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Post by invention1 on Mar 13, 2019 9:36:03 GMT -8
Here's a photo of the stove while it was still operating. I was just about the start adding the final layer of tile. Good thing I didn't
View inside the bell after teardown. What you are looking at is the opening to the bench, and the opening to the flue. Bench opening is 8"X16", flue opening is 8"X8", flue is 8" round. Heavy creosote deposits.
Metal of the 5 minute riser showed som indication of heat patina in this area. I'd guess the blue would be about 550F if I'm correct. Some of the pipe was against the bell, which resulted in cooler temps.
Inside of the 5 minute riser, good shape.
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Post by Dan (Upstate NY, USA) on Mar 13, 2019 12:54:35 GMT -8
"MORTAR IS DOOMED. "
Mortar's purpose is not to stick bricks together.
It's purpose is to keep them apart.
Every brick is a little different. Without mortar the pressure on these irregularities would be high pressure spots.
These high pressure spots from weight of stuff on top or heat expansion would cause the brick to crack. Then without mortar in between the bricks would be able to dance around each firing and then eventually come loose.
Mortar is a pillow that evens this all out.
I am sure if I took my J-tube apart I would find several cracked bricks in the tunnel roof and all the mortar would come right off by rubbing the bricks together.
But those cracked bricks are locked in the structure with the mortar that is left between them. Even though the mortar no longer sticks...
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Post by drooster on Mar 14, 2019 10:23:55 GMT -8
Dear Mr Invention1, everyone has been super-helpful in comments on your thread but I think your previous few posts are in danger of misleading stove builders with blanket statements which only apply to your strange stove which was never a standard J=tube or batchrocket running on just dry wood with correct dimensions and airs. It does look very nice though, top of this page
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Post by coastalrocketeer on Mar 15, 2019 0:14:29 GMT -8
It is however, a good example of why cob, clay slip and sand mortar and the like are so lovely to build with, ESPECIALLY with experimental designs. Pull it apart... Break up cob with hammer, wire brush off clay sand mortar, rehydrate, rebuild.
It’s that easy!
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Post by DCish on Mar 15, 2019 5:04:24 GMT -8
Refractory mortars of any sort will eventually crack in the hottest areas because of thermal cycling, as they are dissimilar from brick and fail because of differential expansion. Does anyone have a counterexample that has worked without mortar cracking for years? Hopefully folks with personal experience will chime in on this part, but certainly I don't get the impression from this forum that "all mortar will fail." Rather, I would say "plan for the fact that in areas of large temperature differential there will be differing rates of expansion / contraction leading to stress cracks no matter what material you use." I think that is a different thing from the fact that you had a specific kind of mortar that failed. Sorry, I missed the fact that yours is an 8" stove . The dry-stacked core is very much the exception rather than the norm. Firebrick cores with a good, thin, fireclay-based mortar are the most common. Yes, a cast roof is common. This thread shows that even high-quality refractory castable will break apart if high temperature differential thermal zones are not taken into consideration.
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Post by invention1 on Mar 16, 2019 21:33:28 GMT -8
Well I can see I've caused a ruckus with this thesis about mortar. My apologies if I've ruffled any feathers.
Let me restate it in non-controversial terms.
"Refractory mortar, whether DIY or commercial, in the upper third of a rocket stove firebox will not provide an adequate bond between masonry. Masonry that depends on a bond for its integrity will fail, such as when bricks are cantilevered in constructing a roof. Masonry that does not depend on a bond, such as when a roof is made by spanning the roof of the firebox with a complete masonry unit or casting, will have cracking in joints but may not fail. It is for this reason that cast cores may be preferable to bricks."
Is there any part of this thesis that isn't correct?
I would have saved a great amount of effort if I had been aware of this info before embarking on building a final prototype with brick. I'd like to leave a few clues for the next guy. It seems this is what folks are telling me, but despite studying this board for quite a while I never found this info. It has nothing to do with the admittedly wierd design I am building - a basic property of masonry mortar is to provide a bond between bricks. If we cannot depend on that bond, then the mason should be forewarned. There is nothing pinned on the materials section that gives any indication of this warning. Commercial refractory mortar claims to withstand thousands of degrees, which is true, but can still crack and fail because of thermal cycling in the upper parts of a rocket stove.
I would argue that the thesis stated above should be a pinned post on the materials section.
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Post by martyn on Mar 17, 2019 0:58:50 GMT -8
You may well be correct about refactory not bonding to brick under extream conditions. I build pizza ovens, dome shaped brick versions are built so as not to expose the bonding cement mix to direct flame but the actual bricks reach 500c and stay around that temperature for many hours. The dome shape is obviously self supporting so if the cement mix does not bond with the bricks, the dome will not lose its integrity .
However on the few occasions I have had to take a brick dome apart, I have found the cement mix has only normally lost its bond to one side of the brick and remained very firmly attached to the other side.
I also make cast refactory domes, these are made in three separate rings that sit on top of each other to form the complete oven. This style of pizza oven does not seem to crack even after many hundreds of firings and many years of use.
On my own cast pizza oven, I can see some hair line crack but I believe it to still be structurally sound after around 10 years.
The casting mix is made for a commercial product rated to 1400c made up from crushed fire brick, fondue cement, stainless steel needles and burn out polypropylene fibres.
I make up my own bonding cement from crushed firebrick dust and fondue cement but it is very differcult to use due to the fast drying times.
It would seem that trying to bond different materials that have different expansion rates is pretty much guaranteed to fail in very extream conditions like a rocket stove.
For cast construction, by adding s/s needles you can add strength and by adding polypropylene burn ot fibres you can help with expansion but I don’t have a cement recipe that would guarantee a good strong bond over time.
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Post by drooster on Mar 17, 2019 11:37:03 GMT -8
I would argue that the thesis stated above should be a pinned post on the materials section. Subscribed. Popcorn.
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