Understanding Calcium Phosphate Scale

In swimming pools and spas, calcium phosphate scale is rare. But in the right conditions, it can form scale deposits that are harder to manage (and remove) than calcium carbonate. This article explains calcium phosphate, how it forms and precipitates, and how you can prevent it.

Covered in this article:

• What is calcium phosphate?
  • Is calcium phosphate soluble in water?
    • Example 1 (82ºF, 350 CH, 8.0 pH)
    • Example 2 (88ºF, 700 CH, 7.5 pH)
  • Calcium phosphate scale is localized
  • Are the flakes in salt pools made of calcium phosphate?
• Types of calcium phosphate
  
  • How hard is calcium phosphate
• Calcium phosphate lab report
• Ways to prevent calcium phosphate scale in swimming pools and spas
  • Cal hypo tabs and buffered acid
• Conclusion

What is calcium phosphate?

Calcium phosphate compounds are various mineral ion pairs that contain calcium cation (Ca²⁺) and some form of phosphate anion (PO₄^3-). This ion pair can be bound with other compounds like chlorine, fluorine, hydroxides, carbonates and can also be in various states of hydration (anhydrous, monohydrate, dihydrate, etc.).

Because of these variations, there are many types of calcium phosphate. We do not yet know how many types of calcium phosphate can form in pools, but thanks to lab tests conducted by Que Hales at onBalance, we know of at least one.¹ 

Calcium phosphate is an integral part of the bones and teeth of most animals (including humans), and is also a material found in certain kidney stones. According to the University of Waterloo, many forms of calcium phosphate are called apatite:

Apatite was named in 1788 for the Greek word apatite, which means “to deceive”, since apatite has a similar appearance to many other minerals. Apatite is not actually a specific mineral, but rather, is a name for a group of similar isomorphous hexagonal phosphate minerals. It can be very difficult to distinguish between the individual members since they partially replace each other, so a distinction is rarely made and they are instead all called apatite. 

Interesting fact: The bones and teeth of most animals, including humans, are composed of calcium phosphate, which is the same material as apatite.²

As far as we know, the forms of apatite found in tooth enamel and bones is not the exact same chemical compound that scale swimming pool heaters and filters. We will discuss the many different types of calcium phosphate later in this article. The only lab sample we have seen indicates the calcium phosphate found in a pool heater was a carbonate-rich apatite called Hydroxylapatite (or hydroxyapatite), also known as Collophane. 

Is calcium phosphate soluble in water?

Calcium phosphate is not very soluble in water.^3,4,5 When formed, calcium phosphate can quickly precipitate out of solution and deposit scale on surfaces, filter media, and inside heaters. The chemistry factors that lead to its formation (and precipitation) are high water temperature, high pH, high phosphates, and high calcium hardness.

While calcium carbonate saturation can be quantified using the LSI to predict carbonate scale formation, calcium phosphate scale is not related to the LSI. Calcium phosphate does not involve carbonate, for instance, so carbonate alkalinity is irrelevant.

There is a known saturation formula for calcium phosphate to predict how much phosphate (PO₄) is needed (relative to calcium, water temperature, and pH) to precipitate calcium phosphate. This formula has been attributed to a study on calcium phosphate in heat exchangers from 1979.⁶  According to Richard Falk, the formulas is most relevant for Hydroxylapatite, even though the linked source says Tricalcium phosphate. Apparently it can be adapted for other forms of phosphate with similar principles. The formula is:

PO₄ = 10^{[11.755 - log(CH) - 2log(t) - (0.65 * pH)]}

Where:

• PO₄ is the concentration of phosphate (ppm or mg/L),
• CH is the concentration of calcium ions, also called calcium hardness (ppm),
• t is the temperature (ºC), and
• pH is the pH_(actual) of the water.

When the phosphate level exceeds this calculated value, the water becomes oversaturated with calcium phosphate, leading to precipitation and scale formation.  If the phosphate level is lower than the calculated value, calcium phosphate should not precipitate.

Example 1

Thankfully, most swimming pools and spas stay well below this calculated value.  Let's show two examples with realistic pool chemistry so you can see just how high phosphate levels need to be.

82ºF
350 ppm CH
8.0 pH

First, we need to convert the temperature from Fahrenheit to Celsius. Most smartphones will convert this easily. 82ºF = 27.78ºC.  But here's the math so you can see it:

t = (T - 32) ÷ 1.8
t = (82 - 32) ÷ 1.8
t = 50 ÷ 1.8 = 27.78ºC

Now that we have the temperature in ºC, just plug in the values.

