3 Types of Calcium Dust in Swimming Pools

There are three distinct forms of calcium carbonate that we commonly find in swimming pools. They are carbonate scale, calcium crystals [calcite], and the topic of today’s article, calcium dust.


Our topic today ties into our core message at Orenda about the Langelier Saturation Index (LSI). Remember, the LSI is a measurement of the saturation of calcium in water...specifically the saturation of calcium carbonate. It is so important, that LSI Balance and Calcium Management is the first of our Four Pillars of Proactive Pool Care.

Low-LSI water seeks more calcium, and therefore dissolves calcium wherever it can be found. For a gunite pool with a cementitious surface (like plaster, pebble, quartz, etc.), the most readily available source of calcium is pretty obvious...it’s the surface. For vinyl liner and fiberglass pools, the sources of calcium may not be there at all, which is why these surfaces can get faded and damaged by aggressive water. The water is starving for calcium.

On the opposite end of the spectrum, high-LSI water has too much calcium carbonate, and deposits it out in the form of carbonate scale. Scale tends to deposit in the hottest places first: like a pool heater, salt generator, and a pretty water feature or tile line that gets a lot of hot sunlight all day.

What causes calcium dust in pools?

Now that we have reviewed the LSI, let’s talk about calcium dust. Calcium dust is precipitated calcium carbonate, just like carbonate scale, but they are not the same thing. Calcium dust comes from a different place, with different chemistry involved. Calcium dust can be formed in a few different ways.

Plaster Dust

The most common form is called plaster dust, where low-LSI tap water fills a newly plastered pool and begins dissolving calcium in the cement. Within the cement, the most soluble form of calcium is called calcium hydroxide, Ca(OH)2. Calcium hydroxide has a pH of about 12.6-12.8, which is really high. When brought into solution, it rapidly changes the pH of the water near the surface, which dramatically raises the LSI in that immediate area.

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Once dissolved, carbonate ions and/or carbon dioxide in the water react with the calcium hydroxide and carbonate it. The reaction creates calcium carbonate, which is not very soluble in water above 0.00 LSI. The calcium carbonate precipitates out in the form of dust. This dust is known as plaster dust, because it has been viewed as an inevitable part of starting up a pool.

In reality, plaster dust is not inevitable, if you follow the right startup procedure. By adding the right amount of calcium or sodium bicarbonate as the pool is filling up, plaster dust can be prevented before it starts. Watch the video above to witness it for yourself.

Plaster dust is hard evidence that valuable calcium is being stolen from the cement in the surface; cement still trying to hydrate and cure. Haven’t you noticed how calcium tends to “drift up” during startup, and the pH spikes high for the first several days you come back to the pool? These things happen because the pool water is stealing high-pH calcium hydroxide from the new plaster finish. Why? Because if you do not balance the water according to the LSI, the water will find LSI balance on its own--at the expense of your brand new surface.

Winter Dust

calcium dust3 zoomLet’s say you winterize a pool, and come back in the spring time to open it. Have you ever seen dust in your pool, or perhaps what you think is “scale” at the bottom of the pool? This calcium may be stuck on your plastic fittings too. This is very similar to plaster dust, except the plaster already cured. This sort of dust can occur with surfaces many years old, and seems to only occur in very cold water (40ºF or colder in the winter).

You will see small white rings around aggregate in with this condition, which indicates the loss of--you guessed it--calcium hydroxide. The rings occur because this calcium hydroxide is most often found in the interfacial transition zones (ITZs) between aggregate. Rings around aggregate mean the ITZs lost material.

This high pH calcium hydroxide spikes the pH of the pool, and eventually the water stops eating the walls because the pH provides temporary LSI equilibrium through the winter. But unlike plaster dust, the precipitation of calcium carbonate does not happen immediately. Rather, the dusting is delayed, and we think this happens because of warming water temperature.

We think the precipitation occurs as the water goes from freezing to say, 60ºF by the time you get there in the spring.  The water already stole the calcium hydroxide and spiked the pH, and high pH in warmer water begins to precipitate calcium carbonate as dust.

For some reason, this form of calcium dust is often mistaken for carbonate scale. This dust formed for a different reason, but technically, as the water warmed up, the LSI changed and precipitated calcium carbonate out of solution. So sure, if you want to call it scale, you can, but we advise against it. Why? Because semantics matter: horrible habits have developed in cold climates, like trying to keep calcium low going into the winter, as to prevent scale. Rule of thumb: if you see it on the bottom of your pool, it wasn't a high-LSI problem like scale. It was a low-LSI problem.

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Calcium dusting over the winter. This dust slowly precipitated as the water warmed up.

Carbonate Clouding

soda ash cloud, cloud pool, soda ash clouds a pool, orenda soda ash, orenda poolA third type of calcium dust is created when adding chemicals improperly. One way is when you add soda ash, either too fast or too much. This drives the pH in its immediate area up, which converts bicarbonate ions to carbonate ions, which then attract calcium and create calcium carbonate. Calcium has a strong attraction to carbonate.

This is why pools sometimes cloud up when throwing soda ash in. Here again, we recommend pre-dissolving chemicals before adding them to a pool, not just throwing dry chemicals in. And this sort of clouding is not only caused by soda ash. Have you ever tried adding calcium chloride to the pool at the same time as sodium bicarbonate?  It clouds up with a similar reaction.

How to prevent calcium dust in swimming pools

The point of this article is to describe three distinct forms of calcium dust: plaster dust, winter dust, and carbonate clouding. We made up the terms ‘winter dust’ and ‘carbonate clouding’ because we have been looking for terms in the industry to describe these two phenomena, and have not yet found any. These forms of dust happen for different reasons, and prevention is easier than remediation.

Sure, acid will dissolve any and all of these forms of calcium carbonate, but it matters how the dust got there in the first place. Once you know that, use the Orenda LSI app to figure out the proper correction strategy before the problem starts. Do your part with the LSI in preparation for changing temperature or other conditions, and SC-1000 can help you do the rest. If you need help, we would love to hear from you.

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