alternative to replacing big and tall posts

i have 6 damaged posts, each post is 6x6 and at least 20 feet high. the bottom of the posts is where the rotting has occured due to termite as well as contact with cement. the best way is to replace but it's too expensive. handyman said that since there's enough wood left, the posts could be treated and pack with mortar.
if this is a valid remedy, what king of chemical is used to treat posts and what kind of mortar?
i am in northern california.
thank you in advance.
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