Can you make an oven dome with a 3:2:2:.5 mix (gravel, refractory, sand and lime)?

[nalbertasr5]

Hey guys. Total noob here. I was interested in making a pizza dome by using a 3 parts gravel, two parts sand, two parts refractory cement and 0.5 parts lime mix. Castable cement is extremely expensive/unavailable where I am so this would be more budget-friendly. Will this work? Or will it not be strong enough? I would like to try packing it rather than casting it.

Thanks!

[Gulf]

Most gravel is a not advised imo. Many types will cause mini explosions causing spalling under the intense heat. david s has answered many similar questions to yours. I'm not sure what you mean by "packing" but the "home brew recipe" is very economical. Check out some of the recent cast ovens on the site for the recipe

[david s]

Any untested mix you design for yourself is an unknown regarding its success. Castable refractory which contains refractory cement combined with fine high temperature aggregates, burn out fibres and other goodies in a secret recipe that companies won’t divulge, is expensive and refractory suppliers e homebrew offer a number of different products depending on its service application. Generally these castables are overkill as they are designed for service temperatures around double what we require for a WFO.
The homebrew (3:1:1:1) has been around for eons and proved itself ideal and in fact superior to refractory mortar for WFOs. For this reason I suggested it’s use as a cheap castable around 10 years ago, not knowing how it would stand the test of time. We’ve not had any reports of massive failure from builders, so I guess it’s proved itself.
The combination of refractory cement (calcium aluminate) and lime is not a good one as the reaction is accelerated and you’re likely to have a barrow full of mix that’s already gone off before you can place it. Unlike the combination of lime and calcium silicate cement which results in a slower reaction, providing extended working time.
on one hand I’d love to hear about alternative mixes, but you need to be aware that any untried mix risks failure.

I’m not sure what you mean about “packing” rather than casting. Anything placed in or against a mould produces a casting. This forum is rich with information and lots of research here should answer most of your questions from folk willing to share rather than keeping secrets.

[nalbertasr5]

Any untested mix you design for yourself is an unknown regarding its success. Castable refractory which contains refractory cement combined with fine high temperature aggregates, burn out fibres and other goodies in a secret recipe that companies won’t divulge, is expensive and refractory suppliers e homebrew offer a number of different products depending on its service application. Generally these castables are overkill as they are designed for service temperatures around double what we require for a WFO.
The homebrew (3:1:1:1) has been around for eons and proved itself ideal and in fact superior to refractory mortar for WFOs. For this reason I suggested it’s use as a cheap castable around 10 years ago, not knowing how it would stand the test of time. We’ve not had any reports of massive failure from builders, so I guess it’s proved itself.
The combination of refractory cement (calcium aluminate) and lime is not a good one as the reaction is accelerated and you’re likely to have a barrow full of mix that’s already gone off before you can place it. Unlike the combination of lime and calcium silicate cement which results in a slower reaction, providing extended working time.
on one hand I’d love to hear about alternative mixes, but you need to be aware that any untried mix risks failure.

I’m not sure what you mean about “packing” rather than casting. Anything placed in or against a mould produces a casting. This forum is rich with information and lots of research here should answer most of your questions from folk willing to share rather than keeping secrets.

Thank you for the information. There is no fireclay available where I live for the 3:1:1:1 mix. I've seen mixes like the ones I was talking about, posted in places like here https://www.traditionaloven.com/tutorials/concrete.html. so perhaps other people have used them with some success? Would gravel be unacceptable to use like the poster Gulf mentioned? I already have 5 bags of refractory cement. Is there anything else I could do with them? I am looking to build a 38" dome.

[nalbertasr5]

Also thought I would mention:

My friend is selling some fire bricks. Is it possible I could go and crush them into grog to use instead of gravel? Unfortunately, I can't find any fire clay or portland cement in my area.

[david s]

Thank you for the information. There is no fireclay available where I live for the 3:1:1:1 mix. I've seen mixes like the ones I was talking about, posted in places like here https://www.traditionaloven.com/tutorials/concrete.html. so perhaps other people have used them with some success? Would gravel be unacceptable to use like the poster Gulf mentioned? I already have 5 bags of refractory cement. Is there anything else I could do with them? I am looking to build a 38" dome.

When you say refractory cement, do you mean cement only, cement plus fine aggregate (refractory mortar) or cement plus aggregate as castable refractory?

Any powdered clay should be suitable. Pottery suppliers stock heaps of the stuff for ceramic casters. Probably the cheapest is Ball clay, but avoid Bentonite as the extremely small clay particle size imparts high shrinkage which you don't want. If you are keen enough you can dig below the topsoil to find a layer of clay pretty much anywhere, but you'd need to dry it crush it, slake it in water for a few days sieve out the coarse material, dry it again and then crush it to powder.

Portland cement is the ordinary builders cement used all around the world, I'm sure you could lay your hands on some.

