QUESTIONToday I am requesting your assistance regarding the best methods to prevent efflorescence in natural stone tile installations. I have recently begun working as an agent for a distributor that sells a limestone that goes by a few different names; Haisa Light, Rockwood and Timber Beige to name few. As I was showing this stone to a client she described a few installation problems she had with efflorescence migrating through this stone. While she didn’t go into great detail about the installation site conditions and the setting material used she did mention that the remedy she was told to use to prevent this in future jobs was to seal the back of the stone before installing. I had heard this remedy once before when another client told me about a similar problem he had with a Chinese basalt tile. In both client’s problems the specified tile size was 12”x24”. Right now my new supplier only stocks this product in 3”x6” tile and several mosaic patterns. I’ve done a little research about efflorescence and so far haven’t found anyone recommending sealing the backs of tile as possible cure for this issue. I am also concerned that too often people lump all sealers into the same category and the result may be someone using the wrong sealer and ending up causing additional installation problems. So now I am reaching out to you to see if you can provide your insights into both efflorescence migration causes and cures, and your opinion of sealing the back of tile as a preventative measure. Can you point me to any articles that discuss this in depth?
ANSWERANSWER - Excessive efflorescence staining (generally a whitish substance) on stone, tiles, grout, or adjacent surfaces is a generally a symptom of an excessive moisture problem. That means the stone installation was not properly designed or installed correctly such that it is allowing the stone to be subjected to excessive moisture. Although in some cases a stone can get some efflorescence from surface water migrating into and evaporating form the stone, which might be unavoidable and not a problem.
The efflorescence precipitates from the moisture that travels through the soil, substrates, mortars and stone picking up minerals/salts that dissolve into the moisture. As the moisture evaporates at the surface of the tile and grout the minerals crystalize leaving the efflorescence precipitate. This precipitation can cause spalling in all types of stone.
Some stones have a greater propensity to be damaged by the efflorescence or the wet dry cycles than others. Generally speaking the more porous tiles with higher absorption rates are more prone to efflorescence and those related damages. Regarding limestone, the ASTM C568 standard for limestone is broken down to low density, medium density and high density limestone, where each respective category has different absorption rate and propensity to damage. There is a test called the Salt Crystallization test that can test a stone to see if it has a higher or lower propensity to damage from moisture and mineral migration and evaporation.
That being said, tile and stone sealers are normally bond breakers and should not be applied to the back of the stones. I have read some claims that certain sealers can be used on the back of stones and that they improve the bonding of the tile, but I am suspicious of those claims. On the other hand, we have applied liquid applied waterproof membranes that meet ANSI A118.10 on the back of stones to damp proof them. We have tested some and they perform well.
Although keep in mind that damp proofing the back of the stone will not solve an improper installation of the stone. For exterior applications the tile must be installed correctly over a concrete substrate with a vapor retarder under it that runs up the sides of the concrete. There should be proper deck drains and French drains as necessary. The tile should have a waterproof membrane under it with an adequate slope to drain.
There is an article I wrote that covers some of this at: Specifying Tiles for Exterior and Interior Wet Areas