You won't be able to send the reminders from the workflow itself.  Instead, you'll want to set up a scheduled job to trigger an event for any outstanding approval records.  Then you could set up an email notification to respond to that event and send the reminder.  There's a nice solution demonstrated here by Chuck Tomasi that makes the process of setting up that scheduled job much simpler.

https://community.servicenow.com/community?id=community_blog&sys_id=ae9caee1dbd0dbc01dcaf3231f96193d