Crushing dense firebrick is hard work, Insulating firebrick is easier. But as you need to make a dense castable you'd need crushed dense firebrick. It should be crushed to no larger than 1/4" and discard all the fine dust.

Much easier to just use the homebrew.

[fox]

I wonder where you live, there cant be many places in the world where standard cement is not available?

I have come across folk who have used crushed clay bricks and refractory cement, even crushed clay roof tiles and refractory cement but from the results i have seen these DIY mixes have not worked particularly well although in both cases the small ovens were both in use and working just with lots of structural fractures.

I make my own refractory mix but use well tested materials that I buy individually ... fire brick grog and Fondue cement with a small amount of fibres.
The mix ratio is 4 parts crushed fire brick 1 part fondue cement.

So if you can get crushed and sieved fire brick grog you could make a effective refractory mix.
However as has been pointed out that type of mix sets very quickly and is best used in a mold and mechanically vibrated rather than packed by hand.

You could try using your refractory cement mixed with course silica sand at a 5-1 mix but at the end of the day the ‘homebrew’ mix will be easier to use and quite possibly a better product if you are hand packing.

[nalbertasr5]

When you say refractory cement, do you mean cement only, cement plus fine aggregate (refractory mortar) or cement plus aggregate as castable refractory?

Any powdered clay should be suitable. Pottery suppliers stock heaps of the stuff for ceramic casters. Probably the cheapest is Ball clay, but avoid Bentonite as the extremely small clay particle size imparts high shrinkage which you don't want. If you are keen enough you can dig below the topsoil to find a layer of clay pretty much anywhere, but you'd need to dry it crush it, slake it in water for a few days sieve out the coarse material, dry it again and then crush it to powder.

Portland cement is the ordinary builders cement used all around the world, I'm sure you could lay your hands on some.

Crushing dense firebrick is hard work, Insulating firebrick is easier. But as you need to make a dense castable you'd need crushed dense firebrick. It should be crushed to no larger than 1/4" and discard all the fine dust.

Much easier to just use the homebrew.

The cement that I have is listed as a castable refractory. If I could get more of it, could that work just on its own? The stuff I managed to find was extremely expensive ($90 a bag). Obviously I would need more of it to cast a dome but would be costly. I live in Northern Canada so I could make a multi day trip to find more to the closest big city. When I've asked around locally about portland/builders cement people have no idea what I'm talking about. Would it be possible to use the castable cement that I have for the oven floor with my fire bricks? I've already got an insulated base made out of proper materials and a supply of fire bricks I planned to use for the floor. Then could I make the dome with the recommended home brew? I would hate for the supplies I have already bought to go to waste.

[david s]

The floor is best with loose laid fire bricks, so save the castable for the dome. There are many types of castable, check the MSDS on it and ensure it is a dense castable not an insulating one. Also ensure that it is still viable. If it has hard lumps in it then it is stuffed, don’t use it. Assuming it’s viable dense castable you could layer it. That is cast a thin dome and then make it thicker with a homebrew castable, but make sure you add some burn out polypropylene fibres to it.

[nalbertasr5]

The floor is best with loose laid fire bricks, so save the castable for the dome. There are many types of castable, check the MSDS on it and ensure it is a dense castable not an insulating one. Also ensure that it is still viable. If it has hard lumps in it then it is stuffed, don’t use it. Assuming it’s viable dense castable you could layer it. That is cast a thin dome and then make it thicker with a homebrew castable, but make sure you add some burn out polypropylene fibres to it.

Thank you so much for your advice! That sounds like a plan. Just a few more questions! About how many lbs of refractory would I need for a 38" dome that is 2' thick?? Alternatively, how much portland cement? Also, can I build it right on top of the loose bricks? Or should the oven be mortared to the base with the bricks just placed inside?

[david s]

That question is a bit tricky. All castable refractories will vary somewhat, but assuming you have dense not insulating castable each litre of dry loose castable will weigh around 1700g. The dry material can be compacted further and when water is added it will compact even more. When about 400ml of water is added to create the "ball up" consistency the volume of the resulting mix is around 970ml and weighs 2086g.
The castable refractory I get comes in 25kg (55lb) bags although invariably they weigh 24kg (I guess that's within their + or- 5% quoted weight) You will need to convert this to lbs to see if it's around the same weight as your bags.
The formula for the volume of a sphere is 4/3 x pi x r3 (halve this result because it's half a dome. The volume lost in the oven mouth = the same amount required around and above it so just work on the hemisphere volumes). Work out the volume of the hemisphere using the outside radius then deduct the volume of the hemisphere using the inside radius. This will give you the volume of your casting.

Alternatively scour the casting builds to see how much castable others used.

[fox]

My 30inch dome requires 9 x 25 kg bags for the dome and 3 for the gallery, my 34inch dome requires 13 bags and an extra 3.5 for the gallery.
Both are approx 2.25inches thick.