All that proves that there is another reason that chains get dull.When I got in the tree biz back in the mid 80's we would come across cement in dead and dying trees a lot.
It was a standard practice before that to fill tree cavities with cement.
It would really 'grind down' a chain. Some of it was pretty soft but it still wore the chain badly.
As the tree trunk decayed away internally the cement would accumulate near ground level and make for some really depressing stump cuts with a distinct cement odor.
I've cut cement hundreds of times sometimes at a pretty good height in the tree.
no prove of dirt not dulling chain-which it does