Following tool may be used for sending email reminders for pending approval in approval process in specific interval. The reminders will be keep sending until the related record be approved.
Can install as manage package from link https://login.salesforce.com/packaging/installPackage.apexp?p0=04t1r0000010tgv or download the git repository
Read more: https://lc169.blogspot.co.il/2017/02/approval-process-reminders.html