Understanding Calcium Nodules in Pools

October 1, 2020

Calcium nodules in pools look like little calcium volcanos or blisters that ooze out of pool walls and floors. In this article we cover two opposing viewpoints about what causes calcium nodules, and lay out the facts that everyone agrees on. Then we'll discuss how to prevent and fix calcium nodules.

Covered in this article:

What are calcium nodules?

nodule on tile grout (onBalance video)

Photo credit: onBalance

According to the National Plasterers Council (NPC) technical manual, calcium nodules are:

"[Calcium nodules are] a slang term used in the swimming pool plastering trade referring to a calcium carbonate formation on the surface of a finish coat, resulting from the percolation of water that is capable of leaching cement compounds from voids, bond failures, cracks, or a weak sub-boundary layer in the cementitious coating, which is then precipitated onto the surface."1

That being said, nodules have also formed from the grout in all-tile pools. But the overwhelming majority of these issues are found in plaster-type finishes.

How are calcium nodules formed in swimming pools?

Essentially, a nodule is formed when calcium or cement compounds–usually calcium hydroxide (Ca[OH]2)–come out from within a surface due to some sort of weakness, like a small crack, hole or void. Once out in the pool itself, these calcium compounds carbonate on the visible surface inside the pool when they interact with:

  • carbon dioxide (or carbonic acid, which is aqueous carbon dioxide, H2CO3)
  • bicarbonate alkalinity (HCO3), or 
  • carbonate alkalinity (CO3).

Carbonates can turn calcium compounds into calcium carbonate (CaCO3). These deposits will continue to grow out from the weakness in the surface until their source is depleted. Some nodules even get large enough to 'ooze' with gravity. And just like that, you have a calcium nodule.

delaminating non-bonding vs non-delamination

Credit: Screenshot from NPC video (embedded below)

Experts all seem to agree that calcium nodules come from within a cementitious surface (with very few exceptions). Those same experts disagree on how and why such nodules occur in the first place, however. One thing is certain: calcium nodules are not a type of scale, because scale is the result of an over-saturation in the water, and nodules are not. Carbonate scale, by far the most prevalent type of scale, is the result of a high LSI, meaning the water is over-saturated with calcium carbonate, so it precipitates out of solution and forms scale. Nodules are a different phenomenon entirely, and are not caused by a high LSI. 

In other words, nodules are not directly caused by water chemistry. Perhaps long-term chemistry can weaken a surface and contribute to the problem over time, but it is not a direct relationship.

Related: Understanding the Langelier Saturation Index (LSI)

What are the underlying causes of calcium nodules?

This is where expert opinions begin to conflict. We at Orenda have done quite a bit of research about nodules and other plaster issues, but we are not original sources of the information. We'll offer both opinions and our take on them, and let you decide for yourself.

The two expert groups that conflict about plaster issues–NPC and onBalance–have both conducted actual experiments and scientific studies. Orenda has not done such studies (yet), and therefore the information we are drawing on is a combination of these two sources and our own in-the-field experience.

We bring this up to remind everyone that we are not scientists, and this article is meant to inform you on both sides of the issue. So let's start with just the facts about calcium nodules that everyone seems to agree on.

The facts everyone agrees on

  • Delamination is a failure of plaster to bond to the concrete substrate, and is not water chemistry related
  • Nodules can either be a consequence of delamination, or can come from within a surface without delamination
  • Nodules form on the surface where voids, cracks or other weaknesses are located
  • Voids, cracks and weaknesses allow for calcium compounds to migrate to the surface2
  • Calcium compounds carbonate and form the nodules

The items of disagreement are primarily...

  • What causes the voids, cracks or weaknesses in the first place?
  • What draws (or forces) calcium compounds into the pool from within the surface?
  • How to remedy and prevent nodules

We at Orenda are not picking sides on this debate, as both sides have rational opinions with evidence to support them. Perhaps asking "whose fault is this?" is the wrong question. Instead, we should be asking 'what steps need to be done to prevent nodules, and who most capable of doing those things?'

The National Plasterers Council (NPC) opinion

The NPC has many videos about nodules, which you can find on their YouTube channel. Instead of showing all of them, here is one video that sums up their position on the matter:

The NPC position is that voids, cracks and weaknesses are inherent to all cementitious materials, and since most plastered pools do not have nodules, that shows that these inherent cracks and voids are not the culprit. Instead, nodules form from voids/cracks/weaknesses that are caused (or at least magnified) by bad water chemistry, or a bond failure between the plaster and the concrete substrate. Even with delamination, there must be a channel for aggressive water to travel into the plaster and cause this issue. And according to the NPC, aggressive water must pass through that channel for nodules to form.

In short, their position is that aggressive, low-LSI water (driven by low pH, carbonate alkalinity or calcium hardness) not only opens up the voids/cracks and penetrates into the cement, but also draws out calcium hydroxide which then carbonates into a nodule.

