Rinse off the sidewalk with your garden hose to remove the loose buildup and also to make the sidewalk a bit more porous so the cleaning solution can soak into the stains and residue.
Sprinkle a heavy coating of baking soda over the stains and let it sit in place for about 15 minutes. The baking soda naturally draws out the stain residue.
Drizzle a thin coating of dish soap over the baking soda and scrub the ingredients into the sidewalk with a wet nylon scrub brush. Continue until all the stain residue is removed.
Rinse off the sidewalk with your garden hose again and let the sidewalk air dry.
Repeat the cleaning procedure if any stain residue is still present after the sidewalk has dried.