Rust on Concrete

This morning I took note of a rust stain on the wall of a parking garage.

Rust on Concrete

Rust 101:
Rust is formed in the presence of iron, oxygen, and water.
(If you want a more detailed breakdown: iron oxide is mix of ferrous oxide and ferric oxide. 3Fe + 2O2 = Fe3O4.)

Why would concrete have rust stains?
 Concrete is a porous material. Water with a high mineral content - also known as 'well' or 'hard' water - would penetrate the surface and leave behind a rust-friendly surface.