Thanks everyone for this very informative thread.
The last heater I build I used some concrete bricks in with my usual clay brick and cob mortar facings. Slip joints were all thorough and clean, but I had a few micro cracks (in low stress areas!) where I transitioned between the concrete and clay materials. So yes, I agree about the importance of using like with like.
Regarding finishing/hardening oils and waxes on high-wear areas like benches… I have grown weary of applying oils on clay plaster finish surfaces. SO much smell and long drying times… Has anyone had success applying only carnuba wax? If so do you preheat it to thin viscosity?