פּירוש אויף בראשית 25:1

קטורה KETURAH — This is Hagar. She was named Keturah because her deeds were as beautiful (sweet) as incense (Ketoreth) (Genesis Rabbah 61). And since she closed her 'opening,' as she did not mate with anyone from the time she separated from Avraham (Genesis Rabbah 61:4).

ויוסף אברהם ויקח אשה, “Avraham continued life as usual by taking a wife;” this is a reference to Hagar who now had a another name, Keturah. We find a similar formulation to the above in Isaiah 8,5: ויוסף ה' דבר אלי עוד, “the Lord continued by speaking to me again;” the Torah wished to teach us civilised norms by reporting this, i.e. that when widower’s sons have grown up but have not married yet, he should wait with remarrying until his children are married. (According to matnot kehunah in Breshit Rabbah the apparently superfluous word ויוסף, “he continued,” is to teach us that it was the same woman to whom he had been married once before, i.e. Hagar.) In order to make the point that a widower under the right conditions is to marry again is why the above line has been written immediately after the Torah had reported the marriage of Yitzchok to Rivkah.

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.