fire mortar vs homebrew castable

[johnedm]

so... we are building a 37" homebrew castable pizza oven... we have a paper mâché dome that we are gonna render the castable onto. I went to buy the fire clay locally and found that most places _call_ stuff fire clay, but it is really "fire clay blend" that is actually called fire mortar... they call it all kinds of things (seemed when I was at the store anything I mentioned he would say, yeah... that's this stuff -- fire mortar, refractory cement, stuff for pizza ovens, etc)... but, it seems to have a very similar recipe to homebrew... fire clay, sand, lime and portland (though, I can't find the ratios)... we are testing a small bit of it by building a dome on a helmet nacho helmet (maybe we'll use it as a small pot for a plant... or maybe to cook some mini bagel pizzas.. who knows)... but really, what am I lookin for... my plan was to see if it cracks a bunch... if it seems sturdy... to see what it looks like after being in the regular house oven at various temperatures (like put it through a mini curing cycle)... the fire mortar is "paragon" brand... am I gonna be screwed if I use this to render my whole dome?... I, of course, could alway switch my plans to bricks... I didn't wanna use bricks cause I didn't want the 2.5" thickness but I was only able to find 2.5" thick floor bricks (they were 18" squares so I couldn't pass up that convenience)... so maybe the brick dome is better for more even heating?

anyway... I think my questions are:
- if I see cracking, what is the fire mortar likely missing that homebrew would have had?
- if it cracks after a curing cycle, what then?
- is fire mortar just not the right thing at all and so I should give up and just use it to build the dome outta bricks?

thanks,

john

[david s]

so... we are building a 37" homebrew castable pizza oven... we have a paper mâché dome that we are gonna render the castable onto. I went to buy the fire clay locally and found that most places _call_ stuff fire clay, but it is really "fire clay blend" that is actually called fire mortar... they call it all kinds of things (seemed when I was at the store anything I mentioned he would say, yeah... that's this stuff -- fire mortar, refractory cement, stuff for pizza ovens, etc)... but, it seems to have a very similar recipe to homebrew... fire clay, sand, lime and portland (though, I can't find the ratios)... we are testing a small bit of it by building a dome on a helmet nacho helmet (maybe we'll use it as a small pot for a plant... or maybe to cook some mini bagel pizzas.. who knows)... but really, what am I lookin for... my plan was to see if it cracks a bunch... if it seems sturdy... to see what it looks like after being in the regular house oven at various temperatures (like put it through a mini curing cycle)... the fire mortar is "paragon" brand... am I gonna be screwed if I use this to render my whole dome?... I, of course, could alway switch my plans to bricks... I didn't wanna use bricks cause I didn't want the 2.5" thickness but I was only able to find 2.5" thick floor bricks (they were 18" squares so I couldn't pass up that convenience)... so maybe the brick dome is better for more even heating?

anyway... I think my questions are:
- if I see cracking, what is the fire mortar likely missing that homebrew would have had?
- if it cracks after a curing cycle, what then?
- is fire mortar just not the right thing at all and so I should give up and just use it to build the dome outta bricks?

thanks,

john

The term "fire clay" as used in the building industry primarily for addition to mortars, is an ingredient to use sparingly tomake the mortar more workable (sticky). Its name is derived because it is also used in higher heat applications like fireplaces, ovens and kilns. It is usually comprised of the cheapest sieved clay available. To a potter fire clay is a different material. It is a powdered clay free of impurities and fluxes like iron oxide that can accelerate materials melting at high temperatures (1100C+). So if you obtain fire clay from a building supplier it will be a different material to that suppled by ceramic suppliers.In Australia Fire clay from building suppliers has now been changed to Bricklayers clay to eliminate the confusion (after my complaint about its mis-labelling).

Mortar is the product used my masons whilst fire clay is sometimes used ingredient of mortar.
Once you mix water with a mortar or castable, any remaining in the centre can rapidly expand violently blowing the casting apart, called steam spalling. All potters have experienced this when firing wares not properly dried, too thick or fired too fast. !00C/hr is standard firing schedule up to 350C to avoid this. The same can occur with a castable if pushed too fast. A further measure of protection can be introduced by adding very fine polypropylene fibres, well dispersed in the wet mix, which upon firing will burn away leaving a network of very fine mini pipes throgh which moisture can escape. This is far more important for a casting that's thick than a mortar course which is thin because the moisture has much further to travel to reach the outside surface.

