Concrete Mold?

Hi, I was wanting to know if anyone can shed any light on what makes concrete eventually turn black... pretty much any concrete surface goes black after a few years making it look awful. A power washer cleans it up nicely but obviously financial and logistical reasons mean not every surface can be done, plus it ends up coming back not long after.. in some instances worse then ever.


Curiously I have noted that at the base of a lot of traffic signs it seems to stay clean as if some run off from the metal is retarding the growth. I don't even know what type of metal it is.


I know there are sealants you can buy but are there any other ways you can keep the concrete looking relatively new for a long time? I have included photos of what I mean.

Brand new traffic island in Brisbane
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The same island after only a few years.. looks almost 30 years old now.
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The clean part at the base of the traffic signs...
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