Fortunately it's drizzling a bit and the pavement is wet.
I rubbed the blade on the edge of a curb til it was cut through enough to break off at the length I wanted.
Then I rubbed the rest of it on wet pavement til it was sharp and there was a bit of a burr on the edge.
It goes pretty quick. Cement is a good abrasive.
Look for smoother concrete if you want a better finish on your knife.
Or rub a rock or piece of cement on your working abrasive surface to smooth it first.
A rock, brick, broken flower pot, or any other rough ceramic item make good abrasives.
Hard wet beach sand can be a miraculously good abrasive.
The blade got a bit hot even though the pavement was wet.
I dipped it in a puddle from time to time to cool it off.