cob clay

Dear folks, I am getting set to build a rocket mass heater and i have a question about the clay. I made a couple of small test bricks to see if my earth is good for building and it looks promising-very sticky and is drying hard and strong. My issue is that, after forming the wet bricks and putting them on a plate to dry, i woke up the next morning to find what looked like two crystal-meth chia-pets! I am assuming this is alkali but i don't know if this is cause for concern when using this for the build. The bricks seem strong and solid but would there be any issues around the barrels and stovepipe? Any feedback is much appreciated. Thank you!

Good Morning David;     Welcome to Permies!
So your wanting to use  local clay  to make bricks.
How and where are you thinking of using these bricks?
Depending on your build design you might need quite a few...
Do you know that clay bricks are .48 each at home depot?
Making your own bricks, could be very satisfying knowing you manufactured most all of your rmh.
However for me the amount of time to make a hundred bricks versus a trip to home D is a no brainer...
At .48  lets load up in the truck and go visit home D.

So a chia pet  brick...   I would like to see a picture of that!

I believe these are just bricks for testing the properties of the clay, not to use for actual building.

As Glenn stated in the post below yours, these were just clay blobs to see if the clay would be useable for the mass infill part of the stove. I will pour my own refractory cement combustion core and riser. I will also do a stone facing integrated with the mass infill and will also use stone as part of the infill. My question still remains-should i be concerned about the apparent alkali content of the clay-in particular around the barrel and stovepipe? Also Thomas, thank you very much for the advice concerning your pop-in door and secondary air channel!

Rust is the natural state of iron (ferrous oxide). Rust occurs when an exposed area of a ferrous (iron-containing) alloy (like steel) drops below a pH of about 8.2 or 8.3. Rust inhibitors provide an environment that combats the pH drop on the surface - thereby protecting and preserving the surface finish. Different types of rust inhibitors provide different ways and durations of corrosion protection. By the way - water is the culprit that starts the rusting process. The water doesn't even have to be in a liquid state - it can be the humidity in the air. That is one reason cars in Chicago rust faster than cars in Phoenix - because Phoenix has very low atmospheric humidity compared to Chicago. Hope this helps.

Ok.  I stole this from a chemistry forum.

First thing you need to do is test the pH of your clay.  That will determine the propensity of the clay to oxidize metal in contact with it.

If your metal stove pipe is coated with a protective barrier, like hi temp paint, and then covered in your cob to insulate it from humidity, I would think it is much less likely to rust.

Hi David,    When dissimilar materials like cob and metal meet, it is often recommended to allow for the expansion and contraction with the use of a flexible material like rock wool, superwool or even a high heat silicone.
This not only helps to keep the cracking at these joints to a minimum, but also from rust that might form on the metal.

Thank you Gerry for your reply. I intend to use ceramic fiber blanket material around the combustion core/riser but i have heard that it is not wise to use this around the barrel manifold so as not to offset the thermodynamics of the exhaust path. I am open to new info if available and welcome this if you can share.

Not sure where your information came from David or what it means but even though we're dealing with rocket science here, its actually quite simple in how it all operates.
If you did want to get an idea of how the heat/exhaust flows in a system, look into "fluid dynamics" which behaves very similar.
As long as you don't restrict the flow of gasses with bottlenecks, create smooth curvy surfaces and stick with the guidelines for lengths of pipe, bends and leaving enough heat to create a draft in the chimney, there should be no problems.


source

Thank you again Gerry for your reply. I don't remember where i heard that info either-but i did hear it. I believe the theory is that if you create an exhaust path that goes from hot to cooler back to warmer (combustion core/riser-exposed uninsulated barrel-insulated manifold) that there could be a sort of thermo-lock that could back up smoke into the combustion unit. I personally do not know this to be true but i guess that is why i'm here-to learn from those that know more than i. If no one else has heard of a problem with an insulated half-barrel manifold then i would love to insulate it as i have the ceramic fiber blanket material (2" thick) and that would take care of my expansion joint issue! Thank you for the info-take the best of care.

No worries Dave. I think your on the right track and are ready to tackle this Dragon build!
Keep us posted on your progress. We're here to help if you have more questions.