sawing posts for buildings

[bandmiller2]

I have taken sections of utility poles set them in the ground then used my own untreated above that. Frank C.

[Don_Papenburg]

You might want to try sonatube for the inground portion of your post . Sonatube is a cardboard tube that will form a nice round concrete pillar inground and above. Put post anchors in the concrete after the pour .  Pre measure where they will go before you call the readymix truck.

[Rockn H]

For using post that aren't treated, I'd go with concrete piers like others have suggested.  Here's a link showing how another member made brackets for securing his post to concrete.  I'd add one step and use a piece of rebar like wwsjr suggest or flat stock to keep the post from sitting on the bracket so air can move under post.
Here's the link and the pic is post #16.  https://forestryforum.com/board/index.php/topic,37670.msg545327.html#msg545327

[diesel pap]

i emailed postprotector this past week end and this morning i got a call from them. they are telling me that the product will keep a untreated post from rotting. idk