Concrete will not react with treated posts; however, treated posts will rot faster if the concrete is brought to the ground level because moisture cannot escape from around the base of the post. Therefore, the moisture is what rots the posts, causing a premature need for replacement.
When the use of concrete to better secure the posts is desirable, be sure to leave 1 to 1½ feet of dirt on top of the concrete up to the ground surface. The concrete should be poured in the bottom of the hole.
Additionally, if you cut the tops of your fence posts off to create an even/level row of posts, you are exposing untreated wood to the weather. Therefore, you should also re-treat/seal the tops of the posts to reduce rot.
