Hi ..We have a white pine which is about 20 years old and the bark is pulling away from it at the base and starting to rot...so, I cleaned off the rotten bark and put a thin coat of cement over the area. Is this okay or should I have done something else ? LeoHi Leo, welcome to AMHD! :)
You did not mention your location, but you will most likely find that after time the differences of expansion, and contraction in the two dissimilar materials (wood, and concrete), your repair will fall away from the tree.
It is usually best to just remove as much of the damaged tree material as you can, keep the wound protected with a screen like material, and then let the tree try to repair itself.
Depending on its location, and condition you may have to consider removal.
See this link for advice:
TCIA | Tree Care Industry Association Staff (http://www.treecareindustry.org/public/About_faq_treecare2.htm#7)