Genesis 25:1

Ketura: This is Hagar, and she was called "Ketura" because her deeds were pleasant like incense. And she tied up her opening so that she was not coupled with a man from the day she separated from Avraham.

And Abraham added and took a wife, and her name was Keturah; she is Hagar, who had been bound to him from the beginning.

Keturah. This cannot be Hagar because in verse 6, Scripture refers to Avraham’s concubines in the plural.

קטורה, according to the plain meaning of the text this woman was not identical with Hagar.

ויסף אברהם, he took another wife although he already had had two wives. He meant to continue siring children in his old age in order to comply with the blessing/command to increase the world’s population. Seeing that the continuity of his own seed [the Jewish people having originated in seed from both a Jewish father and a Jewish other, Ed.] had been assured, he was not concerned with the antecedents of Keturah. We can be certain, however, that he selected a woman who personally possessed all the good qualities he would have desired also for a wife for his son. The only thing he did not insist on was such a woman’s national background. He was certainly not looking for experiencing disappointments with any children from Keturah, having experienced enough disappointment with Ishmael. We may safely assume that Keturah was not of Canaanitic descent, seeing that even Hagar the Egyptian had not been of such descent. (compare 17,3) Keturah was not a concubine, This is why the Torah writes ויקח אשה, as apposed to the mention of פלגשים, concubines, (verse 5). The sons of the concubines are not mentioned by name whereas the sons of Keturah are all mentioned by their names. This fact alone clearly shows that their status was superior to the sons of Avraham’s concubines. The essential difference between a wife and a concubine is that though both are exclusive partners of the men with whom they live, the former, when becoming wives, underwent the ceremony known as chuppah, and the union was celebrated with a wedding party, whereas a concubine was not accorded all this pomp and ceremony. [after the Torah was given, the essential; difference in status was that no financial settlement, ketuvah, was made when a concubine was taken by a man as his companion.

ויקח אשה ושמה קטורה, “he married a wife whose name was Keturah.” Although in years to come, Israelites were not permitted to marry Egyptians, seeing that the Torah decreed in Deuteronomy that only commencing with the third generation after conversion were Egyptians acceptable as potential marriage partners, even if it was Hagar who had been converted, the fact that her father was Pharaoh that made her only a second generation even if we consider him as a first generation convert. According to B’reshit Rabbah 61,4, Avraham married her at the express command by G–d.