Hard water contains calcium salts, which can cause unsightly white stains on your plain concrete, colored concrete or stamped concrete patio, porch, or basement or garage floor. Although concrete is a very durable material, it's also porous, which makes it susceptible to stains. Many things may cause the salt deposits on the concrete, including water from hoses or leaky flower and plant pots. You can remove the stains using a few different techniques, so test them out to see what works best.
One effective way to remove white salt deposits from your concrete is with a solution of hydrochloric, or muriatic, acid. Prior to working with the toxic substance, put on rubber gloves to protect your hands, as well as a respirator and eye protection, and move to a well-ventilated area.
Create a cleaning solution of 10-percent hydrochloric acid and 90-percent water in a bucket. Pour plain water into the bucket prior to adding the acid. Wet the concrete stain with a bit of plain water and then rub the acid mixture into it using a coarsely bristled scrub brush. Allow the solution to fizz and then rinse it off with plain water.
Repeat this process as necessary until the stain is gone or until sand begins to be exposed on the concrete surface. Protect neighboring plants with tarps.
Another way to remove hard water marks is with trisodium phosphate, otherwise known as TSP, which is recommended for use on decorative, or stamped, concrete. Put on protective rubber gloves and then combine 1 pound of trisodium phosphate with 5 gallons of warm water. Next, work it into the concrete stain using a stiff broom or brush. Then, rinse the area with warm water, and repeat the process as necessary.
A chalky white salt residue on concrete is referred to as "efflorescence." This is sometimes caused when moisture carries the calcium salts found in concrete to the surface, creating a stain.
Efflorescence is especially noticeable on colored or black concrete, and you can remove it using a mild acid or a commercial efflorescence-removing solution especially formulated for colored concrete. To avoid future staining, cure and seal the concrete after using the solution.