This Bible contradiction is from the Skeptic's Annotated Bible.
She was his wife.
Genesis 25:1 View context
Then again Abraham took a wife, and her name [was] Keturah.
She was his concubine.
1 Chronicles 1:32 View context
Now the sons of Keturah, Abraham's concubine: she bare Zimran, and Jokshan, and Medan, and Midian, and Ishbak, and Shuah. And the sons of Jokshan; Sheba, and Dedan.