Would wrapping a wood fence post in plastic help protect it?
Most concrete and cement these days do not contain lime. When it contains lime its called Limecrete and generally it is used as a type of mortar for older buildings. Anyway If you wanted to protect the posts you are best soaking the post in creosote it's a type of oil that is used specifically to treat wood for this very reason its also used in tar distillation and is commonly available. You can also soak the wood in general motor oil for a week as it absorbs it well and repels water. The plastic would in fact trap moisture in the wood which in effect would increase the rot. Ideally you want to dig the hold deeper and put gravel under the post to allow water to drain easier and then fill the rest of the hole with concrete