Carbonate Scale is a buildup of hardened calcium carbonate (CaCO3) on pool surfaces or equipment. Scale can be a big problem for a pool and its plumbing system (and other water systems besides pools, like fountains). Like most of our articles on the Orenda blog, this article is meant to simplify chemistry so that the chemistry can be better understood . If you want more detailed information, just click the links to our sources to read more.
To adequately cover this topic, we will discuss what causes scale, and the treatments to prevent, control and remove it. Up front: there are many types of scale, and while we will touch on others, the focus of this article is calcium carbonate scale–the most common form.
What causes scale?
The short answer is simple: scale is the result of an over-saturation of calcium carbonate in your water. The water cannot hold more calcium carbonate in solution after it is completely saturated, so it deposits calcium carbonate.
In other words, scale occurs as a result of a high-LSI violation, when calcium carbonate precipitated out of solution, and deposited on surfaces or equipment–most likely in the hottest places first. Why the hottest places first? Because water temperature is one of the driving factors of the LSI. Higher temperature makes it more likely for calcium to fall out of solution.
Scale and Etching are opposite consequences when it comes to LSI balance. We talk about corrosion and etching when the LSI is negative, below -0.30, meaning the water is under-saturated with calcium carbonate. In salt pools, corrosion is usually accelerated by electrolysis, which is not a direct LSI factor, but nevertheless matters. Without saltwater in the pool, however, corrosion is primarily a factor of the LSI being too low. The pH is just one of the six factors that matter to the LSI.
Water is incapable of over-saturating itself with calcium. So when water gets over-saturated (say we adjust chemistry too high, or the temperature rises too much), water must get rid of some calcium to stay in LSI balance. The calcium the water gets rid of? Yes, you guessed it, that excess calcium becomes scale.
Carbonate scale occurs in nature, such as Luray Caverns or the Travertine Pools in Pamukkale, Turkey. So it's not just limited to swimming pools and fountains.
Factors that lead to scale formation
Again, it gets back to the LSI. There are four factors that, when raised, also raise the LSI. They are pH, calcium hardness, alkalinity and water temperature. In swimming pools, the other two factors of the LSI actually decrease the LSI when they are raised: TDS and CYA.
Contrary to popular belief, total hardness is usually not the driving factor leading to scale, unless the numbers are insanely high. Even with exceptionally hard water, it usually takes a combination of other LSI factors to drive calcium out of solution. Water hardness is not necessarily a bad thing, but according to the World Health Organization (WHO), its impact on drinking water must be taken into consideration. Take those travertine pools in Turkey...they get beat on by the sun all day, which raises the water temperature. That could help precipitate calcium carbonate. Who knows what the pH of that water is, but chances are, it's high.
Related: Calcium Flakes in Saltwater Pools
In general, calcium carbonate scale occurs when hard water meets high alkalinity and warm temperature, which allows for the dissolved minerals to leave their dissolved state and re-solidify. In other words, when the pH and temperature are high enough, hard water’s minerals can harden and attach to surfaces (like metal pipes, tile, gunite, concrete, etc.). Once minerals have begun to attach and re-solidify, the buildup begins. Water tends to become scale forming above +0.30 on the LSI.
How do minerals and metals get into the water?
Water—known as the ‘universal solvent’—has the ability to dissolve just about anything natural on earth. Look no further than smooth, round pebbles on the side of a river bank. Those rocks were once jagged and rough, but flowing water is incredibly powerful over time. When water flows by minerals and metals (naturally found in the earth’s soil), it dissolves them along the way. This is the natural way that minerals like calcium get in the water. And since water treatment facilities do not necessarily remove minerals from the water (for various reasons), your pool’s fill water likely has those minerals already in it. That's one of many reasons it is so important to test a pool's fill water. Pool chemistry starts at the tap.
For pools specifically, there is another way to get minerals in the water. Your pool products themselves often have minerals in them. You can also buy calcium chloride (CaCl) to raise calcium hardness in your water directly...which we strongly recommend doing if you have low calcium hardness. One of the most popular types of chlorine is calcium hypochlorite, which gradually raises calcium hardness over time. Saltwater pools use electrolysis to turn saltwater into chlorine and sodium hydroxide. Liquid chlorine, sodium hypochlorite, leaves behind a byproduct of salt, which raises TDS. Everything has a consequence.
