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.
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.