PO₄ = 10^{[11.755 - log(CH) - 2log(t) - (0.65 * pH)]}
PO₄ = 10^{[11.755 - log(350) - 2log(27.78) - (0.65 * 8.0)]}

^{log(350) = 2.544
log(27.78) = 1.444.  1.444 x 2 = 2.888
0.65 x 8.0 = 5.2}

^so

PO₄ = 10^{[11.755 - 2.544 - 2.888 - 5.2]}
PO₄ = 10^[1.123] = 13.3 ppm

Since most of us measure phosphates in parts-per-billion, instead of parts-per-million, we multiply by 1,000.

PO₄ = 13.3 x 1000 = 13,300 ppb

With this relatively-normal summer pool chemistry, phosphates need to be extremely high (over 13,300 ppb) for calcium phosphate to form and precipitate.

Example 2

Now let's look at pool chemistry that resembles the pools we have found calcium phosphate scale in. These tend to be heated commercial pools (and spas) with acid feeders and cal hypo chlorinators. It is not uncommon for a commercial pool using cal hypo chlorine to have over 700 ppm calcium hardness:

88ºF (31.11ºC)
700 ppm CH
7.5 pH

PO₄ = 10^{[11.755 - log(CH) - 2log(t) - (0.65 * pH)]}
PO₄ = 10^{[11.755 - log(700) - 2log(31.11) - (0.65 * 7.5)]}

^{log(700) = 2.845
log(31.11) = 1.492.  1.492 x 2 = 2.984
0.65 x 7.5 = 4.875}

^so

PO₄ = 10^{[11.755 - 2.845 - 2.984 - 4.875]}
PO₄ = 10^[1.051] = 11.24 ppm = 11,240 ppb

The higher calcium (700 ppm) and temperature (88ºF) lower the amount of phosphate required by a little bit compared to the previous example. But the lower pH (7.5) largely negated that. If the pH was 8.0 like the previous example, the result would be only 5,321 ppb phosphates. That's a level that is quite common in swimming pools that do not address rising phosphates. 

Related: Phosphate Removal (Pillar 3)

Calcium phosphate scale is localized

Much like calcium carbonate scale, calcium phosphate does not scale anywhere and everywhere in a pool or spa. It precipitates in the highest-saturation parts of the system first. For calcium carbonate, which we measure with the LSI, we know that areas of high temperature tend to scale first. This is why heaters and salt cells scale before the sunny tile line in the spa and spillway, which will scale before the lower depths of the pool.

Calcium phosphate is similar in this regard. It is largely driven by water temperature, as you can see in the formulas above. This explains why heaters are the first place people tend to see calcium phosphate (if at all). In a heat exchanger, temperatures can exceed 140ºF as water passes through it. Within that heater, both the calcium carbonate and calcium phosphate saturation will be higher. It makes sense why heaters scale so often.

Looking beyond heat, the concentration of calcium also matters.  Calcium hypochlorite (cal hypo) chlorine introduces a high concentration of calcium to the water, which then dilutes into the pool. As mentioned before, all instances of calcium phosphate scale we have seen have been in heated pools using cal hypo chlorinators.

Are the flakes in salt pools made of calcium phosphate?

Based on everything we have learned in the field, researched, and the samples we have had lab-tested, no, calcium flakes in saltwater pools are not calcium phosphate. At least, we have yet to see it. As far as we know, calcium flakes are calcium carbonate, and easy to prevent. Learn more here.

There is a widespread myth in the pool industry that says calcium flakes are calcium phosphate. Theoretically, if phosphate levels are exceedingly high, it could happen. But if it did, it is highly unlikely that calcium phosphate would flake out of the cell and blow into the pool..If calcium phosphate really did deposit in a salt cell, good luck cleaning it...muriatic acid does not dissolve it much at all.  Sulfuric acid can soften it, but that creates the bigger problem of calcium sulfate scale. More on that later in this article.

Types of calcium phosphate

We have been researching this topic for years due to the numerous types of calcium phosphate, making it challenging to identify which ones affect swimming pools and spas. Recently, Que Hales at onBalance provided lab results from pool heater samples, shedding light on the issue.⁷ With Que's permission, we will share these findings in this article.

