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The best cleaner is going to be powdered laundry detergent TIDE brand. Dry up all the diesel fuel you can with paper towels, then coat the area with the detergent so there is a layer of detergent you can not see through. Let it soak this way for an hour to pull the surface fuel out of the concrete, then apply the hottest water you can and scrub the area with a hard bristle brush, then wash the detergent away.
This is the same method Nascar uses to clean the track of fuel spills.
This is also the same product I use at home for stains and my hands covered in grease. It has to be powdered Tide and nothing else.