A.T. Robertson Commentary Matthew 3:12

"whose fan is in his hand, and he will thoroughly cleanse his threshing-floor; and he will gather his wheat into the garner, but the chaff he will burn up with unquenchable fire." — Matthew 3:12 (ASV)

Will burn up with unquenchable fire (κατακαυσε πυρ ασβεστω). Note perfective use of κατα. The threshing floor, the fan, the wheat, the garner, the chaff (αχυρον, chaff, straw, stubble), the fire furnish a life-like picture. The "fire" here is probably judgment by and at the coming of the Messiah just as in verse 11. The Messiah "will thoroughly cleanse" (διακαθαριε, Attic future of -ιζω and note δια-). He will sweep from side to side to make it clean.