First, here are various forms of calcium phosphate:⁸

• Apatite (Ca₅(PO₄)₃)
  • Hydroxylapatite (Ca₅(PO₄)₃(OH))
  • Chlorapatite (Ca₁₀(PO₄)₆(Cl))
• Tetracalcium phosphate (Ca₄(PO₄)₂O
• Monocalcium phosphate, anhydrous (Ca(H₂PO₄)₂)
• Monocalcium phosphate, monohydrate (Ca(H₂PO₄)₂(H₂O)
• Dicalcium phosphate (CaHPO₄)
• Dicalcium phosphate, dihydrate (CaHPO₄(H₂O)₂)
• Dicalcium phosphate, monohydrate (CaHPO₄(H₂O))
• Dicalcium diphosphate (Ca₂P₂O₇)
• Calcium triphosphate (Ca₅(P₃O₁₀)₂
• Tricalcium phosphate (Ca₃(PO₄)₂)
• Octacalcium phosphate (Ca₈H₂(PO₄)₆∙5H₂O)

Hydroxylapatite is in bold because it seems to be the one that forms in swimming pools and spas, and is what the formula in the previous section calculates. It is also prevalent in animal bones and teeth, and is used for synthetic bone replacements.⁹

How hard is calcium phosphate?

Compared to calcium carbonate, Hydroxylapatite is much harder to manage and remove. For one thing, calcium phosphate is a 5 on the Moh's Hardness scale, whereas calcium carbonate is only a 3.^10,11  On the absolute hardness scale, Hydroxylapatite is 9-12 kg/mm², compared to calcium carbonate, which is only 6-8 kg/mm² absolute hardness.¹²

If calcium phosphate forms scale in your pool or pool equipment, it does not clean up easily. It's often easier to replace heaters and filters than to clean this stuff out.

We have seen commercial pool filters that needed to be cut open and had the sand broken apart with a jackhammer to remove. See the photo on the left (below). These sand boulders were solidified with calcium phosphate, and this photo was taken after them being jackhammered out of a commercial pool sand filter.

The photo on the right is from a commercial pool's regenerative DE filter that had its elements coated in calcium phosphate (mixed with perlite, the filter media used in this case). The pool also had a copper issue, which is why the scale formation appears to have a tint of blue or green to it.

Muriatic acid can easily dissolve calcium carbonate (CaCO₃), but it does very little to calcium phosphate. It is slower to dissolve, but according to Richard Falk, dissolving can be accelerated when used in conjunction with an acidic phosphate remover (like PR-10,000) and good water flow. Even so, trying to chemically dissolve it is often impractical due to calcium phosphate's low solubility.

Calcium phosphate can be slowly dissolved by sulfuric acid, but that would create calcium sulfate and phosphoric acid...and nobody wants calcium sulfate in their pool either.

Ca₃(PO₄)₂ + H₂SO_4(aq) → CaSO₄ + H₃PO_4(aq)
Calcium phosphate + sulfuric acid_(aqueous) → calcium sulfate + phosphoric acid_(aqueous)

Calcium sulfate scale crystals in a pool cartridge filter

Obviously we don't want to solve one problem by creating another one. Calcium sulfate scale is just as problematic as calcium phosphate scale, if not worse. Neither are easy to remove without physical labor, draining the most (or all) of the pool, and oftentimes replacing equipment.

Calcium phosphate lab report

Before showing these images, they are used with permission from Que Hales and onBalance. They sent in the samples to the lab and paid for it. You can learn more about this directly from them.

The samples were removed from a pool heater and sent to a laboratory that analyzed them using Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS). The samples sent in were curved, matching the shape of the inside of the heat exchanger. The outside (convex) side were darker in color, and the inside (concave) side was off-white colored.

"EDS analysis of the dark-colored side of the flakes, the side in contact with the tubing, show the widespread presence of copper oxides. These deposits are responsible for the reddish black color of the convex side of the flake. Greenish blue patches mainly consist of copper, chlorine and oxygen.  Minor elements detected in these regions included nickel, calcium, phosphorus and silicon. Colorless, transparent crystals mainly consist of calcium and sulfur. The crystalline shape resembles gypsum." - WJE Lab report

Image credit: Que Hales, onBalance

We suspect the heat exchanger was copper or cupronickel. We are surprised to see calcium sulfate (gypsum) crystals within these flakes. This indicates an accumulation of sulfates, likely from pool products like copper sulfate (algaecide), potassium monopersulfate (non-chlorine shock), sodium bisulfate (dry acid), sulfuric acid, or many others. We do not know exactly what products were used, but we do know the sulfates were enough to form these calcium sulfate crystals in the heater.

