what is the reason/cause of the invention of ceramic cores for RMH ?

recently tried to mortar together a  core with firebrick and 3:1 sand:fireclay with no success. It fell right apart.  In any case, got me wondering about ceramic cores. Have been looking into masonry heaters for 20 yrs and only recently began a first RMH build. Chose firebrick and the mortar that is light and allows for expansion/contraction. Almost all other research on more stiff mortars such as high temp cement etc seem to be passed over because they crack. Correct me if this is incorrect, as I'd like to use this as a solution to my build.  But it started me thinking that these RMH are possibly run too hot and is why they crack? the high temp refractory cements are rated for 2000F and yet they seem insufficient?  Is the switch to ceramic cores a solution to this high temp RMH? and is another solution to run them a bit less 'efficiently', maybe with less insulation around the cores, so that high temp cement is a viable solution?

or is there other info that I have not come across as a good mortar for brick builds?

Carol,

What does it mean it fell apart? By itself?
I have a test stove built outside from not too straight red bricks with no mortar and nothing is falling apart.

First of all, the mortar in masonry structures acts as leveling agent, not a glue. Properly designed structures should stand and be stable with no mortar help.

Also, I would not settle rigidly on 1:3 ratio, especially if the clay is not coming from some commercial source that was lab tested. There are thousands of various clays and the ratios should be determined empirically.
Finer sand will work better with richer clay-to-sand ratios. Thicker mortar with less water will also be less crack-prone. Thinner joint will also crack less.

Please do not abandon building from bricks. There are millions of heating devices on the planet built from bricks and they work fine, both in domestic and industrial settings. Joints between bricks act as expansion joints, so the structure will not crack within the body of firebrick.

Also, I would never make the firebox to be less efficient just to fit the cement type.

How big are you joints?  I hope they are not stacked - it would help cracking.

i let it sit for 3 days and when i went to finish the top section of the riser it just came apart...it was 30-40 degrees for these days. that is the only reason i can think? but i've never used this 'mortar' so dont know if the sand was wrong or what? but in general it seems like a precarious mortar  and i dont know how others get it to work.

the joints where 3/16" 'ish  or smaller, and the sand I used was general construction sand with silica. I knew quickly it had larger sized particles in it than desired and if i try again, it will be sifted finer...the clay was commercial fireclay

maybe if I build it in 50+ temps, sift the sand, and use a 1:1 or 2:1 ratio resulting in less joint size, I could get it to 'work'. Meaning maybe it would hold together while I wrap it with insulation and then when the bell is built over it, the strength of teh core is not an issue? as it will never be 'touched' ? that is the only thing I can think as to how people get it to work. The core must be precarious by nature and the next outer layer of brick is what protects it?  but no one ever seems to mention this? unless im missing something....

I just went outside with a caliper and measured the grains of in the pile of masonry sand I bought by the truckload.
Most grains are 0.6-1.2 mm, some bigger are 1.8. If yours is similar it would be nice to do the following:

1. Eliminate particles larger than 1.2 mm.
2. Make richer mortar.
3. Reduce joint size.
4. Wet with a sponge, only the parts of the firebrick that will receive the mortar. Firebricks are usually quite porous and if too wet the delicate bond will be weakened.
5. Do not lay when the temperatures are lower than 45 F with possible freezes or when it's too humid for complete drying.

When I build for example in December, when I get night freezes, I always cover the structure with blankets, just in case. And the mortar is usually cement/lime based and the structure is at least 40 cm thick, so enough heat will be accumulated during the day to prevent freezing at night on top of the fact that cement will make the mortar set rather sooner than later.

In case of clay mortar without additives it has to dry on its own. When it freezes in the meantime, a delicate structure between joint and brick created by capillary action, gets broken.
Winter is not the best friend of a mason.

Hi Carol,

I have the same question as Cristobal. I just completed a test burn before I build the heater in my shop. The picture below shows my entire core dry stacked, no mortar or anything other than gravity to hold it together. Can you perhaps post a picture so others can comment on your build plan?

I have a thread started for the build, but I took it apart to redo later. But i've dry stacked to test various cores with minimal smoking and realize that not much strength is needed if the idea is just to create a burn tunnel and riser that keeps most of the smoke out....but shouldn't a core do more than that? or is that delicateness an inherent part of all cores?

plus, the bricks have been used and have some wear and tear, so are not pristine. I will need some good mortar solution for those parts if nothing else. Plus I'd like some inherent stiffness/structure for at least the burn box and tunnel, as this may end up as a single skin with no protective outer shell.