I have no idea what the ingredients or their proportions in your "fire mortar" are. You could call the manufacturer. It may contain calcium aluminate cement (higher heat tolerance), rather than OPC and or lime. The homebrew recipe is 3:1:1:1 (by volume) sand, Ordinary Portland Cement OPC, hydrated lime and powdered clay. Because of clay's extremely fine particles it does impart shrinkage. For mortars it is recommended not to exceed 7%. if I use homebrew I halve the clay content for this reason and it seem to work better with no shrinkage cracking.

[johnedm]

well... it doesn't say on the bag or anywhere I can find, other than a warning that it does contain portland cement... but I think, if i'm reading your answer right, that if I add the fibers to the mix, and cure it slowly, I should be fine....

I saw something on youtube from a pizza oven manufacturer that had a 5 day fire schedule of fires made with an increasingly larger amount of wood over the five days (keeping the door closed after the fire burns out each day to keep moisture out)... not sure if that was just for their ovens or if that would work for a castable like i'm planning...

john

[johnedm]

I see the curing schedule in the pompeii oven instructions... I know that doc is for brick ovens, but would that same schedule work for castable oven domes as well? it seems to closely track to the youtube video I saw

thanks,

john

[david s]

Yes it is important to eliminate the water slowly use 7 fires in 7 days getting progressively bigger. It is also important to damp cure a homebrew casting for a min of a week, two would be better. The industry standard for OPC concretes is 28 days.
Do a search on Other Oven types and look for homebrew casting ovens, there are quite a number of well documented builds.

[johnedm]

thanks so much... sorry I posted in this area cause, except for the dome, i'm mostly using the pompeii plans...

also, i'm building in a gas burner... can I cure the oven with that as long as I closely monitor the temps or is ti better to stick to wood for the curing... also, I think I read somewhere the curing should be at least 7 days after the dome is done and at least 2 days after the exterior is finished... whichever comes last...

[johnedm]

ok... so my helmet nacho test with the fire mortar failed... I looked at it this morning, it was still damp, but cracked all the way through in several places... this leaves me with a couple questions:

1. I hadn't added the firbers yet... would that have made a difference keeping it together (doesn't seem so).
2. I didn't cover it with plastic, but it was still damp when I got up so i'm guessin that wouldn't make a difference
3. is there anything I can possibly add to make it not crack that way

shifting gears to use bricks instead and totally follow the pompeii instructions, I see it calls for walls that are 4.5" thick... this seems crazy thick to me... seems it would take a long time to heat up... also, i'm not sure I have the space for walls that thick and insulation... is there a way to build a pompeii style out of bricks that uses the 2.5" side for thickness... like cut the bricks into thirds or something? is that not advisable from a structural perspective?

thanks so much,

john

[johnedm]

I think I realized why it cracked... it is caused by shrinkage... as the stuff cures, it shrinks and cause the plastic of the nacho helmet is unforgiving, it has no where to go but to separate... so if I repeat the experiment again, but this time divide the helmet into 3 or 4 sections (maybe with duct tape fins) and a top section, those lines will be where it shrinks. as a bonus, that will also provide expansion joints for when it cycles later... then I can separately mortar (just on the outside) them back together after they are cured... just like the pre-cast kits come in several pieces...

in the assembly instructions for your pre-cast ovens, it says the gap in the joints should be 1/4" ideally but no less than 1/16" ... seems like that would be hard to achieve this way cause it means each piece needs to stand on its own without support or leaning against the other pieces... this would be particularly impossible if I do a "cap" piece...

thoughts?

john

[david s]

Apart from the ease of building a sandcastle mould for a simple one piece cast dome, it also has the advantage of having a little give which is sufficient to allow for a little shrinkage. The other alternative is to use a mould upside down so the cast can shrink away freely. However this requires the fabrication of a solid mould, hardly worth it for a single casting. It also requires lifting turning and replacing the casting onto the build. The sandcastle method allows for casting in situ, (no repositioning required), removing the sand through the oven mouth as soon as the casting has set.
Shrinkage is caused by water take up in the hydration process as well as evaporation. Excess water means more shrinkage, so keep the mix stiff. Also clay imparts shrinkage so a reduction of the clay content will help, hence the halving of the clay in the 3:1;1;1 recipe to 3:1:1:0.5