Burn chamber; cracks/ refractory stones moving

Hello all,

My rocket mass heater has some issues at the burn chamber level, the cob around it always cracks in the same spot and some refractory bricks keep getting pushed inwards, partly blocking the burn tunnel.
Has anyone else encountered these problems?
Would using refractory cement as mortar for the refractory bricks work as a solution?
I have tried using regular cob as mortar for the bricks but it just burned away so now I had them dry stacked and encased in cob/ straw.

Thanks.

You can make your own fireproof cement using waterglass.

Hi Robin,  Welcome to Permies!

Some cracking is going to be a part of life when it comes to solid large pieces of cob in a place where expansion and contraction happens all the time. There are ways to mitigate them, but nothing will ultimately stop them from happening if the 'Crack Gods' say so.  :)
You can wet the cob down and smear it back to looking pretty only to find the next firing it opens up again (or not if you get lucky).
Building the stove with an expansion/contraction layer like insulation can really go a long ways to keep them under control.

As far as your moving bricks, do you know what is causing them to move?