Grout can become organic since the binding cement, the ballast sand –with
larger amounts than 4%- and water, that may have organic qualities,
is the constituents.
On the grout's surface dirt, damp and the very small fungus spores accumulate, which supplies the fungus spores with nutrients.
After that the spores have gotten hold and begun to grow, the affections continues to recur until the surface is totally clean. The only way is to sterilize the growth and to disinfect the surface –thus stopping is all old and future affections.