Grout is organic since it consists of binder –either cement
which contains organic substances, plaster, lime or plastic- and ballast
which most often is sand -with higher organic percentage than 4%- and
water which consists different organic subjects.
Grout has varying coarse surface –depending on which binder that has been used- and the pores in the surface therefore gives different conditions for the algae to get hold.
Dirt that has been collected on and in the surface binds damp which provides nutrients for the algae.
Independent of the surfaces coarseness the algae spores can get hold everywhere and therefore the affection recurs as long as spores, or rests of them, are still left on or in the surface. Algae must be sanitized.