Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

[Tscarborough]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

That is close, Dave, I would only add that the clay and sand are there to reduce shrinkage cracking.

It is important to remember that mortar is not used to hold things together, it is used to hold them apart. That is to say, it allows units that do not fit perfectly to be used to build a cohesive structure. Joint size and aggregate size are tied together, and joint size is tied to unit size and regularity.


Concrete (castable) is not mortar and the requirements are totally different, having in common only some of the same ingredients, refractory or not.

[david s]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

I work with proprietary castable and home brew all the time so shall throw in my .02c worth. Most castables have been designed to withstand temperatures way in excess of the temperature range in which we fire. My product is rated to 1450 C which is around three times hotter than we require. Attempting to find their recipe will run you into a dead end as they are closely guarded secrets, but they contain aggregates that are stable at very high temperatures and not required in our case. The stuff is however designed to be "fired" ie taken to a temperature w hich sinters the product. Failure to do this results in a weaker product and in addition the repeated cycling through the 500+C range is damaging as considerable chemical changes and rapid expansion of materials takes place in this range. Both home brew and castable do work for our application although I would have no idea how long home brew will last as it has not undergone industry testing.
Regarding reinforcement for both castable and home brew, the strength can be enhanced with a number of methods. My preference is stainless needles, although rebar and chicken wire may be effective too. The disadvantage of rebar is that as it is relatively thick and much more heat conductive then it gets hotter than the refractory that surrounds it resulting in greater expansion and therefore damage to the surrounding refractory. Needles allow the heat to dissipate more easily to the refractory that surrounds them. Rebar is also subject to rust in the presence of heat and moisture of which there is plenty in an oven. The problem with chicken wire is the ovens temperature. Zinc melts at around 400C so who knows how protective it will be over time. For these reasons stainless steel needles are the preferred and recommended reinforcing for any castable refractory and although expensive are probably the best solution.

[cobblerdave]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

That is close, Dave, I would only add that the clay and sand are there to reduce shrinkage cracking.

It is important to remember that mortar is not used to hold things together, it is used to hold them apart. That is to say, it allows units that do not fit perfectly to be used to build a cohesive structure. Joint size and aggregate size are tied together, and joint size is tieid to unit size and regularity.


Concrete (castable) is not mortar and the requirements are totally different, having in common only some of the same ingredients, refractory or not.

Gudday
Like the bit
"it allows units that do not fit perfectly to be used to build a cohesive structure"
I think it describes my oven to a tee
Regards dave

[stonecutter]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

I'm not discounting the fact that commercially available engineered products work, just that a much cheaper more readily available home brew version does as well.

so once again

one part portland
one part lime
one part fireclay
three parts sand
mixed dry
add minimal water.

This mixture once cured will form into a rock hard structure that will survive many years of scary fires and can achieve the desired 90 second pizza. It will not crumble, crack or disintegrate.

Like it was said, I'm glad it worked for you...for your needs, circumstances, location, etc. However, it is not a good choice to use that mix to cast a dome for the reasons given already. Anyone considering an oven like this should do the research and consider their goal with the build....you stated yours was an oven on a shoestring...good job, mission accomplished. And since you are convinced of it's quality, I am not really trying to reason with you anymore.

But before advocating 3:1:1:1 as a bombproof "refractory" mix ( it isn't) to people that want to cast an oven, maybe you should consider what ceramic engineers and refractory mix designers have to say about high ratios of Portland in high heat castables. Portland degrades from high heat, so a castable with a high percentage of Portland will not..should not be used. And THAT is the point that is being made....not that the mix doesn't make an oven.

I can dry in a roof with tarps, but that doesn't mean I made a good decision because my house stayed dry for 5 years.

[thickstrings]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

.From above......Don't know if this will work,http://www.skylinecomponents.com/Cer...ercoating.html but I have thought to use this over CF board in the heat riser section.....could be viable, just "hot face"heat and no mechcanical abrasion....Can any one coment on this product?

[stonecutter]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

.From above......Don't know if this will work,Ceramic Fiber Rigidizer but I have thought to use this over CF board in the heat riser section.....could be viable, just "hot face"heat and no mechcanical abrasion....Can any one coment on this product?

Sure it will work. I did something similar to the ceramic gasket used in my thermal break, to encapsulate the fibers. I used a slurry of Portland cement/fireclay mixed with an acrylic admix, and painted it on. It works well, it's cheap, and easy to apply.

I didn't look at the price of that stuff, but for what it does, you can achieve good results with a portland/fireclay slurry, and the material is already on hand.

[david s]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

.From above......Don't know if this will work,Ceramic Fiber Rigidizer but I have thought to use this over CF board in the heat riser section.....could be viable, just "hot face"heat and no mechcanical abrasion....Can any one coment on this product?

Last time I looked at ceramic fibre rigidiser I found it contained carcinogens so I steered clear of it.

[thickstrings]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

My intent was to use it in a rocket mass bell heater....http://www.stove.ru/index.php?lng=1&rs=3 this may help to explain how they work.... a heat pump,sort of.

[RocketStoveBuilder]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

FYI: Inspected our first rocket stove heater built used to heat our shop yesterday. We've had around 20-25 total hours of fire in the stove, none of which had a high amount of fuel at one time - so the fires weren't as hot as they could be.

