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.

The buildup you're seeing might also indicate that the weep holes in the shower drain are clogged. If the water moving through the mortar bed to the weepholes can't exit, the mortar bed will become saturated with water that will wick upwards into the wall setting bed materials, or through the floor tile grout joints where it evaporates and leaves its mineral cargo behind on the surface of the tiles.

Hard water is another possible explanation for the buildup. You mentioned the water is from a public source, so if the water is hard, it would be common knowledge. Evidence of hard water is easy to find: Look for deposits and crust on showerheads and tub spouts, and for visible waterline marks around the inside of toilet bowls. The best solution is to install a water softener; otherwise, the buildup will continue and may eventually clog the shower drain's weep holes.

Cleaners strong enough to remove lime, salt, or other mineral deposits, yet safe enough for use with tiles are available from most tile supply stores.

Contributing editor Michael Byrne is an expert tilesetter and consultant in Los Olivos, Calif.