Genesis 25:1

“Then again Abraham took a wife, etc.] Three years after the death of Sarah, and when his son Isaac was married, and he alone, and now one hundred and forty years of age: and her name [was] Keturah; who she was, or of what family, is not said. An Arabic writer f1343 says she was a daughter of the king of the Turks; another f1344 of them calls her the daughter of King Rama; and another f1345 the daughter of Pactor, king of Rabbah; but there were then no such people in being. Very probably she was one of Abraham’s handmaids born in his house, or bought with his money, perhaps the chief and principal of them. The Targums of Jonathan and Jerusalem say she is the same with Hagar, and so, Jarchi;…”

“SONS OF ABRAHAM. (Gen 25:1-6) Abraham took a wife--rather, "had taken"; for Keturah is called Abraham's concubine, or secondary wife (Ch1 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.”

“25:1-11 Though Abraham had sons by another wife, he safeguarded Isaac’s inheritance and blessing. 25:1 Exactly when Abraham married . . . Keturah is unknown. It was probably, but not necessarily, after Sarah’s death.”