Mixing refractory

[fox]

Hi guys, I have not been around for some time due to illness and my retirement from building pizza ovens.
However, I have been busy building rocket stoves and I am soon to be building a new pizza oven mold.
in the mean time, I do have a couple of videos that might interest some of you….

[david s]

Thanks for posting those most useful videos. One small point that might confuse members is your use of the term “refractory cement”. There is often confusion when the terms cement and concrete are used to describe a mix. Cement is the binding agent, but when mixed with aggregate becomes a concrete.
The fondu (calcium aluminate) cement is the cement component. When mixed with an aggregate it becomes a concrete mix known in the industry as castable refractory.

Like you I use a vibrating table, but home builders can also use an orbital sander held to the outside of the mould. If casting over a sandcastle mould, which is far simpler than fabricating inner and outer moulds, vibration is not possible and a wetter mix makes application and void reduction much easier.

I was interested in in your experiments with fibre additions and have also found the basalt fibres difficult to disperse evenly (agglomerate). AR fibreglass fibres do not have this problem. Both are resistant at a WFO service temperature range (I've melted glass fibres in my kiln at 900C) and produce similar strength enhancement according to testing data.

Breaking samples with a hammer is not particularly accurate, but does provide an indication. Here’s a sample bar breaker I made to test concrete sample bars strength. It uses a torque wrench to apply the force. It’s not quite as good as an industrial lab’s hydraulic press, which applies a steady increase in force rather than a stepped one, but still gives pretty accurate results.

[UtahBeehiver]

Fox, glad to see you back and feeling better. Always am interested in you rocket stoves...........

[fox]

Hi thanks Russel, after two bouts of major surgery I am left with 3/4 of one kidney, I have to stick to a careful diet but, otherwise I feel OK.

So, I have an old mold that I started to make for a 34” dome, it has been in my shed for about 10 years but, I thought now is the time to finish building it.

David re the refractory, in my rocket stove community we are always striving for recipes to be able to cast integrate shapes with sharp edges that can withstand 2000c.

Using fine grain aggregates and carbon fibres are offering the main successes, the biggest issue is distributing the fibres.

There is an American company producing basalt needles, they are 50mm long and very stiff.. They are designed for concrete reinforcing, they work very well and are very easy to mix in but, are too long and thick for fine detail casting, so what we are working on, is finding a way to rigedsize carbon fibre so we can mix them more easily?

[fox]

I have this video showing my tiny 4” vortex rocket stove that I have been working on for a couple of years, it runs around 750c in the afterburner but I have designed the hot plate to run around 325c for cooking purposes.

[david s]

Hi thanks Russel, after two bouts of major surgery I am left with 3/4 of one kidney, I have to stick to a careful diet but, otherwise I feel OK.

So, I have an old mold that I started to make for a 34” dome, it has been in my shed for about 10 years but, I thought now is the time to finish building it.

David re the refractory, in my rocket stove community we are always striving for recipes to be able to cast integrate shapes with sharp edges that can withstand 2000c.

Using fine grain aggregates and carbon fibres are offering the main successes, the biggest issue is distributing the fibres.

There is an American company producing basalt needles, they are 50mm long and very stiff.. They are designed for concrete reinforcing, they work very well and are very easy to mix in but, are too long and thick for fine detail casting, so what we are working on, is finding a way to rigedsize carbon fibre so we can mix them more easily?

That is most interesting. A rocket stove with a more concentrated flame and fuel toxygen in a more ideal ratio produces a far hotter flame than a WFO with its large open mouth. The best way to trial stiff is by trial and error I think, so you’re on the right track. Both vermiculite and perlite are good for 1100C and for interest I’ve fired samples of both in my kiln to 1100 and confirmed that they turn to dust. Crushed firebrick that has been fired to over 1200C is more stable. I use basalt fibres for reinforcing mixes of various concretes, but mine vary in length and diameter and suffer from agglomeration (clumping) AR glass fibres , being thicker and longer don’t suffer the same problem. I Ali’s use melt extract fibres (stainless needles) , which are the industry standard for castable reinforcement. They disperse very well, but they’re not called needles for nothing and need care in application bring hard to handle. Generall the smaller the fibre the greater the problem of agglomeraion. I also use nano fibre reinforcing which utilize graphene or carbon nano tubes to reinforce a mix on a molecular level. The manufacturers claim the agglomeration problems have been solved via chemical additions to the liquid concentrate, but as they're not visible only testing will provide the answers as to strength enhancement and even dispersal in the casting. I’ve tested sample bars for flexural strength and it shows a 50% increase in a standard high strength concrete mix.

