Carbonate Scale. The unwelcome buildup of hardened calcium carbonate (CaCO3) can be a big problem for a pool and its plumbing system (and other water systems besides pools). Like most of our articles on the Orenda blog, this writing is meant to simplify chemistry so it can be understood at a very basic level. If you want more detailed information, just click the links to our sources to read more.
To adequately cover this topic, we will discuss the causes of scale, and the treatments to control/remove it. Because it often builds up inside pipes and heat exchangers, salt chlorine generators, and other components in the filtration system, we recommend regular testing for pH/alkalinity, and calcium hardness.
What causes scale?
The short answer is simple: scale is the result of an over-saturation of calcium carbonate in your water. In other words, it is a high-LSI violation, so calcium carbonate precipitated out of solution and deposited on surfaces or equipment–most likely in the hottest places first.
There is a difference between scale and corrosion. We talked about corrosion in salt pools being accelerated by electrolysis. Without saltwater in the pool, however, corrosion is primarily a factor of pH and the LSI being too low. The lower the pH (more acidic), the lower the LSI, and therefore, more corrosion occurs. This is why airborne chloramines cause so much corrosion in and around the pool. They are highly acidic, and when they combine with moisture in the air, they condense on metal and eat away at it.
Scale, on the other hand, thrives in a more alkaline (basic) environment. The higher the pH, the more scale-friendly your water becomes. Alkalinity plays an important role, and should be regularly tested by operators. Since calcium carbonate scale is the most common, let’s focus on that.
Side note: if you want some extreme cases of natural calcium carbonate scale buildup, have a look at Luray Caverns and The Travertine Pools of Pamukkale, Turkey.
The other major factor that should be tested for is hardness, sometimes referred to as calcium hardness. It’s essentially the way to measure dissolved minerals (calcium and magnesium) in water. The more mineral content in your water, the ‘harder’ it is. You can read more about hard water on Wikipedia, and its impact on drinking water from the World Health Organization.
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. These conditions can be measured on the Langelier Saturation Index (LSI). Water tends to be 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.
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. Saltwater pools use electrolysis to turn saltwater into sodium hypochlorite. You can read more on these types of chlorine here.
How does dissolved calcium carbonate harden?
While there are a few factors that must take place, primarily it’s the pH and temperature. High pH drives calcium carbonate out of solution to form scale. Temperature also plays a role, in that calcium hardens with heat. Odd, right? The colder your water, the less calcium wants to harden. The hotter the water, the more scale you will have. This is why scale is most prevalent in and around water heat exchangers.
Remember phosphates? Well, many metal-removing pool products are phosphate based. When phosphates are in a pool with calcium, calcium phosphate scale can occur, which behaves differently from calcium carbonate scale. 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.
How to clean up carbonate scale
You can go about reducing 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). The problem with both of these 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 treat the water itself 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 takes a couple of 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 or the inside of your pipes. Lowering the pH of the pool to get slightly negative on the LSI can accelerate this process too.
But those are reactive measures. How about proactive?
Proactive pool care
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.