Jamieson, Fausset and Brown on Genesis 25:1

CHAPTER 25

Ge 25:1-6. Sons of Abraham.

1. Abraham took a wife--rather, "had taken"; for Keturah is called Abraham's concubine, or secondary wife (1Ch 1:32); and as, from her bearing six sons to him, it is improbable that he married after Sarah's death; and also as he sent them all out to seek their own independence, during his lifetime, it is clear that this marriage is related here out of its chronological order, merely to form a proper winding up of the patriarch's history.