Carol,

A lot will depend what are your expectations for strength - any structure may be destroyed with enough force used.
If the main firebox is built from bricks laid on shiners or on stretchers will also matter a lot. I have noticed that heater masons in central Europe always lay bricks on shiners. If they are laid on stretcher (for easier future brick replacement) they lay two layers of bricks on stretcher. They have to be heavy enough to be idiot-proof. Nobody would pay premium for a heater that will not last at least 30 years and break by poking it with a piece of wood.
The type of ceiling of the firebox will also affect the stability of the walls.

carol othdac wrote:plus, the bricks have been used and have some wear and tear, so are not pristine. I will need some good mortar solution for those parts if nothing else. Plus I'd like some inherent stiffness/structure for at least the burn box and tunnel, as this may end up as a single skin with no protective outer shell.

I just read your edited post. It looks like you are building the J-tube and now I understand your concerns better - bricks in the feed channel do not want to hold together.
If the bricks are not new, usually the edges are broken/eroded so there is even less adhesion surface in the joint, especially if laid on stretchers. In this case just build it from bricks laid on shiners and it will be way more stable. You will see how it performs. You can always rethink and change it later. Experimenting is fun.

so that is laying them 'on edge'. Which is how I originally built it and how i was planning to rebuild it.

I still need to find out why high temp cement is not viable. If it is because it cracks, why is that a dealbreaker since that is not much worse than a dry stack at that point, plus the leaks are still contained within the insulation and/or the bell. So the piece of mind of having a factor of strength to the core may be worth the potential cracks. This is my current assumption. If that proves to be innacurate and there is no other menthod/mortar, then I will have to refine the sand/clay method as outlined in the early post of mine regarding ratio, sifting sand, temp etc

I meant to lay ON shiners not AS shiners. You are already laying them as shiners. If laid flat the structure will be more stable right away.
If you want to use a ready mix for firebricks you can do it to check if it works for you.

regarding mortar mixes. Searching has not revealed much about this; but the idea of mixing in a portion of high temp cement with a mix is the idea I am considering.

Maybe a 1:1: 1 sand/fireclay/refractory cement

There may be a good reason why I havent discovered much of anything on such a mortar (such as the possibility that adding any percentage of high temp cement into the mix negates the flexibilty or other qualities of the sand/clay ?)....., but perhaps it just hasn't been tried. But if you'all have ideas as to why that may or may not work, I'd be interested in hearing, so that I don't waste too much $ and time on high temp cement additives, but if it is a possible idea, it may be worth the risk.

Clay-sand mortar is strictly a leveling/sealing agent, not any kind of glue at all. A brick structure built that way needs to be stable if it were to be dry-stacked. Refractory cement mortar may crack at stress points, but it will maintain a somewhat jagged joint that will not slide easily, and much of the structure will stay rigid making the whole stronger. I think a lot of the preference for clay-sand is the ease of changing it if needed and reusing the bricks. If you are not going to be changing it, that becomes less of a consideration. The firebox or J-tube feed and burn tunnel if outside a bell or mass need a second layer both for structural safety and for keeping the fire hotter. I understand that a batch box firebox can lose some heat as long as the riser is well insulated, but a J-tube must stay extremely hot.

Another factor that may matter to you is that the International Building Code for fireplaces and for masonry heaters requires refractory cement, and specifically forbids clay-sand mortar.

So sand/clay has no adhesion. This is what I found out. Had not read that anywhere in years of searching these topics. I was working under the assumption that the $50 bag or fireclay was a moderate version of refractory cement, when in reality, the free clay in the yard would likely have performed similarly.

The other realization that compounded my problem, is that the stability was further compromised by laying all the bricks on edge/soldier style. That is rickety. I did that because I want to keep the heater narrow and as small as possible because it is a small work space. Also the reason I'd like some rigidity to it as it will likely take a hit from a stray piece of lumber etc. So it sounds like I can get some refractory cement and use it alone? and no need for sand/clay as their properties may likely be negated entirely once the cement is used?

I'd  like to get confirmation on that point because if sand'clay still do something, I'd use them, as they are paid for and on site....additionally I was planning and still will double up the bricks on the feed and burn tunnel regardless...thanks

Putting bricks AS sailors will be the most unstable structure.
You can:
-add 10-20% of Portland cement to you clay mix - if the temperature not exceeds 500 C it will be ok
-buy some refractory mix, add water and use it

I still think that building a stable structure that will perform structurally without any joints is the core of good masonry.
If you still want to lay the bricks the same way, use some angle iron and weld the frame around them so they will stay in place.

You might try wrapping the burn tunnel in sheet steel. The steel will seal and provide structure. I've carried this thing all over and the bricks stay put. I too am having second thoughts on the riser as I'm having difficulty sourcing materials.