Note - I've read Should I insulate my pizza oven between the firebrick base and concrete? and Will a slab poured with mortar stand up to service under a pizza oven? but they aren't rerally what I need. Strength isn't the issue, heat transmission is the concern here.
I have just poured a heavy duty retaining wall in the back garden. It's really heavy duty, due to soil/slope issues - about 500mm wide at the base tapering to 250mm at 800mm height. It contains steel reinforcement, and is faced by expanded metal (EML) + render on the patio side.
The concrete used was professionally placed and is C40 grade (UK - a mix with much more cement powder than usual, approaching 500kg per m^3). It was low water and used superplasticisers and a higher proportion of sand/fine aggregates, instead - i.e. a self compacting/self consolidating mix.
The wall has a right angle corner, and we have now decided we want to put a steel tray for a briquette BBQ or pizza oven in that corner. To do it, we plan to pour 500x500 mm concrete in the inside of that corner, to create a total 700x700 mm flat concrete surface for use.
My concern is the intense heat of a BBQ or oven resting on this concrete. Especially with steel in its structure, and cycling over time. I dont want any issues. I could just airgap it and have a 5mm steel tray resting on a couple of rows of firebricks, it probably won't move due to weight. But I'm not convinced about safety and heat issues. I want long lifetime too, and ideally no movement or maintenance.
If I want to have it resting directly on the corner of the wall, perhaps on some kind of mortar bed or insulation layer, is there a specific insulating material one uses to stop heat conduction through the fabric damaging what it's resting on? Rockwool or something else with extremely low conductivity?
What should I do, and what's the risk of damage or weakening over time?