By default rails escapes the content before printing it. You have to explicitly declare the content is safe to ouput:
You can do it using the raw helper, or the string method html_safe:
content_tag(:input, nil, :type => 'button', :value => 'Redo ↻'.html_safe)
or
content_tag(:input, nil, :type => 'button', :value => raw('Redo ↻'))