The safest solution rather than making your own mix is to purchase some castable refractory, available in dense (hot face) or insulating forms that has the high temperature aggregates and the burnout fibres included.Any casting made from proprietary castable or a made up custom brewshould be very carefully kiln fired to the correct firing schedule or it can suffer destruction from the very vulnerable mechanical water removal (100-300C), chemical water removal (400-600C) and the alpha change (500-600C) The vulnerable changes that occur a re not possible to done with a fired in place method, particularly with wood which results in uneven temperature fluctuations.

[fox]

There are a few manufactures of vermiculite board, the best ones will survive 1000c for several hours every day for years, but yes you are right they detonate at around 1100 - 1200c!
I have concentrated on the 4” vortex stove mainly because of its low operating temperatures compared the 6-7-and even 8” stoves that sore to over 1500c in the vortex chamber.
At those temperatures we generally use fire brick backed with insulation but it makes the stoves very heavy and much larger and much slower to heat up compared to a stove made from purely insulating material.
So we are always trying to find hight temp mixes that can be made very thin, 20-25mm like kiln shelves, some of the more expensive refractory mixes are very good but still fail when cast that thin.
Even my little 4” stove can reach over 800c in the central air space, the sides of the chamber can be pushed to 750c but the ceramic glass starts to melt at that temp, so I have designed it to run at around 650-700c.
At that temp the glass will only last about 20 hours before is goes opaque, although it will last for a long time in that state, I change it every 15 hours or so.
In some of my other rocket stoves, the vermiculite board will last for many years even in the loading fire box but they operate at around 500c in the fire box area.

[david s]

If you are seeking a low thermal mass solution then you may need to be prepared to compromise. Dense refractory does lead to high thermal mass, desirable for a WFO, but not necessarily so for other applications. Lowering density leads to an equivalent lowering of strength which may be undesirable. Modern kils are lightweight, utilizing hot face (not insulating) ceramic fibre blanket, which is held in position by ceramic buttons and high temperature wire. This leads to far less fuel consumed because of the low thermal mass. The density of the kiln furniture and wares provides adequate thermal mass for stable temperature retention.
For my kiln I took a compromise to use calcium aluminate cement, clay, stainless needles and crushed insulating firebrick, from fines to 6mm, as the aggregate, which gave me a casting of low density, but with sufficient strength to withstand bumps and abrasions, while also having the high refractory qualities required. The insulating firebrick are far easier to crush than dense firbrick, but the fine dust created should not be added to the mix. My casting was fired in place with its gas burner, following a very conservative firing schedule.

I'm sure with a number of folk pursuing the problem, you'll end up with a decent solution. Can you post some pics or plans of your rocket stove. Some members have built ovens powered by rocket stoves, but there are very few reports on their success or performance.

[fox]

We (as a group of enthusiast across numerous forums and platforms ) wont use ceramic fiber products anywhere that is exposed to the flame path due to the high health risk oft the super heated, sintered fibres!
I have several build videos showing how the vortex stove (and the vortex J tube) are made but as it is still in development ( getting close now) I have not finalised the dimensions but will make them public as soon as I am content with the stoves consistent function.
There are lots of folk who have built what they consider rocket powered pizza ovens but, as a rule they are not really able to charge enough heat into a large floor mass.
I have often considered building a hybrid pizza oven that could be heated with an internal fire and then be maintained with a rocket stove or re heated the following day.
There are even rocket stove kilns powered by duel 8” stoves that can fire pottery but the main benefit of a rocket stove is to power a large mass with a very clean burning fire.
The mass is most commonly, a double brick bell (box) built over a compact design rocket stove, the stove will be fired for one or two burns (batches) to warm the brick bell to around 70-100c and slowly release the heat over the next 10-12 hours. So not that dissimilar the a pizza over without any insulation over the dome.
So you get an extremely clean burning stove that will heat a large house with only one fire a day or two in cold climates.
The down side is cost to construct (similar to a pizza oven) and making it blend in with the home!
The very latest designs, namely the “Shorty Core” is very simple and easy to build, fully scalable from 5-10” . It wont smoke from start up to charcoal, one load will last upto 2.5 x 3 hours. They are very efficient and very economical to run and pass all european pollution regs.
I can think of ways to heat a pizza oven dome or the floor but not both at the same time unless you use two separate stoves!
However, only last week we saw the first Chinese made, mass produced rocket stoves, they are presently being tested for the tough European safety and pollution regulations. So with big company backing we might see much more scientific development ……there is only so much one man can achieve in his back yard!

