Why Colored Plaster Turns Whitish

A sample of a blue pigmented plaster and pebble finish where the cement portion has lost the blue color and turned white. Note the absence of any calcium scale deposit over the pebble aggregates, which eliminates that cause of the plaster whitening effect.
A sample of a blue pigmented plaster and pebble finish where the cement portion has lost the blue color and turned white. Note the absence of any calcium scale deposit over the pebble aggregates, which eliminates that cause of the plaster whitening effect.
Photo courtesy onBalance

Having a beautiful pool with a colored surface, especially one with a quartz or pebble plaster finish, is a popular choice among pool owners, and understandably so. The color adds ambiance to the setting and can make the water wonderfully attractive and inviting. That’s why pool owners are willing to pay extra to have that special color enhance their water and the entire backyard.

Because the interior pool surface is such a dominating visual element, it must be disappointing for everyone involved on those occasions when they watch their pigmented, colored pool become unsightly with whitish blotches, streaks or small spots. Later on, it must also be disappointing — if not more so — to learn that the cause of the unsightly color problem was misdiagnosed, and worse, the remedial action taken to resolve that problem caused more serious damage in the long run. 

Plaster colors can fade or turn whitish for a handful of preventable reasons. First off, calcium scale can be deposited onto pigmented plaster surfaces by out-of-balance pool water. As is widely known, calcium scale makes the surface rough and turns it white, and acid washing is often the remedy. 

However, it is also common to have colored plaster (including quartz and pebble) surfaces lighten in color due to mistakes in plaster workmanship and material selection. Because changes in color resulting from these defects are also a lighter or whitish color, it is easy to mistakenly believe that the problem is calcium scaling, and that the pool water has been out of balance. Once you make that misdiagnosis, it is then easy to mistakenly assume an acid wash is the best remedy. But when there’s no scale there to remove, that will only cause etching. It won’t fix the problem and will shorten the life of the plaster. 

If it’s a plaster application problem, most likely the whitening is a result of soluble calcium ions in the plaster dissolving and fleeing the scene into the pool water. When calcium dissolution occurs, it means the plaster surface is deteriorating, losing density, and becoming porous. Yet it usually remains smooth. And when porosity develops and increases (which can take several months or longer), a whitening of color/pigmented plaster results. 

A porous surface appears lighter in color because of the way it reflects more light than a non-porous surface. According to Dr. Boyd Clark, senior materials specialist with Construction Technology Labs, it’s the same reason that a head of foam is lighter than the beer; the foam is lighter in color because it throws off more light.

“The fact that we have a spot, or a lightened area, is due to the fact you have an increased porosity,” he says.

It’s helpful to note that even common white pool plaster can develop white spots and streaks that are whiter than the surrounding plaster because of increased porosity in those areas. There is a direct connection in how white and pigmented plaster develops color-shading changes. 

Scale vs. Porosity

Calcium scaling, which is caused by out-of-balance water (an overly positive LSI value), usually deposits a uniform layer of calcium carbonate over the plaster finish and whitens the entire pool. Even the surface of pebbles (in pebble finishes) will be covered by calcium scale along with the cement surface that surrounds and binds the pebbles together. Calcium scale is generally rough and is generally easy to remove by hand sanding with sandpaper.   

On the other hand, if the whitish discoloration is smooth to the touch, manifests itself in streaks, blotches, or small spots, and if diluted acid or hand sanding does not easily and quickly remove the white discoloration, then one should realize that the problem is probably not calcium carbonate scaling, but a porous surface instead that is deteriorating and turning white.

Additionally with pebble finishes, a very close examination is needed to determine if the whitish discoloration is only within the cement portion of the finish that surrounds and binds the pebbles. Porosity (a loss of material) only occurs in the cement portion and not the pebbles or quartz aggregates themselves. 

What leads to a porous, yet smooth surface? As mentioned above, plastering mistakes can cause two relatively soluble components to slowly dissolve away, creating a very porous surface. Those soluble components are calcium hydroxide (a by-product of the cement/water reaction) and calcium chloride (if added to the mix to accelerate the hardening process).

It’s not uncommon to encounter the misconception that calcium hydroxide and calcium chloride can only be dissolved by aggressive (negative LSI) pool water. That is not true. In fact, both plaster elements can be dissolved by positive LSI water. Aggressive water is not the root cause of porosity and the whitening effect. 

Other Factors 

There are other reasons for color loss: some plasterers or suppliers use organic pigments in their plaster mixes. Organic pigments can be bleached and turn white by the presence of chlorine or other oxidizers, and even by the sun. It is recognized that organic pigments are cheaper than inorganic pigments, but they generally cannot withstand a swimming pool environment for very long. 

Some plaster mixes and finishes contain two pigment colors. If one is organic and becomes bleached, that color will simply disappear leaving the other non-bleachable pigment visible as the sole color.

For a detailed explanation of the plastering issues and other aspects, see this link: https://www.poolhelp.com/home/onbalance-research/education/


An acid treatment is often tried to restore the original color of plaster. However, such acid treatments often etch the plaster and make the surface rough. That in turn, leads to balanced water penetrating an etched surface and dissolving more of the soluble calcium hydroxide material away over time which then increases porosity and surface deterioration. That reduces the lifespan of the finish, and the plaster color often fades away and turns whitish once again. 

With white plaster, an etched surface will more easily and quickly become stained with trapped dirt and minerals such as copper and iron.  

A better alternative for removing a porous and whitened plaster surface would be to sand/polish the plaster. That process removes the porous material and restores a hard, dense, and smooth surface.  Unfortunately, when dealing with discolored pebble finishes, sanding and polishing is virtually impossible. 

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