, you can get fire clay from our big, local masonry supply company, SBI -- who is also a Forno Bravo oven dealer.

Here is the recipe for fire clay mortar, where you measure by volume (use a bucket or shovel to measure):

1 part portland
3 parts sand
1 part lime
1 part fire clay

In between Refrax and fire clay/Portland cement mortar, you can make your own aluminate mortar. It is hard to work with, as calcium aluminate can be challenging. If you get the mix, or water wrong, it won't set correctly. It partially sets very quickly, and you cannot re-hydrate it, so you have to mix it and use it in small batches. Still, if you are trying to save money and want/need the heat resilience, heat conductivity and longevity of a true aluminate mortar, it works.

1 part calcium aluminate
3 parts sand
1 part lime
1 part fire clay

-James
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[njsicilian]

I have a question. I have been putting my firebrick up with which i guess is your original fireclay mix of 8 parts sand, 3 parts portland and 2 parts fireclay. But now i am reading about the new mix of 3 sand, 1 portland, 1 fireclay and 1 lime. Am i going to be ok with the other mixture? I am up to my 7th chain. I will change over from this point. please inform.

[james]

Don't worry. The original recipes works exactly the same. The main different is texture in application. The two mixes are very close. Also, 1:3:1:1 is so easy to remember.

James

[james]

An update. I have created a High Heat Mortar page within the Pompeii Oven plans, with links to it from the Materials, Oven Dome, Overview, etc. pages. This should point all of the new builders to the same place for the same alternatives and recipes.

Let me know if you find reference to any older information on other Pompeii pages, and I can fix it.
James

[james]

I can't help myself, so here is Nick's buried testimonial for Refrax from an earlier posting. From the source...

"I've used the 1,3,1,1.. The heatstop 50, and have found (Forno Bravo's) Refrax to dry way harder than both. If I slow cure it with damp burlap it'll take a bunker buster to break it I reckon. "
-Rebrick (himself)

You can take that to the bank. Thanks Redbrick.
James

[tdibratt]

Question about the home brew. According to the book "Bread Builders" by Allan Scott, MASONRY CEMENT is Portland Cement with lime added. So instead of taking portland cement and adding lime, could I use Masonry Cement? If so which type and would it then be 1 part masonry cement, 1 part fireclay and 3 parts sand?

Thx

[karl]

Hi,

I am using the attached specification fire mortar for the complete oven. I have no idea what the constitutes are in the "home mix" which is recommended (fire clay, portland cement etc.). Would it be comparable to the shown specification?

karl

[brokencookie]

I'm no expert but this looks similiar to Heat Stop 50

40% alumina
50% silica
2% iron oxide


A little higher in Alumina and a touch lower in Silica but with a rating of 1400 degrees C, my guess would be that it should work fine. Heat Stop 50 is being used by a lot of builders with no problems. Search the forum for alumina and you'll get a bunch of info. There was a discussion about matching the alumina content of the mortar to the alumina content of the brick to match expansions but I don't think a definite conclusion was reached.

Bruce

[david s]

When using calcium aluminate cement I found that adding lime in the mix made the mixture "go off" really quickly. On researching this I found, from the manufacturers (can't remember which one) that they said "do not add lime as it acts as an accellerant, which is exactly what I had found. Do not add lime if you want to keep it workable for a reasonable period (eg 1/2 - 3/4 hr or so) .The calcium aluminate cement is very temp dependant. Use chilled water if using on a hot day and don't leave the stuff in the sun to get hot.

[tdibratt]

Question still remains, "instead of taking portland cement and adding lime, could I use Masonry Cement? If so which type and would it then be 1 part masonry cement, 1 part fireclay and 3 parts sand?"

Thx