If its just diesel then i would just burn it off with a blow torch?
We cleaned some block paving last year and removed
black oil stains. Customer called back later on and said there is more stains than before.
Turns out van diesel tank had been brimmed and it leaks (common on transits with non modified tank restraining) and the van was parked on the drive outside where we wasn't to clean!!!
It rained for a two days after the call.
Turned up and it was clean/slightly dirty diesel. We had a portable blow torch and decided to try burning it off.
No damage done and you couldn't tell where it had been done.
I would try burning then absorption.
Edit - Just seen its concrete, may be too late then