Concrete contains sand, crushed stone, water and cement, which are
the natural nutrients for affective algae. Concrete is often subject
for weather and wind and therefore exposed to the large amounts of spores
that float in the air.
Affective algae are furthered by damp and dirt which accumulate in the
concretes pores, which give good hold to the spores. This speeds the affection and growth of the algae.
The affection will recur as long as there are any algae and alga spores left in the concrete's pores.