We were surprised to find two 1 1/4" fire bricks cracked - we assume from the heat. One brick was cracked completely through, the other almost through. With a little pressure we believe we could break the brick.

I've had many wood stoves in my lifetime, many of which we had filled up the fire box full to get a roaring fire going - but I do not remember ever observing a cracked brick. My last stove had 1 1/4" bricks lining the stove and even though we burned "hot" fires all through last winter, I did not notice any cracked bricks. This leads me to believe the fire within our rocket stove gets much hotter than the fire in the wood stoves, which is to be expected.

Taking everything into account, we've concluded we will use a commercially available castable mix for the lining of our rsmhs - at least for now - as we continue to test our shop "home brew" mix.

We also "cured" two home brew bricks we made by placing them on the rsmh for a few hours while a fire was burning.

For those of you interested, we used a mix of alumina cement - perlite - silica sand - fireclay and minimum water. The test bricks hardened up nicely. When we cut one brick, the 1" x 1 1/4" thick piece did crumble easier than we would have liked. This, in part, leads us to believe we need to keep testing while we use the commercial mix.

We are going to reduce the thickness of our lining and use more perlite to increase the insulation qualities of our stoves, while decreasing the weight and the cost at the same time.

As an aside, we were able to raise the very top of our pea gravel mass (not the best mass we admit - due to the air gaps between the gravel) from 55* F before the first fire to 85* F with 2 1/2 hrs of medium fires with mostly low btu soft wood. I say the top because we have an 8" stove pipe that is doubled over upon itself due to space constraints, and most of the highest heat is in the lower pipe not the top pipe. We've been able to raise the temp. of the lower pipe to 120* with a 2 1/2 hr fire. The top seat of our mass bench was warm to the touch, but not as warm as we expect to eventually achieve with hotter fires and longer burns.

[stonecutter]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

I hope you continue updating your progress..this is very interesting. As to your test brick, perhaps the mix needed more hydration..after all not enough water in the mix is as bad as too much.

[david s]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

The calcium aluminate cement only requires 24 hrs curing, it does not continue to increase strength over time like Portland cement. It is advisable to cover it to hold it the moisture while curing though.

I think your cracked brick is almost certainly caused by temperature difference of which there will be plenty in a rocket stove. It wouldn't matter how high the quality of your fire bricks are they will still crack if the temperature difference and hence the expansion is too great. The only remedy here is to heat them slowly to allow the temperature and expansion to even out and this is not really an option. Either live with the cracks or engineer joins where the cracks were.

[RocketStoveBuilder]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

"I hope you continue updating your progress..this is very interesting. As to your test brick, perhaps the mix needed more hydration..after all not enough water in the mix is as bad as too much."

We made two test 1 1/4" bricks - one with more water - but didn't cut that brick. Will check Monday. I've been told too much water is not good either - so don't know what to make of it. Maybe we heated them too soon......

We also made a 4" thick brick. Will check it Monday also.

"Either live with the cracks or engineer joins where the cracks were. "
We can live with the cracks as they are not that significant.

thank you both for your interest.

[RocketStoveBuilder]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

Put the "home brewed" brick inside the rsmh yesterday right where the fire burned most of the day. This was after we had the bricks set on the stove itself to cure for two days. Checked it last night - didn't seem to be any damage.

We also put another home brewed brick, one with more water added to the mix, deep inside the stove right below the burn riser (chimney). Will remove and check today.

We place our 4" brick on top of the stove on the hottest part of the metal - between the fuel feed and the outer shell for the last two burns. Checked the temperature of the brick - at times the brick was close to 500* F. No signs of damage.

With the cost of commercial refractory castables for each individual stove priced at $550, (including shipping - shipping is expensive isn't it?) we are going to proceed with our stoves using our own mix and continue testing as we go. We cannot justify spending over 25% of the cost of the stove on these components of the rs when we believe we can make these components ourselves in house with our own mix with satisfactory results.

As promised, I will post images as time allows. Thanks for your input and suggestions.

[RocketStoveBuilder]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

...... The refractory manufactures are in the business of manufacturing and selling
refractory. I already know the answer they will give me.

I have proven that the homebrew does work as a refractory casting material. Why is everyone so adamant that it wont work? It does work. Its is what is used to hold a traditional brick oven together.

What are your opinions to using a portion of Portland cement along with alumina cement in our mix? Not much - but enough to aid in the binding.

Are there any advantages, disadvantages?

I understand Pcement needs time to cure, but that really isn't a factor in our case.

Any comments welcomed and appreciated.

[stonecutter]

Re: Your Advice Re: HomeBrew Castable Mix Greatly Appreciated

What are your opinions to using a portion of Portland cement along with alumina cement in our mix? Not much - but enough to aid in the binding.

Are there any advantages, disadvantages?

I understand Pcement needs time to cure, but that really isn't a factor in our case.

Any comments welcomed and appreciated.

The thing is, a lot of refractory cements and castables are made with a small percentage of portland in it. But it is understood that the more portland in the mix design, the weaker it will at sustained high temperatures. What you should consider doing, is developing a mix with different levels of portland, but at very low percentages.

That is the issue with using 3:1:1:1, the portland ratio is very high...and to avoid repetition, that's all I will say about that.

I also believe that one or two test fires are not sufficient to determine a quality product....but that is an opinion and ideology, and everybody is different. Take that with a grain.....