A.Michael Byrne responds: There are a couple of possible culprits. The first is efflorescence, which occurs early in the life of a mortar bed installation when minerals from the sand-cement-lime mix get deposited on the tiles. It usually happens because too much of a particular ingredient — hydrated lime or Portland cement, for example — has been used. The shower water brings salts and minerals to the surface of the tile, and a white deposit is left behind when the water evaporates. If materials within the mortar bed, adhesive mortar, or grout are the problem, the efflorescence should go away after 28 days — the curing period for Portland cement products.

Efflorescence can also be caused by salts or minerals being carried by an outside source of water. This happens frequently when ground water seeps through a foundation wall or slab. The cure here is to stop water before it can enter a structure.

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