The consistuents sand, stone, water and cement contains organic substances
which supplies the lichen with nutrition.
Damp and dirt that get hold in the concretes pores are nutrition sources for affection of lichen and the lichen spores easy gets hold in the concretes pores. By that the affection establishes and grows fast.
The affections does recur as long as the surface pors contains spores and old remains of earlier affections. To get –and hold- concrete free from lichen the surface must –both on and in the surface- be totally pure. The lichen must be sterilized and the surface must be disinfected