Outdoor concrete turning black (algae?)

[Lsbcal]

The products mentioned (Wet-n-forget, Spray & Forget) sound ideal for my concrete patio surface. This is a pebble type concrete surface which gets a lot of moss on it due to the oak tree shading the surface. The moss is maybe 1/2 inch thick in spots and I think I will try to scrape some off with a flat edge shovel before applying this stuff.

Any other suggestions appreciated.

Here is a pic of the surface. Moss is pretty now but gets black in dry weather:

 

[Chuckanut]

Thanks. I will give it a try as soon as we have a few days of dry weather predicted.

I have a power washer but as others have noted, it seems to leave the concreate a big grainier. And it definitely digs into some of the pebbled concrete I have. The chemical way sounds a lot better and easier.

 

[tfudtuckerpucker]

I've fought black algae on concrete and on my roof for many years. Yes, wet and forget (or similar products) can work well, but they take time, and you have to retreat after several years. I've also used the round pressure washer attachment for concrete, which makes that work easier. However, I noted the removal of the top layer of concrete from the pressure washer. So I've been looking into a better long-term solution. Here's the best answer I found:

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.

There's also a good writeup here.

 

[Chuckanut]

We have a few warm 70+ days with sunshine coming up so I got out this morning and sprayed Wet and Forget on my painted deck surface. The deck is filthy after a good 8 months of no care in the winter. I swept off the surface dirt, and sprayed it with W&F. It's slowly drying as I write this.
Time will tell how effective it is.

 

[audreyh1]

We use wet and forget.

Works well for us.