What can be done to eliminate a white film or powder in grout joints?
The white powdery layer you describe that most often reforms again after you brush it away is called efflorescence; which is simply white salt deposits. These salts remain on the grout joints when the salts in water solution are wicked to the surface and then the water evaporates, leaving these salts in powder form. It seeks out the porous grout joints because water cannot travel through glazed tile to the surface. Because it is difficult to say exactly where efflorescence comes from, it is just as difficult to say when it will stop appearing. These salts can originate in the grout, the mortar bed, the concrete slab beneath the mortar bed, or even in the ground under the slab. They have also been traced to unwashed sand containing soluble salts, sometimes to free alkalis in cement, occasionally to chemical concrete admixtures, and more recently to high alkaline floor cleaners which absorb into the joints and reappear as efflorescence. That is the reason they call your substrate, (ex. s
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