Red stains on concrete

iplogger1

This is just a general query. I've noticed on various buildings,
chimney stacks with no rendering, pavements a sort of leaching
of a red colour. What causes this ? What I've seen is a streaking
of a red colour which really doesn't look good.

I'm really only asking because it has puzzled me for a while.
I've not been affected by it (yet) but the curiosity was getting
to me to know what generally causes this discolouration or
bleaching effect.

~ipl

Kashkai

One possible cause that I've come across is rust - iron or steel nails rust over time and as the water trickles away, it carries the rust with it and leaves a reddish brown stain down the side of buildings.

No idea what would cause the stain on the ground.

Qwerty?

afaik it's a type of algae. Mossgo kills it off.

Mellor

A redish stain can also be caused my rebar being too close to the surface of concrete. But that wouldn't happen in pavements obviously.