Love for Construction
I used to be a superintendent for a local home builder in an area, where the dirt had a lot of clay in it. We would often get stains on a newly poured sidewalk and driveways of our homes, we would use muradic acid with a plastic bristled push broom. It etches the concrete if left on too long without rinsing with water and requires some muscle with the broom, but worked for us on several occasions. Be sure to follow the warnings on the bottle and read the MSDS sheet that should be provided with it.