Why Sidewalks Turn Black

The Quick Answer

Concrete is heavily porous, meaning it acts like a giant hard sponge. Water, microscopic dirt, and airborne algae spores settle into these tiny pores. Over months of rain and humidity, the algae begins to feed on the limestone in the concrete, creating the thick black or green layer you see on top.

Gloeocapsa Magma and Organic Growth

The primary culprit for the black streaking you see on sidewalks (and often on home roofs) is a bacteria called Gloeocapsa Magma. To protect itself from UV rays from the sun, this bacteria creates a dark, heavily pigmented "shell." By the time your sidewalk looks totally black, you are looking at millions of these tiny protective shells.

Why Regular Rinsing Doesn't Work

If you just spray a black sidewalk with a garden hose, the dirt on top will wash away, but the biological roots dug into the concrete pores will remain. Within weeks, the black algae will simply bloom again.

The Correct Way to Remove Black Algae

To truly get rid of black algae, a professional must lay down a sodium hypochlorite based pretreatment mix. This actively breaks the cell walls of the algae, killing it at the root. Once the algae is dead, high-pressure surface cleaners easily lift the dead matter from the concrete pores, leaving it bright white for the long term.