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Q: How should I clean the slate in front of my house without damaging the mortar or grout?
A: Marble and limestone are in one big group of natural stones, those made up mostly of calcium carbonate. Slate, along with sandstone, quartzite, soapstone and granite, is in the other big group, with stones composed mostly of silicates, such as quartz and feldspar.
Acids eat into calcium carbonate, which is why you should never use acidic cleaners on stones such as marble or limestone. Alkaline cleaners, though, are okay. Silicates are much hardier, resistant to most acids as well as to alkalis. So, in theory, you could use pretty much any cleaning product for slate. It’s not so simple, though.
First, any kind of silicate stone is likely to contain other minerals that might be acid-sensitive. And even silicates will be damaged if they are cleaned with a solution that includes hydrofluoric acid, which is commonly found in rust-removers and toilet bowl cleaners, according to the Natural Stone Institute, an industry group.
Also, as you point out, you need to clean the material between the stones. On outdoor projects, such as your entryway, stones are usually placed in wet mortar, then the joints are filled with more mortar, usually a cement-based product. Strong acids eat into cement, so you wouldn’t want to use an acidic cleaner unless you were trying to remove mortar smears after a messy installation. Instructions on cleaners designed for that purpose generally warn users to rinse thoroughly after just a few minutes so only surface scum dissolves.
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The picture you sent looks like firewood, a planter or something else was left on the stone for a long period. It’s hard to tell from a photograph whether the dark patches are just dirt, mildew or some other kind of stain. You might need to try a couple of things before you hit on what works.
With any cleaning task, it makes sense to start with simple, low-cost measures that can’t do any damage. So for cleaning your entryway, you’d start by sweeping away all surface grit. Or you could use a shop vac or even a leaf blower. Then mop or scrub with warm, soapy water. For the soap, you could buy a stone cleaner or use something you probably already have on hand, such as a squirt of pH-neutral hand dishwashing detergent, which is safe enough to use with bare hands. Or you could use a little laundry soap or dishwasher detergent, both of which are strong enough alkalis to warrant wearing gloves to protect your skin.
Use a nylon scrub brush — not one with wire bristles — so you don’t scratch the stone. Or you could try a power washer equipped with a fan tip, but when stones are so close to a door, you run the risk of getting water where it shouldn’t be or blasting paint off the wall.
Once you rinse off the soapy water and whatever grime it loosened, if the stones and mortar still have dark stains, they are likely from mildew. You should be able to get that out — or at least significantly reduce it — with bleach. But there are different types of bleach, and it’s wise to test a small area first to confirm that you are dealing with mildew rather than a stain caused by something else. Treating with bleach can actually make rust or other metal stains more difficult to remove.
Chlorine bleach is cheap but spatters can burn your eyes and skin and ruin your clothes. Wear goggles, protective gloves, rubber boots and old clothes. Mix ⅓ cup bleach per gallon of water, spray or mop it on, then scrub. Wait 10 minutes, then rinse. To protect nearby plants, mist the leaves with water before you start, which will dilute any bleach solution that might spatter, and spray them again after the final rinse.
Another option is a non-chlorine bleach, such as OxiClean versatile stain remover ($17.97 for a 7.22-pound box) or Scotts outdoor cleaner ($12.97 a gallon). With these products, you don’t have to worry about damaging your clothes or plants, but you still need to protect your eyes and skin. If you’re using OxiClean, mix 4 scoops per gallon of water. Dilute the Scotts formula by adding 3 cups of water per 1 cup of cleaner.
A third bleach option is a mildew remover formulated for use on stone, such as Stonetech mold & mildew stain remover for natural stone ($22.50 for 24 ounces). It contains sodium hypochlorite (chlorine bleach) plus ingredients that help buffer bleach’s caustic properties. Take the same precautions that you would with bleach, though, and make sure the runoff doesn’t go into a waterway or storm drain.
If bleach in any of these forms doesn’t remove the stains, you’re dealing with something other than mildew or dirt, such as stains from metal objects. Iron or rust stains can be brown or orange, and copper and bronze stains can be brown to green. In either case, apply a poultice — a paste spread ¼- to ½-inch thick — and cover it with plastic wrap to keep it damp, then let it sit for a day or two. Make a poultice by mixing an absorbent powder such as baking soda, diatomaceous earth or kaolin clay with water, or buy a premixed product such as Stone Care granite and stone stain remover ($8.48 for 10 ounces). If the stain lightens but doesn’t disappear, repeat the procedure until it doesn’t make the stain lighter. Deep rust stains can be especially difficult, often impossible, to remove completely.
Source link: https://www.washingtonpost.com/home/2023/05/26/tips-stain-removal-slate-walkway/