Cast Double Shoe-box Rocket using User-Friendly Cheap LTGS

[sksshel]

In this thread, I will be building a 6" cast Double Shoe-box Rocket (DSR) mass heater for my home using user-friendly cheap Low-Temperature Geopolymer Setting (LTGS).  Geopolymer (Polysialates)
can be defined as 1 Oxide, 1 Alumina, and 4 Silica atoms.  The structure is some variation of this:
Na₂O - Al₂O₃ - 4SiO₂ - 11H₂0  

Phew, now that I got all that chemistry stuff out of the way, here's the formula that I've tested and decided upon.   You can read more about the testing results HERE   Many thanks to all that participated and especially to Karl for his valuable insight. 

Geopolymers are roughly twice the strength of cement, half as dense and quite heat-resistant (greater than 1670°C, > 3000°F) (update #2)

The formula I will be using is as follows:
monopotassium phosphate 29g 7%  I Purchased Here
Soda ash 26g 6%   I Purchased Here
Water 4oz 1%
acetic acid 3oz 1% $2 at Big Lots for a gallon
portland cement 59g 15%   I Purchased Here
clay 200g 50%   I Purchased Here
fiber 80g 20%   I Purchased Here

- it results in 36 cubic inches of product, .59 liters
- it weighed 15.4 ounces or 436 grams
- extrapolating the results: 1 liter will weigh about 739 grams, 1 cubic foot will weigh about 99 pounds
- it requires 1.7 times the formula to produce a liter of product
- 104 times the formula to make a square foot
1.5 cu ft of the product is needed to make the batch box portion.  The material cost will be less than $50.  (update #1)

Procedure:
- Mix the monopotassium phosphate and Soda ash powders

- Add boiling water This causes a reaction to drive the carbon out. Look for No new bubbles on the surface, no fizzing sound, no volume increase. This should conclude within a few minutes.

- Add the citric acid. Wait until the reaction comes to a halt. This should conclude within a few minutes.

- Add the Portland Cement and Clay.

- Add the Rockwool a little bit at a time, stirring as you go.

- Add only enough water to get the desired consistency

- Let the end product cure until hard. 27°C/80°F is only slightly above ambient temperatures (20-25°C).  (approximately 2 weeks)

- Gradually increase firing temperatures to keep the heat stresses low and to ensure that the ceramic bonds can be established before the destruction of the prior bonds. The refractory bonds which are not fully complete will be replaced by ceramic bonds in the initial firing.


Updates:
1 - added note about batch box cost 2/26/2018
2 - originally "about 1000°C or 1832°F" - further research to be done, but initial findings suggest the product will easily support the temperature requirements of a rocket heater. 

[sksshel]

I plan to build a DSR out of poured LTGS.  Initially, I will be making 5 forms with some being reused multiple times.  The list of initial forms includes:
- Batch Box floor
- Sides (used twice)
- Back
- Batch Box Top 
- Shoe-box Top

Initially, I will be using refractory bricks and duct pipe to prototype and test the Double Shoe-box Rocket. 

Once, tested, I will pour the remaining forms including:
- Shoe-box sides (used twice)
- Shoe-box back
- Riser (used twice)
- Bell Panel (used 12 times)

I will be using angle iron around all the corner edges for structural support.  It will also support the 2 doors.  1 door for the burn chamber and 1 door for the viewing window to the "Ram's Horns" burn process.  The latter door is needed for periodic cleaning of the window. 

The Bell's LTGS components will be externally supported by cement blocks.  See pic below. 

You can read more about Batch Box Rocket (BBR) mass heaters HERE
You can read more about Double Shoe-box Rocket (DSR) mass heaters HERE and HERE

[sksshel]

The ultimate end goal will hopefully look something like the pic below.  I will add a facade made of fire-resistant walls with a decorative stone face.  The top will be open to air flow.  The vent above it will allow the air to flow upstairs.  

