The binding cement in grout has organic qualities, as well as the ballast
sand and the water that has been mixed in. These contain nutrients that
further the moss spores to establish themselves on grout.
The different kinds of binding agents give the grout a more or less smooth surface where the moss' spores and dirt gathers –which binds damp and provides nutrients. The affections grow.
The moss establishes a good hold on grout and when it has begun to grow, sterilization is the only way to stop the process.
As long as any remains of old moss or moss' spores are left in the grouts pores the affections will continue to recur. Treatment with Bensaltenside stops all growths and all affections within 24 hours.
And it is guaranteed to protect for at least four years. Often longer.