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You didn't say how long it's been, but I had something similar happen to me in the past. First thing, kitty litter or Sorb-All as soon as you can will help. Leave that on for a short while, sweep it off and squirt the whole area with Dawn dish soap. Brush that into the concrete some, then pressure wash it off. I did this after my grill spewed grease all over my deck because I didn't have the drip tray all the way in and you couldn't tell it happened afterward.
Oh man, I’m sorry I didn’t get to this sooner. Those grease buckets can be a pain. I’ve heard some people say that you can use kitty litter to clean up things like that, but I’ve always kept some spill absorbent products around the house—just in case. I use SpillFix because it’s a natural product for use outside.