[josephcrawley]

Is 1832 a pretty exact max temp? Cause my understanding is that's about 300 degrees to low for the box or riser. This is very exciting and I hope it works!

[sksshel]

Feb 21, 2018 6:48:53 GMT -8 josephcrawley said:

Is 1832 a pretty exact max temp? Cause my understanding is that's about 300 degrees to low for the box or riser. This is very exciting and I hope it works!

Good question.  Thanks.   When I first began to research geopolymers for a rocket stove, I looked closely at the temperature ratings.  I haven't double checked the temperature rating since then.  I'd like for the experts to chime in here.  If I need to adjust something, now is the time to do it. 

[pigbuttons]

Feb 21, 2018 8:12:45 GMT -8 sksshel said:

Feb 21, 2018 6:48:53 GMT -8 josephcrawley said:

Is 1832 a pretty exact max temp? Cause my understanding is that's about 300 degrees to low for the box or riser. This is very exciting and I hope it works!

Good question.  Thanks.   When I first began to research geopolymers for a rocket stove, I looked closely at the temperature ratings.  I haven't double checked the temperature rating since then.  I'd like for the experts to chime in here.  If I need to adjust something, now is the time to do it. 

You are on your own as far as temp rating goes. Each recipe will yield a different result as geopolymers are not a single chemical entity. There will be many factors which will affect the heat tolerance. I've had a few attempts that crack just over 700F while others will stand anything up to and including a MAPP gas torch at 3650F 2010C. So here is my list of possible variation candidates; the exact chemistry (the clay, NaOH vs KOH vs all the acids); air entrainment; fillers such as fiber, non reactive clays, grog, sand; excess water trapped inside; if cement products were used as accelerators. The long and short of it is you just have to test it out, and don't depend on the results of just one test for a particular recipe. We humans tend to introduce variations without noticing them sometimes.

[esbjornaneer]

There are a lot of factors to take into consideration. A test of the core may work when the finished heater with heat extracting bell does not (the case in my first build) even when using the same materials.

And the geopolymer aspect of it... this is what we have to prove (and report on) that it is coping with the heat generated in these DSR/BBR/j-tube cores.
And report even where there are failures!

[sksshel]

What would be a rigorous test to verify the results? How long should a torch be held against a sample? What metrics should be captured? Once the heater has been cast, what metrics should I capture at that point?

[esbjornaneer]

sksshel your last post makes it look like pigbuttons asked those questions

It would be great if peterberg and briank would comment on the specifications for what confirms that the completed build is a success.

[sksshel]

After doing some research on buying a tester, I've ruled that out.  It's simply too expensive for my goals.

Without a tester, is it feasible for me to assist?

Is this a case where "the perfect is the enemy of the good?"  Peters testing seems to target the absolute best configuration to produce the best possible burn.  Are there simple observations that can be made to determine "Good, Better, Best?"  ie: smell, visible smoke, flue temps, burn chamber temps, etc

[briank]

I’m no expert but I’m certainly willing to share what I do know, which has come directly from Peter’s expert advice. These are the only specifics I know to share with you about the 6” DSR configuration.

Use the exact dimensions Peter has already recommended for a 6” BBR. Make the Venturi port 2.5”x7” in the very rear center of the firebox, right up against the rear wall. Secondary air supply tube must be placed right up against the rear wall and centered in the top port. I’m using a 2” stainless steel tube and the size of the secondary air tube must have a cross sectional area of at least that of this tube (3.15 cu in.). My secondary air delivery recess is a 4”x1” recess in the 1” ceramic fiber board underlying the split thickness firebrick floor of my firebox. The secondary air tube must extend 1/2 into the thickness of the roof of the firebox where the port is cut, and the area where the port is cut needs to be 2 1/2”-3” thick. I’m using 1” thick ceramic fiber board for my firebox roof, then placing another 1 1/2” of ceramic fiber board directly around the port and extending out about 2”.

I’m making the expansion chamber/riser directly above the firebox the same dimensions as the firebox and 7” to 8” tall then a 6” exhaust port (round or oval) at the opposite end (firebox door end).

