Bell materials

Hi everyone !  I'm experimenting with  a rocket heater. I'm using a 6 inch j tube. I'm looking at building a bell for the heat storage.  My question  is can I use  8x8x16 concrete block and fill the open cavities with a cob mix to remove the air pockets?   Most of the ones I've seen are built with red brick.  I have a cheap supply of block available.  Thanks for your advicell and time

Concrete blocks are full of air within the actual structure, brick is far more dense so would more effective.
Vibrated concrete can be very dense but if you pour water over a concrete block it will just sink in and ultimately flow out the other side.
However the blocks will hold heat so I would say it depends on your situation.

Brian, simple answer, no!

Longer answer, it depends.

Simply put, concrete starts to spall at 400C°

So it can be all right or not.


But there is a simpler way!  My method.


Covering "home heating fuel tanks" with whatever mass you have.

These ensure gas tightness and stability. And the mass can be pretty much what you want. You can have lousy joints, it doesn't matter. It's a double skin.

Check my thread, you can see the pics of just this stage, at the begining of this post  https://permies.com/t/44806/Cobbling-workshop-heater-cooktop-oven#787967

And i think, read it all. It's worth it.  Combination of dray construction, and a few tricks.


HTH.

Max.

You could also do it with barrels.

Using this method.



And pavers, for example.

I would say the cob-filled concrete blocks would be adequate for about the bottom half of a bell, if you can get then really cheap. The top will be hotter, and for that you need the durability of firebrick or other refractory material.

You need to decide what balance of cost versus efficiency/effectiveness is right for you.

Thank you for the inputs guys.  Max you have given me a couple more ideas to explore.
I'm thinking of maybe trying to use some steel barrels and adding brick and limestone mass around the outside.

Cool Brian, glad i could be of some help.

One detail, your mass should have a tiny air gap with the barrel or whatever container you use.

3 or 5 mm, to let the metal expand. Otherwise, it pushes it, or buckles inwards.