Spray power washer over stains
Spray liberally power washer on the stained area and let it stay for a while before wiping it off with a rag or paper towel. This will remove excess of mortar from the surface.
Dilute muriatic acid with water
Take a plastic bucket and pour one gallon of water into it. Take 1 ½ ounces of diluted muriatic acid and add it slowly to the bucket containing water. Mix the solution well; however, be careful not to splash the acid, since it can burn your skin.
Make cleaning solution
Now mix this acid solution with water in a one to ten ratio, that is one part of cleaning solution and ten parts of water. Cleaning solution of this strength is effective in removing light to moderate mortar stains. Increase the strength of the cleaning solution gradually if the stains are tough and stubborn.
Prepare the surface for cleaning
Make sure the stained area is wet before you start treating it with the acid cleaning solution. If not, wet it with water using a garden hose with a spray nozzle. Meanwhile, take a stiff brush and place it in the bucket containing the cleaning solution. This will saturate the brush before you use it on the masonry.
Pour the cleaning solution on the affected area and scrub the stains using the stiff-bristled brush you had soaked in the solution. Keep scrubbing the surface thoroughly for almost three minutes and then rinse off with the garden hose. Repeat this process if required.
Rinse the area thoroughly
Once you are sure all the stains are removed, rinse the area thoroughly using an excess of water to remove the acid residue.