If you have ever opened a winterized swimming pool in the spring and found calcification on the walls and floor, it is most likely not scale. The odds are it's actually calcite crystals in your pool. This article will explain the differences between scale and crystals, as well as how to treat and prevent the crystals from coming back.
NOTE: This article has been revised from its original version due to learning more from laboratory results. We still have more unanswered questions about crystals than we have answers. Why do they harden? How many forms are there (we know of at least 3)? Why are some crystals easy to clean up, and others stubborn? What common denominators are involved, beyond cold water and lack of calcium hardness?
Let's start with some history. In 2017, we were in the Northeastern United States making sales calls and visiting customers. We kept hearing of these horror stories about "scale" when they would open their pools in the spring. It seemed like everyone in the pool business had a story to tell about it.
At the time, we had recently released the Orenda Calculator™, and we were learning about the LSI ourselves. Something didn't make sense. Low temperatures and low calcium hardness lower the LSI...but calcium carbonate scale is a high-LSI violation. We soon learned that semantics matter, because calling this stuff "scale" was leading to horrible winterization behavior. Because people assumed the calcium they were seeing in the spring was scale, they had developed habits of reducing calcium via dilution in the fall. The thought was less calcium must lead to less risk of "scale" in the spring.
As it turns out, the "scale" was not scale at all. It was the opposite problem. Every single case was calcium crystals. Here are some common factors in all of the cases we encountered:
All these observations told us that this calcium had not come out of solution (scale). It had grown out from the plaster. This led us to start doing research. At first, we thought it was calcium hydroxide because we know that calcium hydroxide bleeds out of plaster when it's curing. As it turns out, we were half right about that. The calcium hydroxide in the cement is absolutely involved, but as it is drawn out from within the cement, it is carbonated into skeletal calcite, which forms the crystals.
The first step in understanding the differences between scale and crystals is to understand calcium carbonate itself. Calcium carbonate (CaCO3) is the most abundant form of calcium on earth. It is the building block of limestone, and it is also called calcite.
On the periodic table of elements, the 20th element is calcium (Ca). In its elemental form, the calcium atom itself is an alkali earth metal that is relatively unstable, thanks to two valence electrons.1 Those electrons are easily lost, which leaves the calcium cation (Ca2+). The calcium cation has a positive valence because there are two more protons than electrons. For simplicity's sake, we'll just refer to Ca2+ as the calcium ion.2
To become a more stable compound, Ca2+ binds to things that have two or more valence electrons (2-) such as carbonate (CO32-), silica (SiO32-), sulfate (SO42-), or phosphate (PO43-). When these compounds oversaturate water, they form various forms of scale.
The most readily available among these is the carbonate ion, and calcium binds to it easily:
Ca2+ + CO32- → CaCO3
Calcium + carbonate → Calcium carbonate
In swimming pools with cementitious surfaces (like plaster, quartz, or pebble), carbonation also occurs from the interaction of a cement compound called calcium hydroxide (Ca(OH)2).3
Ca(OH)2 + CO2 → CaCO3 + H2O
Calcium hydroxide + carbon dioxide → calcium carbonate + water
Based on the evidence we have collected and lab tested, we believe the calcite crystals are created by the carbonation of calcium hydroxide as the water pulls it from the cement in the pool surface. See the video above.
Like anything else in nature, water craves equilibrium. The equilibrium it cares most about is the saturation of calcium carbonate. The international standard for measuring this saturation is the Langelier Saturation Index (LSI). If you have read our website and used the Orenda app, you know that we teach the critical importance of the LSI. It is the first of our Four Pillars.
0.00 on the LSI is perfect equilibrium. Below -0.30 is considered aggressive because water does not have enough CaCO3 in saturation...so the water will dissolve calcium to pull it into solution and get back to equilibrium. On the other end of the spectrum, if the LSI is over +0.30, water has too much CaCO3 in the given conditions, and it will get rid of some of it to bring itself back into equilibrium.
The LSI, however, only tells us the saturation of calcium carbonate, not other calcium compounds like calcium sulfate, calcium silica, or calcium phosphate. Keep that in mind as you read further in this article.
As we just mentioned, over the past several years we have collected samples of crystals from various pool owners and service companies from around the country. The crystals we are referring to in this article are formed during the winter in cold, stagnant water that is low on the LSI. These calcite crystals are not to be confused with calcium sulfate crystals, which are technically a type of scale that we cover in depth in another article.
Thanks to lab tests and research, we now know quite a bit about these crystals. When we originally published this article in 2018, we did not yet know much about them. Here are some things we have learned:
Related: How to winterize a swimming pool
We cover calcium carbonate scale more in-depth in its own article. Here is Orenda's definition of scale:
Scale is a buildup of mineral compounds (usually calcium-based) that have precipitated out of solution and have formed on surfaces in and around the water system.
There are several types of scale, as mentioned earlier. This is because calcium can bind with other ions with two or more electrons. Carbonate (CO32-) is the most abundant in swimming pools, therefore calcium carbonate is the most common form of scale. The LSI tells us how saturated water is with calcium carbonate, so an LSI value over +0.30 is scale-forming. It means the water is oversaturated and needs to get rid of some CaCO3 to get back to equilibrium.