The DSR configuration requires a lot more secondary air than that of other BBR configurations, about 1:1 primary to secondary air according to what Peter has told me in the past.

These are my best recollections and I’m certainly open to corrections/revisions. I do not own a Testo unit so I can’t provide any data for that. I can say that when I built my first 6” DSR my secondary air supply was only 2”x1” and it burned dirty until I opened up the secondary air recess to 4”x1” and drilled more holes for secondary air intake in my firebox door frame.

[sksshel]

Excellent information.  Thanks.  

I have some questions:
- the primary air on a 6" BBR is 5.64 cu in.   If the secondary air channel that goes into the port needs to be the same size, that would be 2 3/8" square with wall thickness added, wouldn't it basically take up a full 2 1/2" lengthwise of the port?
- if your stainless steel pipe is 2" (3.15 sq in), isn't that the size of your "secondary" air?  I'm getting a little confused by the semantics.  
- if you put additional holes in the door of the batch box, wouldn't that be adding to your "primary" air?  again. . . I know semantics.  I'm just trying to get a grasp on the concepts. 

Thanks again, very helpful info. 

[briank]

Feb 22, 2018 14:43:16 GMT -8 sksshel said:

Excellent information.  Thanks.  

I have some questions:
- the primary air on a 6" BBR is 5.64 cu in.   If the secondary air channel that goes into the port needs to be the same size, that would be 2 3/8" square with wall thickness added, wouldn't it basically take up a full 2 1/2" lengthwise of the port?
- if your stainless steel pipe is 2" (3.15 sq in), isn't that the size of your "secondary" air?  I'm getting a little confused by the semantics.  
- if you put additional holes in the door of the batch box, wouldn't that be adding to your "primary" air?  again. . . I know semantics.  I'm just trying to get a grasp on the concepts. 

Thanks again, very helpful info. 

I agree, it’s confusing. I originally had a 2”x1” recess for secondary air feeding into a 1.67” round stainless tube. Peter recommended a larger tube and widening the secondary air recess to at least 3”x1”, preferably 4”x1”

Plus he said the ratio of primary to secondary air for the DBR configuration was closer to 1:1 than the (closer to) 3:1 (?) ratio of a traditional 6” BBR. Much of his advice I gleaned from this discussion: permies.com/t/72909/Shippable-core-kits-January

From talking to Peter late last year, I really think the DSR configuration is still under development and thus an experimental design. I don’t think he’s had the chance to standardize it yet, so exact dimensions for all these parameters might not be established yet. 

[pigbuttons]

Since we're on the geopolymer sub forum I'm going to try and answer the question of success vs failure from that point of view. Experience is always 20-20 and I started my rocket stove experiments with portland based construction. It turned out strong (structurally), heavier than expected, insulated far beyond my expectations, and ran without smoke or smell. The heat however began to cause spalling after about 50 fires and then some deeper cracks began to form. It was obvious that portland was a no go.

That's when I found this site. My next attempt was with Calcium Aluminate cement and it has performed even better than the first in all of the good characteristics and has lasted for two winters of intermittent use. But after a couple of hundred fires it is starting to "dust" a bit. This one is not going to last more than about 5 years at best.

Now I'm heavy into finishing my house but still dabble in geopolymer recipe quests. I won't know how successful any of them are until I have built a stove and run it for at least 3 years, or very hard for one year.

Just my opinion of the materials side of "success".

[esbjornaneer]

Thank you @pigbutton, it was more on the material side of the conversation that I was asking briank and peterberg to comment as their joint comments in another thread asked @karl to build a batch box stove to proove that the geopolymers are up to the job. I am on that quest and wonder what the specifications are, from their point of view, for a material to cope with the rocket environment.
Is it so many fires? So many years of use? Such abrasion resistance? Such outside wall of core temp?
What else that needs to be shown before it can be recommended for use in an indoor rocket installation?