Building a RMH without cob

Hi all,

I'm venturing in the construction of my first RMH, but I have a big obstacle, in the area where I live in Canada, there isn't much clay. Not in my property, impossible to find with the landscaping suppliers and very costly to ship or pick up from far. In the other hand we have a incredible supply of rocks, this is the Canadian shield, a vast area of the world composed of mainly rock, and a thin layer of sandy top soil, mostly acidic due to the boreal forest vegetation (conifers/evergreens).

What type of mortar can I use to build the thermal bench for heat exchange, I'm talking about the perimeter masonry work to join the stones, and to fill the space in between the fill of rocks around the tube?

The burning chamber and heat riser I'm building with fire brick and stove cement, and will insulate the riser with fire felt, but my concern really is the "mortar" for the rock work.

Thanks in advance for your comments and ideas. Have a great day!

Greetings fellow Canuk Andres! Welcome to Rocket Central - Where fire and science merge.

There are certainly other ways to go without the need for cob in a rocket mass heater. Clay is sure nice to have though as its an ideal material not only becasue its natural and safe to use, but can be re-wetted and used over and over again if you want.
There have been threads talking about where to find clay and how to identify it as I too once thought there was no clay where I lived. Certainly not abundant, but I now have several sources where I can travel a short distance to get what I need.
Another way to obtain it, is through stores. Bagged and labelled as "fireclay" or even scraps from pottery studios may be enough.

I know of a few people that have used compacted gravel as their "mortar" but is not as good since it contains tiny air spaces between the gravel and acts like insulation rather than a solid homogenous mass which is better for heat retention.

Something to consider also is to use bricks to make a bell chamber rather than a piped bench.

So much to consider I know. Let us know a little more about your build and which directions you'd like to go and we can point out the most relevant information for you to ingest.  

Hi Gerry,

Thank you for the warm welcoming.

I've done some research in the area with some excavating companies and aggregate suppliers and so far they have assured me that there is not clay in the surrounding areas. But also they have pointed out that the  closest place to get it is one hour away, which does not appeal to me having on site one of the best source of thermomass (rock).

I also practiced soil types identification with water in a jar and the closest I've gotten to clay is silt, that once dried it crumbles apart with finger's pressure.  

My questions are; Is it possible to use ordinary cement mortar in the rock bench? Taking in consideration that this part of the RMH does not take the direct heat of the burning tunnel and the reburn of gasses in the drum cascade. And can the burn tunnel and drum be separated with a fire felt blanket before covering with rock work? I read in the 6" annex RMH document from Erika and Ernie that a 1/4" corrugated cardboard layer can be use as expantion barrier in some cases.

Again, my burning tunnel/chamber is being built with fire brick and refractory cement

Are Erika And Ernie linked to this forum? Can they suggest anything with their expertise?

Thanks again,

Andres Maslowski wrote:My questions are; Is it possible to use ordinary cement mortar in the rock bench? Taking in consideration that this part of the RMH does not take the direct heat of the burning tunnel and the reburn of gasses in the drum cascade. And can the burn tunnel and drum be separated with a fire felt blanket before covering with rock work? I read in the 6" annex RMH document from Erika and Ernie that a 1/4" corrugated cardboard layer can be use as expantion barrier in some cases.

Again, my burning tunnel/chamber is being built with fire brick and refractory cement

Are Erika And Ernie linked to this forum? Can they suggest anything with their expertise?

It is possible to use ordinary cement in the bench, however do know that its a one time deal. Most folks here get a few bags of fireclay for all their needs, but understand it if you are pressed for materials.
In the book The Rocket Mass Heaters Builders Guide there is a segment (pg 180) on using clay vs cement based products, the pros and cons. If you don't have a copy, it is well worth a look into it as there is a lot of information that I know would be beneficial and interesting to you that is not covered in the Annex plans.

When you say "Fire felt blanket" are you meaning ceramic fibre blanket or Morgan superwool? If so, then yes, it makes a wonderful expansion joint material.

Ernie and Erica are the founding fathers so-to-speak of all things rockety here on the fourms. Its been a while since I last saw a post from either of them as I think they are busy with their own projects and have passed on the torch to young pups like me to help out where I can. Perhaps they might make a guest star appearance but don't hold your breath!

Hi Andres;
If you cannot get clay easily then yes, you can use concrete in the mass. This is not normally done as you would need a jack hammer to modify it.
I highly recommend  that you build a brick bell in place of a piped mass.  It can be built as a bench to sit on if that is something you were looking forward too.
No clay required. Regular concrete is used to cement the bricks away from the core unit and refractory cement (very expensive) is used near the core.

I don't understand your question about the barrel .  The burn tunnel is covered, the riser sits on top and the barrel covers it all. Normally the barrel is mudded down. Sealing could be done with your fireblanket but I don't get how your thinking here? Something must hold the barrel down tight.
Were you hoping to cover part of the barrel with stone?
Were you thinking of using concreted stone to hold the barrel in place?
I suspect that even with fireblanket any regular concrete around that barrel will crumble in no time. Now refractory cement can be used anyplace you like... but it is permanent!

There is lots of new information available beyond the builders guide. Including ceramic fiber boards, ceramic fiber blanket (Morgan super wool) brick bells and the new batch box's.
If you wish to  contact Ernie & Erica you can try using Permies Purple moosage.  Click on Erica's name and then click on the envelope.

Hi Gerry,

I dont know yet what kind if fire blanket insulation I'm using, I'm searching places to see what I can find, still haven't found the material, very difficult times to order stuff here in Canada.

Hi Thomas,

What I meant is to wrap around the fire brick feedinf chamber and burning tunnel with fire blanket up to the barrel edge, then to built with rock over it, using the fire blanket either Ceramic fiber or Morgan superwool to separate the rock work around the feeding chamber and burn tunnel as insulator and expansion material. In terms of the barrel, I was planning to seal it with refractory cement. What do you think?

Hi Andres;
Yes , if I understand correctly then the ceramic blanket will work to insulate the mortar around your rock work.
And definitely yes, about using refractory to seal the barrel.

Do you have or can you easily get perlite?
Using morgan super wool to insulate the firebrick core will work, but its expensive.
A brick box built around but 4" away from your core and filled with perlite between would be much less expensive and insulate perfectly.

Youre asking about mortar for the bench? Benches don't get that hot. Otherwise it couldn't be used as a bench. Everyday normal cement/mortar would be fine in a bench application where temps would have to be less than 140F - otherwise your butt would get burned.
That's for the exterior.
I'm pretty sure you'll want a loose material for the mass, pea gravel etc. You wouldn't want to make anything solid behind the retaining wall necessarily.

On a related note concerning a higher temp mortar I found a couple different recipes online for that

recipe 1
10 parts sand, 6 parts fire clay, 2 parts cement, and 3 parts lime.

recipe 2
10 parts sand, 3 parts fire clay, 3 parts cement, 1.5 parts lime

I haven't tried either so I can't say for sure.

The sand needs to be a particular type - fine grained silica, very clean.