Alternatives to concrete in tiny rmh (Poelito)?

Hi all,

First post here, I've been checking around and I haven't found a post that answers my question, so excuse me if I'm wrong.

I'm starting a construction of a Poelito - a tiny rocket mass heater that is designed for yurts:
https://www.instructables.com/id/Semi-removable-Mass-Stove/

It's a quite ingenious design, with everything fitting inside an oil barrel  - nice when you don't have a lot of space! The basic idea is to make moulds and pour refractory concrete around this, then fill up with sand in the top part. However, the refractory concrete is based on fire cement and I'm wondering if there isn't an alternative to this? Could I potentially use a clay mix of some sort instead of the cement? I guess I would still mix it with refractory chamotte (grog), even though here I would also prefer for instance breaking down old red clay bricks and make a mortar with this.

So basically, does anyone have a suggestion for how to make an alternative to the refractory concrete? Using what can be found second hand/in nature, rather than new products?

Thanks a lot in advance!

Hi Thor, and welcome to permies. You should be able to use fireclay and refractory grog to cast the firebox of this little guy. Just make sure it's fully dried out before the first firing.

Hi Phil

Thanks a lot for your quick reply! I actually didn't even know that such a thing as fireclay existed! So just to make sure I understand, you mean that I can replace the refractory concrete with fireclay, correct? Thus making a mortar of fireclay and refractory grog. Why would one choose refractory concrete over fireclay anyway?

I'm also looking for possibilities to substitute the refractory grog. I suppose I would be able to just use semi-refractory grog (aluminum content 10-20%) for instance, or even broken down old firebricks? If I understand correctly, what I would lose is a bit of inertia, thus less heat storage, if I'm correct?

Sorry for all these anwers, the world of mass heaters is still just opening to me (and it's very exiting!).

Hi Thor;  Welcome to the exciting world of Rocket science!

Folks who can easily get and afford refractory cement use it for solidity.
The downside to using refractory is the permanence. It does not come apart very easy and it can not be reused.

When you use fireclay . Any sudden impact can break apart your build.
On the plus side of using fireclay. You can disassemble your RMH anytime. Hit it with a mallet, scrape off the bricks & simply save and rehydrate the fire clay mix to reuse!

EDIT)   I should add that critical areas near the firebox should be smooth clay slip.  
Also using fireclay rather than cement. You need very thin mortar joints , just enough to fill any irregularity's.  It is not like cement.
Cement of course you would use a 1/4" of mortar  on every joint.

Thanks a lot to you also, Thomas - again very useful information! Would you be able to also enlighten me regarding my second question? Am I correct that I lose some heat storage capacity if using semi-refractory grog with less aluminum than refractory grog?

Refractory grog is probably less valuable in general as thermal mass than quartz sand, but still decent. The real tradeoff would come if you were using material with lots of voids, such as pumice or perlite. Is there a separate mass for heat storage in this design which is not subject to the high temperatures of the burn chamber and riser? If so, then you don't really need refractory materials in that portion of the build.

Note that you won't be able to reconstitute and reuse fireclay pieces that have been fired as opposed to just heated and dried. It's pottery, after all. So anything in the burn tunnel or heat riser area, if you sacrifice it in a future teardown and rebuild, becomes grog.

Hi Thor;
I just watched your video.  Now I understand what you are needing.
You need insulating mass, and you need thermal mass.

In my opinion, crushed fire brick mixed with fire clay will make a fine thermal mass.
For an insulating mass, perlite or pumice would be my choice.

Awesome, just the info I need! Thanks so much, both of you. I think I will try and use as much sand as possible and cast the burn chamber and heat riser with fireclay and crushed firebricks/pumice/perlite depending on what I find. That seems more like a permie solution to me than buying new concrete and refractory grog. Off to the calculations, it is!

Hi again folks,

I've been doing a bit more research and now I'm worrying a bit about the shrinkage of the fire clay mortar when it dries. Basically, in the Poelito design I need to make some quite precise casts for the burn chamber and the heat riser. I've attached a photo of each cast. The casts are normally done by making moulds using hard cardboard tubes which are then covered with cellophane. When the mortar (refractory concrete, in the design) has dried, the tubes are removed and/or burned out. But if I'm using a mortar based on fire clay, vermiculite/perlite/pumice and water, I worry that the mortar will shrink so much when drying that it will break? Do you have any ideas about this? Would I not risk that the casts will "implode"/break around the cardboard tubes?

Hope you can help me (and my partner...) with these worries on our quest to a concrete-free tiny rmh!


 

The aggregate, whether it's vermiculite/perlite/pumice in the insulating section, or grog/chamotte in the mass section, will prevent shrinkage. Pure clay is what you don't want, because that will shrink at least 10%. You want just enough clay to stick things together, and that's it.

I made my RMH riser with perlite and fireclay and made the mix too clay-heavy. The big issue with that was more a matter of slumping and heaviness in the casting, and the finished product doesn't insulate as well as it should. This just makes the draft a little bit touchier than I would like on cold starts.

BTW, I finally got to watch the video and I am going to suggest this to the guy who will be parking a caravan on our land in the near future. Looks like a cool build.

Thanks Phil. And yeah, I look forward to see what this little one can do. I'll post an update here when it's finished and I've had a chance to test it a bit!