The cement in mortar contains resin which is organic, which supplies the lichen
natural nutrition. The affections can therefore starts quickly after
that the spores have got hold, and they grow fast.
The spores create strong holds in the mortars pores and gets nutrition from dirt and damp that have been bound into surface pores. The developing speed is therefore increasing.
Lichen will constantly recur as long as there are remainders of growth in the pores –therefore it is necessary that all old remainders and all spores get sterilized, and that the surface gets disinfected –both on the surface and in the pores.