thermal mass over exhaust duct in ground

I asked this question in another thread, but didn't get a reply.
I have my 6" exhaust duct in a trench in the dirt covered and surrounded on the sides with cob. I am wondering whether I would be better off for the top few inches to use small gravel (the main floor covering) or use concrete cap blocks "set" in the cob to get that area up to surrounding floor level. Or only because I have several bags possibly use quickcrete?
I have a little bit of cob left to mix, but not enough to get up to floor level. I don't think that it would wear well being in a highly travel area.

John Adamz & the' Permies Cloud' : How thick is your cob? The one greenhouse build I worked on was all new territory,so we made everything raised beds, and added a little
structural cob (with straw) in likely high wear areas, Our thoughts were good work height, most heat radiation kicks in around midnight, so things cool down enough to stimulate
plants natural cycles straw buffers the heat in the starting beds, more cob exposed to help regulate amount of water vapor in the air, did I mention great work height,no buddy
would ever try to walk on the raised beds,and great work height!

Latter on hanging baskets were used but only in the areas with structural cob because people climbed on the raised beds to get at the baskets ! Basically I have nothing for you!
I tryed very hard to make sure that there would never be any foot traffic and no one would try to use a potato fork to try to turn over soil and spear a lenght of stove pipe ! If
you put pavers down people will be sure that that is where you want them to walk ! Big Al !
!

I've got around a 1000lbs of cob on and around the duct. About 2-3" on top and filling the trench on the sides. Since I had to shorten the duct under the floor I had no other option but to have it run in the middle of the 5' path right in between the aquaponics grow beds

Concrete has better heat storage characteristics than clay/cob. It is not used close to the fire because it doesn't tolerate temperatures over 400°F. But, if you want to use it further away where it won't be exposed to continual temps over 400°, it should work. The Dragon Heaters blog has a reference page on this topic.

All the above are good reasons to consider.
I'd go with pavers (or at least a scored pour) before I'd do a solid cap, in case you need to get in again later to adjust something.
With 2 to 3 inches of dry cob, you are probably safe from collapse - if that's also at least 3-4" on all sides. But I think a 2" concrete paver that extends past the pipe would do more to protect against accidental collapse than the gravel (e.g. someone puts a ladder foot down there).
If the gravel is the drainage base for your greenhouse, the cob will likely stay damp, therefore it's softer (cob does not 'cure' in damp conditions, as it is a purely physical drying set, not a chemical set).
It will remain somewhat soft mud. It does provide some lateral pressure to help support the sides of the pipe, but it's worth your while to make sure that tools, ladders, feet, and other point-loads can't get to the pipe to deform it. And to be able to pull things up and inspect if you think this has in fact happened.

Best of luck, and thanks for posting the project updates! I hope things are working well for you now.

Yours,
Erica W