Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

[RocketStoveBuilder]

I hope you don't mind my asking for advice for non-oven related uses, but many of you most likely have the correct and accurate information needed. This forum seems like the best place to get solid advice about what kind of "home brew" mix to use for a 1500* F burn chamber and heat riser (chimney) for our rocket stove mass heaters:

We've built our stoves with fire bricks and 8" chimney flues - and they work fine. We also wrap our stoves with metal so they basically look like a woodstove as you can see in the above image.

The problem is, using these bricks and flues requires too much metal and too many welds resulting in higher labor and material costs; hence we want to build these two components - the burn chamber and burn riser (chimney) from a mixed (refractory) castable.

Pre-mixed commercially available materials - such as Sparlite 80 - are expensive as is the shipping.

We'd much prefer to create our own "home brew" castable mix, but we can't find solid dependable advice. Maybe some of you could help?

So far, we've come up with a mix of fireclay, perlite (for insulation purposes) and a refractory cement. We need a chamber that is 2-3" thick.

The mix ratio I've come up with so far is 1:1 and a bit ? of refractory cement with just enough water to keep the mix together but a mix that breaks apart with a "pop" when squeezed together between your fingers.

If we use this refractory cement (or mortar) any idea of the mix ratio? Some say "as much as we can afford" but that isn't much help.

Is there any reason we can't use Portland cement instead of the refractory cement?

We understand the mix needs to be gradually heated to prevent excessive cracking. None of the mix will be seen - the resulting castings will all be hidden by metal.

The castable will get some abuse from the wood fuel being fed into the stove, but nothing significant. So, some abrasive wear is to be expected.

Bottom line is we need a low cost durable castable mix we can produce with consistency.

My family comes from a long line of masons from Europe dating back several centuries, but none of my ancestors who could provide the answers are alive today.

Ours is a start-up business and we hope to produce these stoves commercially. Any help / advice /suggestions some of you experts would care to give would be most appreciated.

Thanks in advance.

[stonecutter]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

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We'd much prefer to create our own "home brew" castable mix, but we can't find solid dependable advice. Maybe some of you could help?

So far, we've come up with a mix of fireclay, perlite (for insulation purposes) and a refractory cement. We need a chamber that is 2-3" thick.

The mix ratio I've come up with so far is 1:1 and a bit ? of refractory cement with just enough water to keep the mix together but a mix that breaks apart with a "pop" when squeezed together between your fingers.

If we use this refractory cement (or mortar) any idea of the mix ratio? Some say "as much as we can afford" but that isn't much help.

Is there any reason we can't use Portland cement instead of the refractory cement?

Yes, Portland quickly degrades from thermal cycling and will not last long.

We understand the mix needs to be gradually heated to prevent excessive cracking. None of the mix will be seen - the resulting castings will all be hidden by metal.

The castable will get some abuse from the wood fuel being fed into the stove, but nothing significant. So, some abrasive wear is to be expected.

Bottom line is we need a low cost durable castable mix we can produce with consistency.

I think your best bet is to experiment with your own mix designs using material with refractory properties

My family comes from a long line of masons from Europe dating back several centuries, but none of my ancestors who could provide the answers are alive today.

Ours is a start-up business and we hope to produce these stoves commercially. Any help / advice /suggestions some of you experts would care to give would be most appreciated.

Thanks in advance.

Some people have made ovens using 3:1:1:1 ratio with sand,lime,fireclay, and portland and have been successful. But if this application is for a commercial business, then you should think twice...and, no disrespect to the forum.....what you are asking is beyond causal builder/user knowledge base. Fire mortar and refractory are two different things, and I doubt you should trust any "homebrew" design without engineering and testing before incorporating it into your product.

[Tscarborough]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

It is not worth the expense and effort for a one-off, just buy the castable refractory. If you are going into production, you can buy the proper ingredients to make your own refractory castables. The homebrew as used on this forum is for mortar NOT as a castable.

[wotavidone]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

You could make an insulating layer with perlite and clay and maybe Ciment Fondu (calcium aluminate cement not Portland) but I sincerely doubt that it would be anywhere near as robust as the comercially available castable.
If money is a big issue, maybe you could line the firebox (the bit subject to impact and abrasion) with the commercial stuff and experiment with homebrew chimney linings?
The 3:1:1:1 mortar mix used extensively on the forum for building ovens is not insulating, and I imagine a rocket stove could subject it to more heat than an oven does - I wouldn't use it in your application.

[michelevit]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

I've made a wood fired pizza oven using the homebrew recipe of portland, fireclay, lime, sand, sand, sand. Its survived many years and has not signs of failure. I recomend it highly.
No need for expen$ive castable refractory when the homebrew works just fine. Ours was cast over a sand mold similar to the clay ovens.

[david s]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

Not sure quite what you are chasing, is it for thermal mass or for insulating? Any addition of insulating aggregates like perlite or vermiculite drastically weaken the resulting mix. You also need to add fibres to the mix to reduce the possibility of the refractory blowing when first fired. Use a proprietary mix and get advice from your refractory supplier as to the best product for your application.

[thickstrings]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

I think he is basically making one of these Wood Burning Heater - Dragon Burner Which uses a cast refractory. ? for O.P. How is the 8" clay holding up? what type insulation are you using?

