I imagine the logs, with their cut ends =
exposed,=20
will expand and
shrink as they get wet and dry out, so that a lime =
mortar=20
would be
appropriate. So I was struck by your use of the term =
"concrete".=20
 
I am told by the pre fab log companies there is a product you can buy : but lime putty(or lime mortar)  3:1 sand to putty , beat well ;then add linseed oil to waterproof  on wire mesh.
leaving space in the back (if your building an outhouse) for air Vent and 6 yr olds to peek in .Py   
 
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