How to remove, and prevent scale
You can go about removing scale in at least two different ways. First, you can physically try to remove it: grinding or bead-blasting can work on tile lines. You could also drop the pH sharply (so the water is more acidic), hoping to loosen the scale so it’s easier to break apart (though this has other consequences). More aggressive yet is to just use acid directly on the scale, which we do not recommend.
The problem with these strategies is that they don’t address the root of the problem, and the scale will come back again. The other problem is the collateral damage done to your pool walls and equipment. For example, you could damage your pool walls or equipment trying to break off chunks of scale.
Option two—the better option—is to introduce a chelant or sequestering agent to the water, and balance the LSI.
Carbonate Scale is fairly easy to fix, as it does not do permanent damage...it's just excess calcium the water could no longer hold. You can try chelating the calcium with SC-1000, which may take a few weeks, and requires the affected area to be submerged with treated water. Over time, scale will dissolve off surfaces in a much less abrasive way than alternative methods. SC-1000 also flows through your system with the water, so it can remove scale in those hard to reach places—like your heat exchanger, salt chlorine generator or the inside of your pipes. Lowering the pH of the pool to get slightly negative on the LSI can accelerate this process too.
So cleaning and treatment are both reactive measures. How about proactive?
As you may know, Orenda's message is about proactive pool care. We have four pillars of proactive pool care, and the first pillar is LSI Balance and Calcium Management. The article talks about how water craves equilibrium, based on how saturated it is with calcium carbonate. With truly balanced water–as measured on the LSI–scale should not occur. If it does, there's something else going on, perhaps locally in a salt chlorine generator. In any case, it helps to understand water's relationship with calcium, because calcium is the most misunderstood chemistry in the pool business.
Be proactive and use the free Orenda App to control your pool chemistry. Doing so can prevent scale from ever starting, even with higher levels of calcium hardness.
BONUS: Other types of scale
Calcium Phosphate Scale
Remember phosphates? When high levels of phosphates are in a pool with high levels of calcium hardness, calcium phosphate scale can occur, which behaves differently from calcium carbonate scale. It is also far more severe and more difficult to clean up. Scott Webb of AQUA Magazine explains:
“Calcium phosphate and calcium carbonate produce similar symptoms — cloudy pool water, damaged heat exchangers and a dull white film on surfaces, but calcium phosphate is not driven out of solution by high pH. It’s barely soluble in pool water at normal temperatures (given sufficient levels of calcium and phosphate), but heat really drives the reaction, causing it to precipitate at the heater.
If you have calcium phosphate scale, you will likely need to physically remove it, or even replace equipment. It is orders of magnitude harder than carbonate scale. The photo shows two boulders on buckets. Those are large chunks of a sand filter that was calcified with calcium phosphate scale, and was broken up by an actual jackhammer. Think about that.
Another type of scale is silica scale. This is sometimes formed on cementitious surfaces, and is apparently immune to muriatic acid. At this time, we do not know much about silica scale, except that it exists and there is no chemical cure that we know of that removes it. Silica scale often looks rounded, almost like kernels of rice, and can vary in color. As we discover more about this phenomenon, we will publish it and update this article.
Calcium Sulfate Crystals
Not to be confused with calcite crystals, which grow from the surface due to a very low LSI, sulfate crystals are actually a scale deposit that is sharp and crystalline. These crystals are almost transparent at first, so they are very hard to see. They are also very sharp and can cut skin with ease. While calcium sulfate may not be directly correlated to the LSI, there is little doubt that they deposit because of an over-saturation of sulfate and calcium. We assume there is some sort of catalyst causing the formation to harden into crystals, but at this time we don't know what that is.
Calcium sulfate was investigated and was determined to be immune to muriatic acid, unless–wait for it–the acid is boiling. Yes, boiling acid can affect calcium sulfate. For everyone else unwilling to boil acid (like us), calcium sulfate needs to be physically sanded/grinded off the surface.