Installation materials
Portland cement products naturally contain lime, which is a source of the salts that migrate to the tile surface. Practically every installation material manufacturer disclaims liability for efflorescence formation with Portland cement products for this reason. Concrete, plasters, mortar beds, dry-set mortars (thin-sets), grouts, and some tiles are manufactured with Portland cements. So, efflorescence is difficult to avoid, but it can be controlled. There are alternative products available that greatly lower the possibility and eliminate some of the sources of these salts. When such products are used as a system, the risk of efflorescence is very low.
The first step is to include a flat membrane (such as a liquid- or sheet-applied type) able to restrict water from entering the substrate. Concrete and other cement substrates are the most likely culprits to provide a source for efflorescence in an assembly, so this can significantly reduce the risk of this problem. Due to their geometric cavities, uncoupling mats may also trap or slow water from draining and result in efflorescence. Adding a drainage mat under a mortar bed in exterior applications instead of using a cleavage membrane will be a great help to move water to a membrane and out of the assembly (Figure 1, page 30).
Second, mortars and grouts made with calcium aluminate (CA) cements or blends of primarily CA are much less likely to contribute to efflorescence, as these cements contain little or no lime. These product formulations are exothermic and cure quickly, using up water at a faster rate. Many manufacturers advertise they will not contribute to efflorescence or effloresce at all. This is especially important in cold and damp conditions. Premixed or single-component grouts and epoxy grouts are another option, as they are noncementitious, typically free of efflorescence, and warranted to be so.
To help prevent minerals from migrating out from cement-based tiles or natural stone, penetrating sealers might be a solution. To adequately accomplish this, a ‘six-sided’ sealing process is employed, in which the stone is immersed in certain sealers that, after removal, are allowed to dry till the next day. Some penetrating sealers will allow adhesion with cement mortar.
Project design options and flaws
Numerous design choices can lead to the formation of efflorescence. One of the most common is reducing the substrate and assembly pitch from 6 mm (¼ in.) per foot to 3 mm (1/8 in.) per foot. This is most often proposed with accessible showers and exterior decks/balconies where the elevations have not been properly accounted for or the design team feels the pitch is too severe. As shown in Figure 2, this reduction severely impacts the margin of error for even the best tile contractor due to acceptable variances in the installation and the tile itself, and actual pitch could be reduced to 0.7 mm (1/32 in.). What makes things worse is the tiles and natural stone recommended for these areas are purposely profiled, rough, or clefted to provide adequate slip resistance. That also means they will slightly slow down water flow or prevent water from draining, resulting in ponding and formation of salt deposits.