The report continues:

"EDS analysis of the light-colored concave side of the flakes shows widespread presence of calcium, phosphorus and oxygen. The colloform morphology of the deposits suggests the deposits may be collophane, which is a water-rich variety of calcium phosphate." - WJE Lab report

Image credit: Que Hales, onBalance

The lab concluded the samples collected by Que Hales were primarily calcium phosphate, mixed with copper oxides and calcium sulfate. It's fascinating. 

Ways to prevent calcium phosphate scale in swimming pools and spas

Chemical remediation is not a reliable solution for removing calcium phosphate. Some types of acid can be more effective than sulfuric and muriatic acid, but these acids are not readily available on the market. And even if they were, the process of chemically removing calcium phosphate is messy and tedious.^13,14

So if you have calcium phosphate scale, it needs to be either physically removed or the equipment replaced entirely. In this way, it's similar in severity to calcium sulfate scale crystals.

Thankfully, prevention is not difficult. Proactive strategies can be implemented:

• Keep phosphate levels low. Our third pillar of proactive pool care talks about all the reasons why lower levels of phosphates are beneficial. We recommend keeping phosphates below 500 ppb (for many reasons), which is well below the threshold needed for calcium phosphate scale. This can be difficult, given the fact most municipally-treated drinking water contains phosphates.¹⁵
• Chelate calcium. SC-1000 is excellent and binding up metals and minerals, which inhibits any type of scale formation. A chelated calcium ion will not be able to form an ion pair with phosphate, carbonate, sulfate, or anything else. Anti-scalants are used effectively to prevent calcium phosphate scale in R.O. filtration systems,¹⁶ so it stands to reason they will do the same in swimming pools. You could also use sequestering agents, but be aware that most sequestering agents are phosphate-based.
• Test calcium hardness and be aware of it. We recommend calcium hardness levels based on water temperature extremes. Pools in colder climates need more calcium than those in warmer climates. Winterizing a pool that is likely to freeze should have over 500 ppm calcium, especially when expecting rain and snow dilution. Calcium becomes an issue when there are also other factors that can lead to calcium phosphate (i.e. high water temperature, high phosphates, high pH).
• Dilute water periodically, especially if the pool's primary sanitizer is calcium hypochlorite. Cal hypo tablets and briquettes, in particular, can contain phosphate-based scale inhibitors.

Cal hypo tabs and buffered acids

It should also be noted that some cal hypo tablets contain a scale inhibitor. While these products are a proprietary formulation, many of our commercial customers using cal hypo have recognized rapidly-rising phosphates in their pools. There is a high likelihood that these scale inhibitors are phosphate-based.

Another common denominator is buffered acids. To their credit, it seems like formulas have changed in the past few years to avoid using phosphate compounds as the buffer. Some of these products are still on the market today, but we know of some that no longer contain phosphates.

Years ago, we visited a commercial therapy pool on cal hypo and a buffered acid that had zero phosphates in the tap water. The pool, however, had over 16,000 ppb phosphates. It was no wonder there was calcium phosphate in the heater and filter. Just be aware of the chemicals used in the pool and their byproducts.

Conclusion

Calcium ions (Ca²⁺) seek anions like Carbonate (CO₃^2-) to bind to. If, however, conditions are right and concentrations of sulfate (SO₄^2-), and phosphate (PO₄^3-) ions are high enough, calcium can bind to them too. These calcium ion pairs are much more difficult to manage in pools. In the samples shown in the lab results above (courtesy of onBalance), they were primarily calcium phosphate with traces of calcium sulfate as well.

From all the field experience and research we have done, calcium flakes in saltwater pools are NOT calcium phosphate; they are calcium carbonate. If your pool has calcium phosphate scale, we do not know of a reliable chemical remedy, and unfortunately the deposits need to be physically removed, or the equipment replaced.

The best strategy is one of prevention. Remove phosphates as needed and stay below 500 ppb, per our Third Pillar of Proactive Pool Care, and dilute water as necessary to keep calcium to a manageable level in your pool to allow for LSI balance year-round.  LSI does not apply to calcium phosphate scale, but it does apply to overall water balance, which is always important to maintain in swimming pools and spas.

