Have you ever seen a white stain on concrete, bricks or tiles? It is not a problem with the tiles but a problem with the foundation and it is called efflorescence. It is caused by moisture with dissolved salts coming to the surface and evaporating, leaving behind the salts.
Causes of Efflorescence
One cause for efflorescence appearing is tiling on top of a newly laid concrete slab which has not yet fully cured. If using a porcelain tile, it is impenetrable so the moisture is not coming from the tile it is coming up through the grout and spreads out over the tile surface from there. To remove the powdery residue don’t mop it with water, as it will just dissolve and will crystallise again when dry, you should scrub it off and sweep it away. In this instance of efflorescence due to ‘green’ concrete there is not much you can do to completely stop it, you just need to wait until it stops coming on it’s own.
Another cause of efflorescence is groundwater with salts seeping up into the concrete or bricks, this form of efflorescence is more damaging and can rot concrete and bricks.
There are also acidic chemicals available specially for removing efflorescence but efflorescence may continue to return so long as the source of the water penetration is not addressed.