For free solutions, there are most probably better options available if you look beside just the Codeigniter ones. However, building sites on top of CMS's are in most cases very limiting. (Solely depending on the demanded level of the site of course.)
Take a look at this template library: https://github.com/bcit-ci/CodeIgniter/wiki/Simple-Template-Library/
Together with the active record class that codeigniter offers, you will be up and developing sites in no time at all. Without any boundaries of course!
