Sand, stone and water in the concrete contains organic substances, as well
as cement, which gives the concrete organic qualities, which provides
the moss nutrients.
Concrete is often exposed to damp and dirt –which contains nutrients, which furthers the moss affections even more.
The spores attaches themselves easily in the concretes fine pores and are therefore hard to treat. As long as the surface pores contains spores the affections keeps recurring. Sterilizing the growth is the only way to get rid of the moss. The concrete surface becomes also disinfected, thus gaining long time protection.