Best fire clay or mortar for alumina bricks

[P2]

Hi,

I am designing an 8" heat riser batch bell style rocket mass heater using high alumina fire bricks:

www.hoganasbjuf.com/sitecore/content/BorgestadFabrikker/Products/Bricks/~/media/Files/BorgestadFabrikker/Products/Brick%20qualities/DB_E_Victor%2095.ashx

I will fit about 2500 kg of bricks in either a round or rectangular shape seen from above but how to best join the bricks? I am fully awere of that these bricks are totally overkill regarding refractoriness but they have high thermal conductivity and density. An other good argument to use them is that they are in my possesion. I plan not to tore the heater apart only because the mortar has degraded after a couple of years, so...what mortar/clay formula to match the refractoriness of the bricks? The heat riser will likely be made of about 2-3 cm thick cut alumina bricks insulated with 5 cm of porous fire bricks on the outer side. The mortar/clay will hold this octaconal structure together.

I have no problem doing the mix myselve, but what to use?

Thanks.

[pyrolyse]

Wouldn't the porous firebricks (insulated firebricks?) be better on the inside of the heat riser? You want the best possible insulation in the heat riser, not on the outside of it.

[P2]

Yes, thats correct. But in this case I am willing to offer some initial efficiency during heat up to gain riser tube longlivety. The inner alumina brick scale will withstand the flame erosion and heat far better than the insulative brick.

[pyrolyse]

I'm by no means an expert on the matter of firebricks. But to my understanding, even the very porous IFB bricks are quite resistant to flame and high temperatures, but not very scratch resistant (ie. not so ideal for the firebox)... But, Matthew Walker actually uses them even in the firebox in his riser less core design and doesn't complain about longetivity.

[P2]

I understand the difficulty in predicting stove life. My wish is to use it daily 3/4 of the year for at least 20 years, hopefully longer. Hence, I try to adress the weakest parts of the construction, starting with mortar and riser tube lining. Thanks for your input so far.

[Vortex]

Even the best quality commercial firebricks can break up over time. If you want it to last that long I'd recommend thinking about a design that allows for easy replacement of the firebrick linings, that way you dont have to rebuild half the stove to replace a few bricks.

[pyrolyse]

I think Vortex has got a point here. Nothing lasts forever.

[coastalrocketeer]

Firebricks made for wood stoves crack after 5-10 years of use when used as baffle bricks in the reburn area.

I think heat shock resistance is probably the most important refractory quality for long life in that area of the rocket design.

With that thin layer edge on, there will not be much area for mortar to join them... And I'm not aware of any mortar that will truly stand the test of time if your stove core regularly gets to 3000F, which can happen in a well dimensioned and insulated rocket.

I'd be inclined to try this:

use masking tape to tie the inside of the firebrick "tile" corners together and use Kast-O-lite 30 (3000F rated) as the insulating layer, with a cardboard tube as the outer form.

Mix and pour the kast-O-lite, and after it has hardened, remove tape inside and cardboard outside.

Can't say for sure how well the kast-O-lite will bond to your fire brick, but if you're set on using the brick, it could be an effective means to both insulate and secure it in the ultra hot-zone of the riser.

[P2]

Thanks for your recomendation, I believe that would be the easy solution. The thing is I already have enough insulative fire bricks I would like to use. Got to measure the chemical composition to estimate refractoriness but the density is 1.1 g/cm3.

This is my thought about high temperature mortar: mix karolin clay and fine alumina powder with water. That way, the mortar would stick adhesivly to the brick surface by hydrogen / vdv-bindings. At elevated temperature, when the kaolin vitrify into mullite and SiO2, it will also sinter and chemically bond to the brick surface and Al2O3 particles. Part of or all SiO2 will merry Al2O3 and make mullite as well. Some elasticity will be lost in the joints but the strength will increase causing small cracks in the mortar exposed to the highest temperatures.

Thoughts about this are welcome.

[coastalrocketeer]

Sounds like a good plan... And like you understand significantly more about the phase changes and chemistry of traditional refractory materials than I do...

You could also look at alkali reacted geopolymer mortar... Which apparently can have much better refractory properties and higher strength than any Portland cement based product, and may be something you can make for a lot less money than conventional hi-temp refractory mortars...

Drawback is it requires significant testing to develop the right mix with the suitable materials you have available to work with.

I believe there are some threads about geopolymers here, and Karl, our resident geopolymer expert has his own forum as well

www.karls-geopolymers.proboards.com

[pigbuttons]

When I built my Rumford fireplace I used "fire mortar" that was sold specifically for use with firebrick by the brick company that sold me the firebrick. The biggest problem with your recipe for karolin clay and alumina is probably going to be expansion rates, especially after sintering. You will then have to re-point the inside face of your structure. The fire mortar is engineered to mimic the expansion rate of the bricks to provide a long term solution. Mine came premixed with the right amount and kind of sand in the mix. I just added water as directed on the bag and it's been holding up fine to some very hot fires. One other thing about this engineered product, there were specific instructions as to mortar thickness to maintain this expansion rate; no thicker than 3/8" no thinner than 1/4". All good to know when you want it to last a long time.

[P2]

My brick suppleant have two types of mortars that could be suitable. Like regular fire clay mortar:

www.hoganasbjuf.com/sitecore/content/BorgestadFabrikker/Products/Monolithics/~/media/Files/BorgestadFabrikker/PDF/Sairset.ashx

And Victor T very high alumina content:

www.google.se/url?sa=t&source=web&rct=j&url=http://cement.hoganasbjuf.com/en/Process-stage/Preheater/Calciner/~/media/Files/HoganasBjufCement/Borchures_English/Monolithics.pdf&ved=0ahUKEwjlzpeko4bTAhWJFywKHSmoDIgQFgguMAk&usg=AFQjCNGT9T0858UntBZd97mkZa1HHCQ0uA&sig2=wcOK74AfZJqjKn-IMM7_Lg

This is really not a money-issue, I just like to know how things work...guess you DIYers understand...

Since my bricks are 95% alumina, I try to find a high alumina clay to match cte.

[pigbuttons]

P2, you are so right about DIYers. We all love to experiment and improve our skills. I look forward to seeing how your project works out.

P.S. "My brick suppleant" should be "My brick supplier", but your written English is very good, better than many college graduates that I've worked with here in America. I just want to help you as friend to improve it even more; I taught English in Japan for 7 years so it's kind of an old habit. "95% alumina, I try " should be "95% alumina, I will try" or "95% alumina, I'll try"

[P2]

Thanks :-) I'll try my best to learn.

Just got an answer from Höganäs, 232 Victor T is prefered for alumina bricks. The price is equal to, or even lower than the raw material individually. Looks like the way to go for the moment even if it is kind of cheating...

[pigbuttons]

It sounds like good news to me, why re-invent the wheel if you don't have to. Cheating would be more like, if you used 232 and told everyone that you mixed it from raw materials. Cheaper, compatible with your bricks, and a known reliable outcome, sounds like a winning strategy. Now you have to get your hands dirty and report back how things are going.