Among the most common defects due to a poor installation, cna be found moulds or efflorescences.
As follows, please find some useful suggestions in order to avoid this unpleasant inconvenience:
First of all use limeless mortars;
When possible, use washed sand;
Use water-resistant adhesives.
In case that material is outdoor installed with high ground humidity conditions, we strongly recommend to interpose inbetween the two existing cement beds a waterproof sheathing (bituminous paper or similar) in order to avoid underneath moisture to continously raise up to the surface showing efflorescence.