I've added 30 concrete blocks around my wood burning stove to add thermal mass for heat storage. Once I've decided on the exact configuration and gotten all the blocks in that I want I'll probably cover it with cob or mortar on some rocks. Does cob adhere well on top of concrete? My only concern with such a covering would be access to the stove should I need to do any maintenance. Still thinking on that. I can report that after burning a couple fires this thermal mass makes all the difference. Granted it has mostly been in the 40s I'm seeing a big difference. I can burn a 3-5 log fire once a day at these temps and the mass stores and releases the heat till the next day. NICE.
My guess is that when we dip into real winter temps I'll be able to burn a morning and evening fire, each 4-6 logs and coast the rest of the time. If that works out it would be a reduction of about 70% from last year! Not only that but the burn time is short and hot which means much more efficient. No more throttling down the burn to control the temp and lengthen the burn time all of which means more pollution. Much less smoke and much less work chopping wood! Not only that but I'll wake up in the morning to a cabin it 60 or 65 degrees rather than 40 or 45 as was the case many mornings last winter.