White grout residue is often minerals left by evaporating water
A whitish mineral film on colored grout is commonly efflorescence. The Tile Council compares it to the powder left when a glass of water dries. Soluble minerals dissolve, travel to the surface, and stay when the water evaporates. They usually come from the slab or the ground under it.
Where the water is not coming from
Except in rare cases, the FAQ says ordinary wash water does not carry enough mineral to show. Thinset under the tile also does not hold enough soluble salt. A thick mortar bed can, but only if moisture regularly passes through it. Cleaning, even soaking the joints now and then, generally does not create the deposit. Exterior rain can, over time, when grout is porous. More often, ground moisture keeps moving up through the slab even when a vapor membrane has penetrations.
Porosity makes it visible
More mineral shows when the concrete and the grout are more porous. Stopping the water path before tile is set is more effective than treating the film afterward. The fetched page says the later options are the weaker ones.
Source
Fetched 2026-10-11 from Tile Council white residue FAQ. The sentences above follow that page. They are not a contractor promise.
Related notes
- Tile notes index
- Cement grout is cleaned with alkaline cleaner, not acid
- Epoxy grout is not the same as epoxy-emulsion grout
- Floor photos
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