Stains on concrete are difficult to remove because the surface is so porous. The substance that is causing the stain can be dissolved or loosened, but if you have no means of removing the dissolved material, while in suspension in the solvent it may end up staining a larger spot with less intensity in its color.
There are two basic methods of lifting the material once it is dissolved. One is flushing the area with water. This can be done with a pressure washing unit and is probably the most effective method of cleaning concrete. There is less physical effort involved in this process. The other is to use an absorbent substance such as Oil-Dri. This product is a bit expensive and does not work any better than an inexpensive kitty litter. Vacuuming does not work very well, even if you are using a wet/dry vacuum cleaner.
If you have an absorbent material handy when a stain occurs, most of it can be lifted immediately using the absorbent material (i.e., kitty litter). Your stains are different in that they have been in place for quite some time. To remove any of these stains you must understand that, first, they have set in the concrete, and, two, it will require some effort to begin to remove them.
Muriatic acid is a relatively inexpensive cleaning agent but may be hard to find in a container smaller than a quart or gallon. Try to buy the smallest amount that you can find. Acids are not the most common cleaning agents for household use so you would not want to keep it around the house.
You will also find that acids are not the easiest thing to dispose of. Muriatic acid is commonly used out-of-doors on construction sites. It is used to clean other porous surfaces like brickwork and concrete. Muriatic acid is a hydrochloric acid, and it fumes when it’s applied to a surface.
If you decide to use muriatic acid you will need to take precautions for your own safety. Use heavy rubber gloves, not kitchen-type rubber gloves. Wear eye protection. Wear old clothing that you are not concerned with destroying if acid spills on them. Muriatic acid is not extremely caustic, but you still need to take necessary precautions. You will also need a stiff bristle brush to scrub the spots.
Another of the older methods of treating fungi that may work on some outdoor stains is to scrub the spot with a solution containing trisodium phosphate. The term “trisodium” simply means that it contains three atoms of sodium in the molecule. Sodium phosphate is a tertiary phosphate used mostly in cleaning compositions. It is or was used in treating water. In treating water it is called tribasic sodium phosphate. It is a fairly safe cleaning agent to use, but be sure to read the label carefully.
Although a power sprayer unit is costly, I believe it still does the best overall job on concrete. You not only blast away the mildew or stain, but you get all the dirt cleaned out of the porous surface. You end up with a good-looking job. Don’t get me wrong — chemicals when used properly can do great things. Usually you’ll still need to rinse after a chemical treatment to wash away the dead mildew and some of the dirt.