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Question: The concrete steps in my house are moldy and a little green on the risers except where I applied concrete sealer. I did it wrong so you can see areas that look like drops. How to safely clean concrete?
Hyattsville, Maryland
A: When you ask for a way to “safely” clean concrete, you are not saying whether your goal is to protect the concrete, yourself or your plants. Fortunately, it is possible to do all three.
The cheapest option is to use a little bleach (only $ 2.78 for a half gallon at Home deposit) diluted in water. The bleach solution won’t damage concrete, but it’s not something you want in your eyes or on your skin or clothing. You might end up with spots of white or weird colors, as bleach unevenly affects the dye colors of fabrics. So before you start retracing your steps, put on some old clothes, long rubber gloves, and eye protection, and have a bucket, sponge, and synthetic scrub brush handy.
[Two enemies of outdoor wooden sculptures: Rain and sun]
(On a small job such as cleaning a few stairs, there is probably little risk of splashing a lot of bleach solution on nearby plants. But just to be sure, you can cover larger plants. near steps with a cardboard box upside down. Do not use clear plastic pots or clear plastic sheet, especially if it is a sunny day. Enough heat can build up under the plastic to wilt the leaves or even kill the plants. Or, instead of covering your plants, you can spray the leaves with water before applying the bleach solution to the concrete. This is the best approach when there is a lawn right next to the concrete that needs to be cleaned. Pre-moistening dilutes the splash, protecting the plants.)
When everything is ready, sweep or vacuum the steps to remove dirt and debris. Then mix three-quarters to a cup of bleach with a gallon of water and blot any areas where you see the green spots. Continue to sponge more as needed to keep the concrete damp for five to 10 minutes. Scrub with a synthetic brush, then rinse thoroughly. Repeat the process if the concrete is still stained. Rinsing with a hose will dilute any cleaning solution that is still on the concrete, so that it will not damage the plants where the water is flowing.
If you don’t want to use bleach, there are other ways to kill mold or algae, but don’t use vinegar, which is often touted in online tips. Vinegar is acidic and acids degrade concrete. Use cleaners that have a neutral (seven) or alkaline (greater than seven) pH. Acids are less than seven, and those with a pH of three or less are especially harmful to concrete. Distilled white vinegar has a pH of 2.4 in 5 percent strength sold in jugs at grocery stores.
One product labeled for use on concrete, as well as many other materials, is Wet & Forget Moss, Mold, Mildew & Algae Stain Remover ($ 24.38 for a half gallon at Home deposit). It is designed so that you can dilute it and spray it on, then wait for the growth to die off and the spots to disappear; no rinsing required. The instructions say to use a pump sprayer, but for a small area, like your steps, you could probably settle for a spray bottle.
Or you can try using a chlorine-free bleach, like OxiClean multipurpose stain remover ($ 13.48 for a 7.22 lb box) or Scotts Outdoor Cleaner Plus OxiClean ($ 10.98 per gallon). These products are attractive because you don’t have to take precautions to protect plants or worry about spills damaging your clothes (although you still need to protect your eyes and skin). However, if you read the Scotts product information, you will see that it does not actually kill mold; it just removes mold stains. Does this distinction matter? Probably not, because killing mold is a rather futile goal. There are so many mold spores floating in the air that when the conditions are right, mold thrives. Whether it’s dead or alive, you just want to get it out of your concrete.
If you already use OxiClean for laundry, see if it works. Mix four spoons of the powder with one gallon of lukewarm water. Apply and scrub as if you were using a bleach solution, except wait 30 minutes before rinsing. Or if you buy a specialty cleaner, follow the directions on the package. If you clean the surface it worked. Otherwise, use a chlorine bleach or cleaner that says it kills and removes mold.
Whichever method you use, once the concrete is clean and dry, apply a sealer to the risers so that they get the same protection you already gave the steps. Although you look at your steps now and notice the drops from your “bad” application, the sealer clearly worked: the drops are not moldy, and neither are the steps.
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