Answer: I've had a similar situation and here's what I did to remove the stains:
Spray the area with the water hose on full pressure. This will wash away any debris and help loosen ground-in dirt.
Sprinkle laundry detergent with bleach or bleach alternative onto the stained areas. Powder detergent seems to work better as it also provides a little scouring action, but liquid detergent is fine.
Let the detergent soak into the stained areas for a few minutes, then begin brushing with the shop broom or scrub brush. For a small area such as a curb, the scrub brush is easier, but for a driveway or sidewalk, the shop broom will save your back.
Rinse with the water hose, again on high pressure, to help lift and wash away the dirty soap and the stain itself.
Cleaning stains only will leave your concrete looking spotted. Follow up the stain removal with a light sprinkling of detergent for the whole area, and sweep it with the broom. Rinse with the water hose and all of the concrete should dry with a uniform color.
Hope this works for you, too.
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