stalactite carbonate scale

Carbonate Scale: the hardened truth

Carbonate Scale. The unwelcome buildup of hardened minerals 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.

pipe scale

This is a cross-section of a metal pipe. Yes, that’s scale in there. Think water can get through there easily anymore?

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?

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. The lower the pH (more acidic), the 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.

travertine pool scale

The naturally-formed Travertine Pools in Turkey. They are made of calcium carbonate.

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 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).


How do minerals and metals get into the water?

Just like minerals and metals, rocks are no match for flowing water over time.

Just like minerals and metals, rocks are no match for flowing water over time.

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. 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?

calcium hardness on salt cell, scale formation

Salt cells generate heat, which encourages calcium to fall out of solution and form scale.

While there are a few factors that must take place, primarily it’s the pH. High pH drives calcium carbonate out of solution and into scale form. 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.

What reduces carbonate scale?

saltwater scaleYou can go about reducing scale in at least two different ways. First, you can physically try to remove it: grind it down with sandpaper or other tools, and replace metal pipes that have scale in them. 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. 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. For example, water softeners exist to neutralize mineral content in ‘hard water’, which should reduce scale. According to the US Geological Survey, natural water hardness depends on where you are geographically located. The harder your water, the more you will have to fight against scale. Read more about the map here.

scale hardness level

Map edited by USEPA, 2005. Modified from Briggs and others, 1977. Source: US Geological Survey.

For scale issues, Orenda can help. Our non-phosphate-based SC-1000 not only treats scale, but metals too. Simply follow the dosing instructions or use our dosing calculator app. Pour it in your pool and let it work. Over time, scale will dissolve off surfaces in a much less abrasive way than alternative methods. It 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.

Preventative care makes a huge difference

Orenda SC-1000 is great for re-opening your pool after a new white-coat/plaster, as it can minimize plaster dust, which is often rich in calcium. It also prevents metal staining on the new surface.  For more information, see the National Plasterer’s Council (NPC) startup guideline. Protecting the new plaster coat in the initial few months of operation is especially important for the longevity of your pool surface.

Bottom line: Orenda SC-1000 gently removes minerals and carbonate scale so you can enjoy the pool and focus on the important things…the people in it.

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