Efflorescence forms due to soluble salts within a concrete substrate, a mortar bed, installation materials, and/or grouts. When water acts as a carrier of the salts through grout, they dry and crystallize on the tile surface as they are exposed to air. Even some tiles and natural stone varieties (typically cement and sedimentary stones such as limestone) can also effloresce when exposed to water. Excessive use of water can easily start the reaction, especially when site conditions are cold or humid or when heat draws moisture from a damp substrate or tile assembly.
Manufacturers recommend an acceptable water range for mixing and recommend waiting until grout gels or firms before finishing. Otherwise, the unreacted grout will be damaged. Washing should always be done with a damp sponge, not a wet or soaked one that may flood the joint and leave puddles.
Mortar coverage of less than a rate of 95 percent under a tile or stone in wet areas, in either a vertical or horizontal application, allows water to flow or set on cement adhesives and substrates. As the water evaporates, it allows formation of efflorescence under the tiles (also known as subflorescence or cryptoflorescence), which later finds its way through grout and sealant joints. This cycling effect of efflorescence formation can also contribute to bond failure, as these minerals expand below and around the tile, creating compression at the bond interface and between tiles.
Additional installation errors in flatness and pitch of components in the tile assembly can easily lead to efflorescence. The requirement for proper drainage on horizontal surfaces in wet areas is 6 mm (¼ in.) per linear foot. This requires preparation work to the substrate (pre-slope), the mortar bed, and the waterproofing or crack isolation membrane. The tile or stone must also be in plane with the pitch. Slight birdbaths in the substrate, mortar bed, or tile lippage will slow or prevent water from draining, resulting in concentrations of salts and efflorescence. Often overlooked is the need for grout to be consistent in depth and smoothness and as close to the tile surface as possible.
Other related components of a tile assembly, such as a floor drain, must have effective weep holes and proper tile edge heights around them for good water drainage. When sealant or grout is very low around the drain flange, water evacuation is slower than the time it takes to dry, and efflorescence forms in these areas. Sealant in other movement joints can also be problematic if not properly detailed with sufficient product forced into the joint to adhere to the tile edges or placed over backer rod per Adhesive and Sealant Council (ASC) recommendations. Since these joints begin either below the concrete or at the substrate level, they are a faster pathway to the surface for efflorescence.
