We appreciate the industry members who emailed us some comments and questions regarding gray (or grey) mottling discoloration in plaster swimming pools. This is written to provide further details and clarification.
Gray ghost, shadowing, hydration, and severe mottling, are terms that may be used to describe blotchy gray or dark discolorations. Sometimes, plasterers also suggest that “entrapped moisture” is a cause of discoloration.
Certainly, a dark discoloration on a cement sidewalk can look like “entrapped moisture,” and the same goes for white pool plaster. All cement products are semi-permeable, meaning that water can be absorbed into the cement matrix. But the better the quality, the less permeable it will be. When pool plaster is under water, it is always saturated with water.
However, it is also known that calcium chloride is hygroscopic, meaning that it “attracts and hangs on to” water molecules. If an excessive amount of calcium chloride is added to a batch of plaster mix, perhaps that condition can enhance a darkening of certain areas of a plaster surface. Some believe that hard troweling can “seal” the surface, not letting water escape. So, perhaps the jury is still out on this. The term “entrapped moisture” is generally not mentioned in cement literature or mentioned as a cause of discoloration.
In the case of adding calcium chloride to a plaster mix, it is known that it speeds up the hydration (hardening) of the silica compounds in cement, but retards the hydration of aluminate and iron (ferrite) compounds. This will cause a darkening color to develop. Incidentally, white cement contains a small amount of iron compounds as compared to gray cement, and may show the dark discoloration more readily. (Interestingly, there are non-chloride admixtures (accelerators) available for plasterers to use that may help avoid gray discoloration).
Trowel “burn” is another cause of extreme dark discolorations of white plaster. Usually, the plasterer removes this discoloration stain by “ragging” or “wetting” the plaster surface while troweling and before he leaves the pool.
A reason that late hard troweling causes a darkening of the color of cement is because it decreases the water-to-cement ratio of the plaster surface. A very low water-to-cement ratio almost always leaves a darker finish color. One theory on the cause of graying suggests that the development and use of non-burn trowels by pool plasterers allows for late hard troweling without the “burning” of the plaster, and thus, may occasionally help cause a gray discoloration to develop.
It is known that adding admixtures such as silica fume and fly ash can also cause a dark discoloration.
It is believed by some that a thicker coat of plaster in certain areas of the pool can result in a slightly darker color.
It is believed that filling a new plaster pool too soon or spraying down fresh plaster with water is detrimental and can result in discolorations developing.
To attempt to remedy and lighten noticeable gray discolorations, some plasterers drain the pool and allow the plaster surface to bake in the hot summer sun. Some also perform a light acid wash or chlorine bleach (wash) prior to sun-baking. But remember, in general, most acid treatments will age or etch the plaster surface prematurely.
Sometimes, “torching” (heating) the gray area with a propane torch device is performed in an attempt to remove the gray color. It is unclear whether the torching process affects the long-term durability of a plaster surface. It is known however, that the plaster surface may “pop” when heated to such high temperatures, so protective goggles should be used. Be aware that the above remedies may only be temporary as the gray discoloration may return after the pool is filled with water.
There is also normal mottling of plaster where the white plaster surface has very slight shade differences from one white area to another. Again, this slight difference still has to do with the fact that plaster is made of a mixture of water, cement, and aggregate, and is a hand-crafted product. At its best, there will still be a slight variation.
As one can see, all of the above discussions regarding dark discolorations of white plaster has to do with workmanship, additives, hydration problems, and/or curing issues, not about whether the water is aggressive or not. Typical aggressive pool water does not cause a darkening of white cement, nor does it affect the hydration rate of the various cement compounds.
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