Asked on Mar 14, 2019
How can I keep my concrete driveway from turning black?
by
Richard
+3
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You can try pressure washing it.
Pressure washing does amazing things, use the round scrubber you can rent. Won't believe your eyes. It's like a vacuum excepts pushes,water out.
It sounds like you have black algae built up on your driveway. If left untreated, it will grow and the situation will get worse. To remove it fill a bucket with 2 gallons of water and 1 cup of bleach. Put on a pair of rubber gloves to protect your skin. Then dip a push broom or stiff-bristled brush into the bucket and apply the bleach mixture to the mildew growth on your driveway. Allow the mixture to sit for 5 to 10 minutes to kill the mildew. Scrub the driveway thoroughly with the push broom to loosen the mildew. Then rinse the driveway with your garden hose. Repeat process if necessary.
Pressure washer or a hard bristle brush. Mine always goes back in the areas that dont get a lot of sun.
New concrete has a high pH level that inhibits microbial growth. That level will drop to 9 due to carbonation, which is CO2 in the atmosphere reacting with the calcium hydroxide in the cement paste that converts it back into calcium carbonate. The black growth is cyanobacteria that feeds upon limestone (calcium carbonate) which is also used in asphalt shingles that develop the same black stains. Your driveway needs to be cleaned before treating it with a silicate densifier which will react with the calcium carbonate to permanently stop the carbonation process. It's the same stuff used on interior concrete slabs to make them harder and to prevent dusting. The two most used densifiers are sodium silicate (water glass) and lithium silicate. Both can be found on Amazon along with their manufacturers' websites that provide plenty of information on how to apply them.
None of those suggestions works. Nothing short of a hammer and chisel has any effect on the black algae on my concrete tiles. I found a chemical once mentioned in an article, but I forget what it was called, and I couldn't find a supplier.