[Lburou]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

During a visit to my local refractory, I spoke with the owner and his engineer about a home made castable for my best friend working overseas where materials are not readily available. This is the home brew recipe they gave me -use at your own risk:

30% Ciment Fondu (calcium aluminate cement I'm sure you can get some)
60% Aggregate (#6 mesh -sizing 3mm down to powder)
5-10% Fine Sand
5-10% kyanite (crushed, Metamorphosed peri-aluminous sedimentary rock, optional, if not available locally-add more fine sand)

HTH

[RocketStoveBuilder]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

Thanks for your helpful replies.

"Not sure quite what you are chasing, is it for thermal mass or for insulating?"

The burn chamber and the "chimney" need to be able to withstand high heat, that is, they are hot faced, and need to be insulating at the same time.

"You also need to add fibres to the mix to reduce the possibility of the refractory blowing when first fired."

I've seen fibers added. What kind of (locally available if possible) fibers could be added? Shredded fiberglass insulation?

My understanding is a low heat slow fire is how this ""blowing" is avoided, with a gradual increase in the temperature of the fire to cure the cast.

"If money is a big issue, maybe you could line the firebox (the bit subject to impact and abrasion) with the commercial stuff and experiment with homebrew chimney linings?"

That is an idea worth pursuing wotavidone.

Money is an issue only in that the commercially available castables, if used, would constitute over 20% of the retail price of the finished product. We'd either have to forego that revenue or raise prices. An in house mix that would accomplish the same end - durable and insulative and less expensive is what we need.

The product itself is not the real problem, it is the shipping that is almost the same cost as the product is the real obstacle.

"You could make an insulating layer with perlite and clay and maybe Ciment Fondu (calcium aluminate cement not Portland) ..."

Yes, just learned that calcium aluminate cement is what we want, not Portland cement - thanks for your excellent advice.

"If you are going into production, you can buy the proper ingredients to make your own refractory castables."[/COLOR]

Yes, that would be an excellent solution. Looks like I'll have to seek the input of someone knowledgeable about the "proper ingredients" or simply bite the bullet and use the commercially available products.

Anyone know of a knowledgeable engineer or chemist - willing to assist us? If so, please PM me.

Thank you david s, michelevit, wotavidone, Tscarborough and stonecutter for your thoughtful comments.

[RocketStoveBuilder]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

I think he is basically making one of these Wood Burning Heater - Dragon Burner Which uses a cast refractory. ? for O.P. How is the 8" clay holding up? what type insulation are you using?

Yes.

The 8" masonry flues seem to work fine.
We use perlite for the insulation.

[RocketStoveBuilder]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

30% Ciment Fondu (calcium aluminate cement I'm sure you can get some)
60% Aggregate (#6 mesh -sizing 3mm down to powder)
5-10% Fine Sand
5-10% kyanite (crushed, Metamorphosed peri-aluminous sedimentary rock, optional, if not available locally-add more fine sand)

Awesome Lburou. I'll need to add perlite to the mix to get the insulative properties we need.

[wotavidone]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

When all is said and done, the 3:1:1:1 (sand:lime:clay:Portland cement) is very very cheap. So, very little money will be put at risk by making a stove lined with it and trying it out.
Same can be said (nearly) for a refractory made with Calcium aluminate cemet (ciment fondu).
Ditto any combination of perlite and cement you can think of, if you need some insulating qualities.

Looks like you are going to have to do some research.

[brickie in oz]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

I think that anyone who comes on here looking for advise from members so they can use it in a commercial environment and hence make money is rude and cheeky.

Are you going to split the royalties with the forum members for their effort, I doubt it.

Go and pay a thermal engineer to come up with something to your requirements.

[RocketStoveBuilder]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

"I think that anyone who comes on here looking for advise (sic) from members so they can use it in a commercial environment and hence make money is rude and cheeky. Are you going to split the royalties with the forum members for their effort, I doubt it. Go and pay a thermal engineer to come up with something to your requirements."

I guess there is another way of looking at our requests: pretty tough trying to start a business in this economic environment, but we are doing our best with limited funds. All the advice offered was voluntary, for which we are very thankful.

Looks to me that Forno Bravo is a commercial enterprise: from their website: "Italian brick pizza ovens for the home and garden, restaurants and pizzerias; wood and gas-fired. Outdoor masonry fireplaces, and accessories, including oven .." thus I saw no problem with seeking advice from those involved here.

And yes, I would be glad to share the money we make with anyone willing to help us get a solid product on the market. Would you like to be involved? I won't even ask that you share in the expense of the shop, the equipment, or the time and effort we spent since May getting this endeavor off the ground. PM me and lets discuss your participation.

Taking wotavidone's advice, along with a few others, we mixed a batch of fireclay, perlite, silica sand and refractory cement yesterday - made two bricks - each with different mixes. After proper curing on the stove itself, we'll place the bricks in the stove to see how they hold up.

Today we intend to make a 4" thick slab more in line with our needs.

Again, thank you to everyone who has offered their suggestions.

[stonecutter]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

I have never worked on developing a homemade a refractory mix design, but I can tell you that using portland cement ( component of the 3:1:1:1 mix) for your application would not be a good idea.

It doesn't appear that you have though. Pics of your test brick would be cool.