concrete patio - sealing suggestions

We have a concrete patio slab under a huge old oak tree. With all the shade, when it rains, the patio dries slowing, and the white concrete is now a dingy dark color from the yuck on it.
I can power wash it - but it comes back next year.
Wondering what I can use as a sealant to help prevent the dark dingy micro growth ? Most of the online blogs don't like the big box concrete & deck sealers.
I don't want the patio to become slick when wet because it has been painted with a sealer... Suggestions ????
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The solvent based concrete sealers are the most effective products. But two problems. One is that like all sealers they will tend to leave the surface slick. You can mix in some fine sand to eliminate that. The other is that you can't buy them in many states because of the EPA. If you live in one of those states you may be able to find someone who will ship it. Kure N Seal 30 is the one I use.
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ps56k wrote:

You don't say where you live, but in my neck of the woods the black yuck is soot from the air, I suspect deposited from automobile particulate pollution.
Further evidence comes from my concrete driveway getting yucky without water standing on it.
That said, a sealer would probably be a good bet in that it closes up the near-microscopic pores into which this pollution lodges.
What sealer, I don't know.
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