If you think you have calcium phosphate, please contact us! We'd love to collect samples and have them analyzed in a lab for more information.¹⁷ We suspect there are more variations in pools than just Hydroxylapatite, but without lab samples, we will not know.

¹  Many thanks to Que Hales for sharing the laboratory results with us. The screenshots in this blog are cited appropriately to Que and onBalance. If you think you have calcium phosphate, contact us.  If it appears to be, we would love to get a sample sent to the lab for analysis. We suspect there are many variations, much like calcite crystals. 

²  Feick, Kathy. (retrieved 2024). Apatite. Earth Sciences Museum. University of Waterloo.

³  Calcium Phosphate (Ca₃(PO₄)₂). BYJUS.com

⁴  Prakash, K. H., Kumar, R., Ooi, C. P., Cheang, P., & Khor, K. A. (2006). Apparent solubility of hydroxyapatite in aqueous medium and its influence on the morphology of nanocrystallites with precipitation temperature. Langmuir: the ACS journal of surfaces and colloids, 22(26), 11002–11008.

⁵  Moreno, E. C., Gregory, T. M., & Brown, W. E. (1968). Preparation and Solubility of Hydroxyapatite. Journal of research of the National Bureau of Standards. Section A, Physics and chemistry, 72A(6), 773–782. https://doi.org/10.6028/jres.072A.052

⁶  Kubo S, Takahashi T, Morinaga H, Ueki H. (1979). Inhibition of calcium phosphate scale on heat exchanger: the relation between laboratory test results and tests on heat transfer surfaces. Corrosion '79.

⁷  These lab studies were conducted to figure out some mystery calcium deposits in a heater. This article from Aqua magazine tells the story:

Webb, Scott. (2012). The Case of the Mysterious Pool Deposits. Aqua Magazine.

⁸  Wikipedia: Calcium Phosphate, and
National Center for Biotechnology Information (2024). PubChem Compound Summary for CID 24456, Calcium Phosphate.

⁹  Xie, Y., Liu, J., Cai, S., Bao, X., Li, Q., & Xu, G. (2020). Setting Characteristics and High Compressive Strength of an Anti-washout, Injectable Calcium Phosphate Cement Combined with Thermosensitive Hydrogel. Materials (Basel, Switzerland), 13(24), 5779. https://doi.org/10.3390/ma13245779

¹⁰  National Center for Biotechnology Information (2024). PubChem Compound Summary for CID 10112, Calcium Carbonate. 

¹¹  National Center for Biotechnology Information (2024). PubChem Compound Summary for CID 57369541, Calcium hydroxide phosphate (Ca5(OH)(PO4)3).

¹²  The absolute hardness scale is different from the Moh's Hardness scale. Here's a decent explanation of the two. Moh's is a comparison between minerals on a 1-10 scale, as determined by which mineral scratches the other. It is a relative (qualitative) hardness scale (minerals relative to one another), not a precise, quantitative measure of "hardness". The absolute hardness scale, however, measures hardness in terms of kilograms per millimeter squared (kg/mm²), gigapascals (GPa), or megapascals (MPa).

¹³  Dorozhkin S. V. (2012). Dissolution mechanism of calcium apatites in acids: A review of literature. World Journal of Methodology, 2(1), 1–17. https://doi.org/10.5662/wjm.v2.i1.1

¹⁴  Margolis, H. C., & Moreno, E. C. (1992). Kinetics of hydroxyapatite dissolution in acetic, lactic, and phosphoric acid solutions. Calcified Tissue International, 50(2), 137–143. https://doi.org/10.1007/BF00298791

¹⁵  US Environmental Protection Agency (2024). Why do water systems add phosphates to drinking water? What are the health effects of drinking water containing phosphates? EPA Question (23002-32407).

¹⁶  Mangal, M.N, Salinas-Rodriguez, S.G., Dusseldorp, J., Kemperman, A.J.B., Schippers, J.C., Kennedy, M.D., Van der Meer, W.G.J.. (2011). Effectiveness of antiscalants in preventing calcium phosphate scaling in reverse osmosis applications. Journal of Membrane Science. Vol. 623.

¹⁷  We will ask that you send photos and speak with us about symptoms before asking to collect samples so we can send them to the lab. The vast majority of scale in swimming pools is calcium carbonate, and we can rule that out over the phone with you.  If your scale deposits pass those tests over the phone, then yes, we want to send them to the lab to learn more.