[fox]

We (as a group of enthusiast across numerous forums and platforms ) wont use ceramic fiber products anywhere that is exposed to the flame path due to the high health risk oft the super heated, sintered fibres!
I have several build videos showing how the vortex stove (and the vortex J tube) are made but as it is still in development ( getting close now) I have not finalised the dimensions but will make them public as soon as I am content with the stoves consistent function.
There are lots of folk who have built what they consider rocket powered pizza ovens but, as a rule they are not really able to charge enough heat into a large floor mass.
I have often considered building a hybrid pizza oven that could be heated with an internal fire and then be maintained with a rocket stove or re heated the following day.
There are even rocket stove kilns powered by duel 8” stoves that can fire pottery but the main benefit of a rocket stove is to power a large mass with a very clean burning fire.
The mass is most commonly, a double brick bell (box) built over a compact design rocket stove, the stove will be fired for one or two burns (batches) to warm the brick bell to around 70-100c and slowly release the heat over the next 10-12 hours. So not that dissimilar the a pizza over without any insulation over the dome.
So you get an extremely clean burning stove that will heat a large house with only one fire a day or two in cold climates.
The down side is cost to construct (similar to a pizza oven) and making it blend in with the home!
The very latest designs, namely the “Shorty Core” is very simple and easy to build, fully scalable from 5-10” . It wont smoke from start up to charcoal, one load will last upto 2.5 x 3 hours. They are very efficient and very economical to run and pass all european pollution regs.
I can think of ways to heat a pizza oven dome or the floor but not both at the same time unless you use two separate stoves!
However, only last week we saw the first Chinese made, mass produced rocket stoves, they are presently being tested for the tough European safety and pollution regulations. So with big company backing we might see much more scientific development ……there is only so much one man can achieve in his back yard!
Here is a good example of a home build rocket mass heater living in Montana with -10 c outside but olny requires one 2 hour fire a day to heat a large area.

[david s]

Thanks for your information. Although a bit off topic for WFOs, there's much that is common. I look foreard to learning about your results.

The ban on RCF refractory ceramic fibre back in 1998 by the EU led to the development of a new generation of fibre that resulted in a much larger fibre that wouldn't lodge in the lungs as well as making the fibres bio-soluble. This new generation of fibre is now exonerated as a carcinogen although still classified, as many products are, as a skin irritant. In a WFO application, because the fibres are encapsulated in the insulation layer they can't present problems in a finished oven anyway. Builders should always check the MSDS for every product used and avoid the old generation of blanket which is classified as a Class 2 carcinogen (ie tumours been discobered in animals, but none in humans)

Regarding the use of hot face RCFs in kilns and furnaces where temperatures exceeding 1000C can create cristobolite "exposure to simulated after service fibres did not produce significant excess tumours in animal studies."
No workers having high levels of exposure to both old and new generation of fibres for many years have developed tumours attributed to RCFs.

Because of the safety concerns I avoided using fibre blanket for insulation back in the early 2000's when I started building ovens, using layers of vermicrete instead. The new generation of safe fibres was ridiculously expensive but by around 2012 it had largely replaced the older generation and the price dropped considerably,

Those wishing to read about this topic shoud check this link.

https://www.hse.gov.uk/foi/internalo...99/267_3v2.htm

[fox]

Ok David, we will have to disagree on this subject, I have no desire to argue or go back and forth trying to convince you with the hard facts.
However I can not support the use of ceramic fibre, so unfortunately I will no longer be posting on this forum.
All the best to you and please be careful.

[david s]

Ok David, we will have to disagree on this subject, I have no desire to argue or go back and forth trying to convince you with the hard facts.
However I can not support the use of ceramic fibre, so unfortunately I will no longer be posting on this forum.
All the best to you and please be careful.

Check my PM