The key here is that scale precipitates from the water and adheres to surfaces. If it's actually scale, the calcium compound–whatever it is–will stick to plastic fittings, lights, rails, and the face of tile. So if it did not originate from the water and land on surfaces, it is not scale.
But if the calcification is only on cement but not the non-cementitious surfaces, it cannot be scale. It is likely still calcium carbonate, but it originated from within the cement itself. Therefore it is not scale.
Many forms of calcification are often mistakenly referred to as scale. Calcite crystals are one such condition. There are also uneven carbonation/mottling issues on plaster, which are even more common than crystals. Just because something reacts and disappears in the presence of acid does not mean it's actually scale. It just means it was likely calcium carbonate. The origin of the calcium carbonate is what matters here. Did the calcium compound come from an oversaturation in the water, or not?
Since 2017, we have collected many samples of crystals and have had them lab tested. As of now we know of at least four types of calcite crystals. They are all calcium carbonate, but they contain trace amounts of other elements like strontium, silicate, and a few others. We honestly still don't know why or how.
Of the four types of calcite crystals, we estimate just two of them account for more than 90% of the crystals we see and hear about.
1. The first we have nicknamed sandpaper crystals:
Sandpaper crystals are stubborn and sharp. These small translucent crystals feel like sandpaper and are notoriously difficult to remove. They tend to be resistant even to acid washes...despite being calcium carbonate (calcite). The oddest thing is they can be scraped or sanded off, then they dissolve just fine in acid. But when they're still stuck to the wall, they are not easily dissolved. It's still a mystery.
2. The second type we have nicknamed needle crystals:
Needle crystals grow from the cement into long and brittle formations. They have a hollow tube in the center of them and the lab called their molecular structure as "skeletal calcite". Compared to sandpaper crystals, these are relatively easy to chemically soften and remove. SC-1000, LSI balance (usually accomplished by raising calcium hardness) and brushing is generally sufficient to soften and remove these as the water warms up over 65ºF.
3. The third type, which is much less common than the first two, we have nicknamed toenail crystals:
Toenail crystals look like...well... toenails. They are usually off-white in color and a bit more opaque than the translucent needle and sandpaper crystals. Toenail crystals are sharp and we have seen them as long as half an inch. They are without a doubt the ugliest of the four crystals. But don't tell them we said that. The good news is, toenail crystals seem to be the easiest crystals to chemically dissolve and remove. Simply balancing the LSI and getting SC-1000 in the water when it's above 65ºF and rising seems to do the trick. Brushing speeds things up too. We have heard of these crystals simply vanishing in a week or two, almost like they were never there. It's wild.
4. Finally, the rarest of the four types of calcite crystals we have nicknamed flaky crystals:
Flaky crystals look like snowflakes or pieces of broken translucent glass. They tend to populate in certain areas of the pool, as opposed to uniformly everywhere like the other three types of crystals. Our theory is these flaky crystals only grow where the cement is most porous and weak. But we don't have enough data to know for sure. We do know they are rare and thankfully they are easy to remove, similar to toenail crystals. They seem to be a bit more stubborn, but relative to sandpaper crystals, flaky crystals are easy to remove.
In either case–calcium carbonate scale or calcium crystals–the LSI is your measurement of success. If your LSI is balanced, neither problem can occur in the first place. Preventative care, anyone?
Beware of the trap: acid products work before your eyes against both problems. That said, if acid is to be used at all, it should only be on carbonate scale, not calcium crystals. In the case of calcium crystals, get the pool's calcium level up (we say 300-400 ppm) to balance the LSI and stop the bleeding, then use something like SC-1000 to chemically dissolve the crystals back into solution. You cannot reverse the damage already done, but you can prevent further damage from occurring.
If you are in a climate requiring winterization of pools, management of the LSI becomes even more critical, because colder temperature yields a lower LSI. Play around with our calculator and watch how the LSI changes in real-time.
For more questions or general advice on how to manage these problems, we are available to speak with you in confidence. Just contact us. Thank you for taking the time to read all this. If you know someone who would find this article valuable, please share it with them.
1 CK-12 Textbook. (2022). 6.10: Alkaline Earth Metals. Introductory Chemistry (CK-12). UC Davis LibreTexts.
2 Calcium ion is technically a cation because its protons outnumber its electrons. We will be referring to it as a calcium ion to reduce confusion. We just wanted to mention this technical detail just in case you're a chemist with free time on your hands who wants to call us and tell us we're wrong. And in all seriousness, if we're wrong about something please do contact us! We want to make sure everything we publish is accurate.
Source: Parikh, M., Webb, S. (2012). Cations: Potassium, Calcium, and Magnesium. Continuing Education in Anaesthesia Critical Care & Pain, Volume 12,(4), pp. 195–198. DOI: 10.1093/bjaceaccp/mks020
3 Vance, Falzone, Pignatelli, et.al. (2015). Direct Carbonation of Ca(OH)2 using Liquid and Supercritical CO2: Implications for Carbon-Neutral Cementation. Industrial & Engineering Chemistry Research. 54 (36) 8908-8918. DOI: 10.1021/acs.iecr.5b02356