Genesis 25:1

There is some discussion as to the exact identity of Keturah, her nationality, and the status of her marriage to Avraham. The Ramban is of the opinion that Keturah was not the same person as Hagar, she was a full wife (as opposed to a concubine), and that she was a Canaanite. It’s confusing that Avraham could marry a Canaanite - and have more children with her – after going to great lengths to ensure that his son Yitzchak specifically did not marry a woman of Canaanite descent! The Ramban answers that it was only important to ensure that Yitzchak did not marry a Canaanite because G-d had told Avraham (Genesis 17:19) that His covenant would be forged specifically through the descendants of Yitzchak. Any later children Avraham might have were not a part of this covenant.