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  #11  
Old 01-09-2014, 04:20 AM
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Default Re: Castable refractory

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Originally Posted by david s View Post
The problem with silica is that it wants to turn into glass in the presence of fluxes.
Yep, if you mix about 0.73 parts of lime, 1.3 parts of iron oxide, 1 part of zinc oxide with 1 part of silica, you can reduce the melting point to 1080 degrees C, give or take twenty degrees.

Leastways, that's what we do with our Blast Furnace. Used to be my job for 7 years, manipulating the feed stocks to achieve those ratios.
BTW, mullite is porcelain clay.
I really must post that phase diagram.

Last edited by wotavidone; 01-09-2014 at 04:41 AM.
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  #12  
Old 01-09-2014, 05:04 AM
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Default Re: Castable refractory

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Originally Posted by wotavidone View Post
Funny isn't it?
The more I look at available knowledge, the more I realise that pizza ovens are a terribly neglected area of science.
Probably because there isn't much that can be improved on...at least, nothing that would substantially improve the performance or extend the service life in a major way...mainly because an oven is such a dynamic structure. I would guess, that a well built occasional use oven, kept dry and fired properly, will last at least 20 years before any maintenance work needs to be done.

Now, understand I'm not saying it isn't worth trying to find a better refractory mortar for lower temps. I would think that research needs to be done with material that has refractory quality by chemically bonding, and not ceramic, like the high temp linings.
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  #13  
Old 01-09-2014, 06:10 AM
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Default Re: Castable refractory

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Now, understand I'm not saying it isn't worth trying to find a better refractory mortar for lower temps. I would think that research needs to be done with material that has refractory quality by chemically bonding, and not ceramic, like the high temp linings.
Reckon you are right. That was what I was alluding to with my tongue-in-cheek comment regarding it being a terribly neglected area.
(The romans managed without the science, of course.)

When the specialists talk about refractories, they talk about furnaces for smelting (ours runs a slag temperature of about 1180 degrees C), or settings for refining pots that have to be able to withstand gas flames constantly impinging on them, or handle molten metal, etc.

The trouble is, then we find that refractory that can withstand 1200 degrees C won't actually help our oven much, unless we can heat it to 1000 degrees C to "cure" it.

Much like making a brick - until it's actually fired, it's just a block of dry clay with some filler in it.

So yes, given that the cements that harden by hydration are not real good in the temperature range we are talking about, it would be cool find a new mechanism for binding the aggregates in the mortar or castable together.

Given that there are some commercial mortars available, then the scope of the challenge needs to be narrowed even further.

It has to be cheap to do a homemade version, otherwise we just buy the commercial stuff.

So there is the challenge, come up with a binder that doesn't work by hydration like the cements, loves 500C and costs bugger all.

The knowledge base I was perusing today spoke of low cement refractories that used three different mechanisms to harden. One by hydration, one by other chemical reaction, one by drying.

The FB Homebrew actually fits all selection criteria.
The ingredients are cheap and there are, arguably, three binders that work in different ways.
You could argue that the OPC hydrates, the lime has a chemical reaction with the CO2 in the atmosphere, and the clay hardens when it dries, and while the mortar cannot really be classified as a true refractory, it handles the temperature ranges we want to use it in.
Now that I have proved beyond all doubt that the FB Homebrew is the best mortar for pizza oven conditions, I'm going to bed.
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  #14  
Old 01-09-2014, 08:04 AM
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Default Re: Castable refractory

It's like the saying goes... If it ain't broke, don't fix it.


Given the wide range of material ratios different builders use, and the success rate they have, ' improvement' on a mix design is very subtle oven to oven. For me, when I make mortar, there is no one type fits all. That carries into oven building too, because there are several different areas in the oven that benefit from slight changes in the component ratios...like aggregate size. I still do that dispute knowing that a generic mortar will work for every part of the construction.

If I get a chance I'll try to find some links that might be of interest.
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  #15  
Old 01-09-2014, 06:03 PM
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Default Re: Castable refractory

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Originally Posted by wotavidone View Post
BTW, mullite is porcelain clay.
Not quite. Mullite is the major and most important component of a porcelain body.
Mullite is the crystalline structure that is formed from Kaollin over 1200 C.
A porcelain body contains kaolin, feldspar and silica.

Last edited by david s; 01-09-2014 at 07:18 PM.
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  #16  
Old 01-09-2014, 07:37 PM
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Default Re: Castable refractory

I need to jump in here. I want to build a very small oven for tailgating. I'm thinking 24 inches max and I don't want it to be brick, obviously cast. Fire it up and cook a few simple pies, bake something for 45 minutes and I'm golden. What would you guy's suggest using for a simple home rolled mix?
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  #17  
Old 01-09-2014, 08:55 PM
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Default Re: Castable refractory

If you are doing it yourself and want it cheap, use the home brew keeping the mix pretty stiff. If you want it stronger and more permanent use castable refractory with 2% (by weight) SS needles.

I make my 21" ovens 2" thick.

Last edited by david s; 01-09-2014 at 11:11 PM.
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  #18  
Old 01-10-2014, 03:28 AM
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Default Re: Castable refractory

Use the homebrew.
I suggest that you pack a mould very tight with a stiff mix. Ram it tight as it sets.
It is very cheap, it should not be expensive to make a mould from medium density fibre board, and if it doesn't work you've only lost a few hours of interesting hobby time.

Last edited by wotavidone; 01-10-2014 at 03:42 AM.
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  #19  
Old 01-10-2014, 03:40 AM
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Default Re: Castable refractory

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Originally Posted by david s View Post
Not quite. Mullite is the major and most important component of a porcelain body.
Mullite is the crystalline structure that is formed from Kaollin over 1200 C.
A porcelain body contains kaolin, feldspar and silica.
Let me rephrase that:
"mullite or porcelainite is a rare silicate mineral of post-clay genesis"
Mullite - Wikipedia, the free encyclopedia

So, at temperatures below 1595, for alumina contents of 10 to 60%, alumino silicate bricks (the white blocks that everyone in the smelting business calls firebricks) consist of silica reinforced with needles of the same stuff that gives porcelain its strength.

Note that alumina silicate bricks are "short" at about 12% alumina. The way I read this diagram, at that point if you heat them above 1600 degrees they go straight from solid to liquid.
Attached Thumbnails
Castable refractory-phase-diagram.jpg  

Last edited by wotavidone; 01-10-2014 at 04:37 AM.
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  #20  
Old 01-10-2014, 04:30 AM
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Default Re: Castable refractory

I've just found another version of this phase diagram. Apparently people make use of this diagram for firing ceramics in the first place. They aim for the temperatures where some liquid is formed, when firing clay, as it means they will vitrify the clay , but hopefully have enough mullite to prevent the piece from sagging.

Once upon a time I cared not one jot for the details of refractories, as long as the engineers gave me a liner that would survive whatever temperature molten material I wanted to put in it.

Now, I find it all downright fascinating.
That's what building pizza ovens does for me - gets me interested in the science behind it.
The Romans did it all without a materials research lab, an assay lab, and a crew of trained refractory masons. Which just goes to show, the Romans were smarter than the average bear.

Last edited by wotavidone; 01-10-2014 at 04:42 AM.
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