Here's our take on the NPC opinion:

It is a proven fact that aggressive, low-LSI water dissolves calcium carbonate and the most soluble type of calcium available in the surface, calcium hydroxide. No argument there. It is also known that calcium hydroxide has a high pH of 12.6, and carbonates virtually on contact with carbonate alkalinity in water, so the science they speak of is sound. The question then becomes whether or not aggressive water actually causes or magnifies the voids/cracks and draws out the hydroxide. We do not know the answer to that, because the opposing opinion from onBalance contradicts it.

The onBalance opinion

Here are onBalance's two-part video explainer on calcium nodules.

The onBalance position is that nodules are not caused by pool chemistry; they are caused by plaster failures. Either it's a bond failure (which NPC agrees can be one of the issues), or the voids/cracks are a result of improper troweling workmanship, water/calcium/cement mix ratio, or something similar. Here's a quote from their website:

"Although some have attempted to associate nodules with aggressive pool water, research has shown that the cause of nodule formation is cracking (from over–drying of the plaster) or bond failure (aka delamination), which occurs primarily in re-plastered pools. Nodules, of course, have nothing to do with pool water chemical balance." 3

calcium nodule

They posit that nodules do not require aggressive water to form. In the case of delamination, there can be enough pressure to push these compounds out into the pool anyway. If there is a void behind the plaster due to a bond failure (delamination), but that void never is exposed to water, if and when water finally gets through a small crack or other weakness, it will begin to chemically interact with the underside of the plaster finish, just like the entire outside of the finish did on startup. This, onBalance argues, leads to nodules too:

"Regardless of whether the void is filled with water immediately or down the road, the process of hydration will occur inside the void just as it did out on the face of the pool when it was new. Only this time the calcium hydroxide is temporarily trapped, allowing it to concentrate – creating a strong, localized body of the material from which a nodule can be made. But it then travels out to the main body of water in the pool where it can react with alkalinity to transform into calcium carbonate. Where does this conversion to non–soluble calcium occur? In our plaster dust example, the conversion occurred uniformly over the entire surface of the plaster, resulting in a uniform fall–out of plaster dust throughout the pool. In the case of super–saturated calcium water oozing out of a pinhole, the conversion occurs just as it exits the void and enters the pool, i.e.: on the plaster surface of the pool."

Here's our take on the onBalance opinion:

We certainly don't know as much as either onBalance or NPC when it comes to cement chemistry, but we do know a few things for certain. One of them is that aggressive water seeks calcium via the path of least resistance. In other words, water is not likely to 'dig a pinhole' sized cavity into cement, when it can etch an infinitely larger surface area lightly. That being said, if such a crack or pinhole cavity already exists, then yes, it makes sense to us that aggressive water can get more soluble calcium hydroxide from within that crack.

While we at Orenda have not done experiments ourselves, our experience in the field suggests that aggressive water damage looks very different than pinholes and tiny cracks in plaster. The exception to this is on startup, because those inevitable micro-cracks are filled with calcium hydroxide not yet carbonated (into CaCO3). Acid treating a new plaster pool steals calcium hydroxide, and if done enough, an acid startup can leave these voids more open, making the cement more porous and vulnerable. This is why we advocate so strongly for an LSI-based startup approach to allow the surface to cure without the loss of this valuable calcium hydroxide.

Related Procedure: The Orenda Startup™ Method

We doubt all of these issues are workmanship caused, but everyone seems to agree that the delamination issue has nothing to do with pool chemistry or startup. Whether hydroxide is forced out or drawn out is unknown to us at this time. In other words, we don't have an opinion either way...we just know that correcting pool chemistry does not get rid of these nodules once they have formed.

Common Denominators

concrete pool weepers, calcium nodules in poolWe have been called to countless pools with plaster issues, and to us, nodule pools usually have at least one of these three factors. First, most of the nodules we see occur along cracks. This makes sense, again, because water always moves through the path of least resistance. In the photo here, you can see water coming from within the surface, all around the nodule. In this case, the wall had delamination issues, and also a major concrete shell problem called weepers.

Weepers are basically leaks where water is pushed from behind the pool shell into the pool itself, which of course then pushes through the plaster. Weepers, in our experience, are almost always accompanied by nodules.

Second, the vast majority of nodule pools are either not waterproofed, are in a high water table area (like coastal regions), or both. We have good customers of ours that have been plagued by nodules, only to discover that their high water table led to pressure from behind the concrete shell. In the past couple of years they have moved to waterproof every shell, and the nodule problem has disappeared for them. We are so happy to hear it, and it gives us optimism.

The third common factor is replastered pools. Yes, original plaster jobs can have nodules too, but if a pool with calcium nodules is not in a high water table area, and/or was waterproofed, there's a good chance it was replastered. Replastering should be done with a proper bond coat so the new layer can adhere properly. You might be surprised at how often this step is skipped. Here's a photo of a spa that was being chipped out and plastered from scratch, and it had two layers of plaster with a proper bond coat:bond coat, plaster bond coat, replaster, how to replaster a pool, calcium nodules

Without a good bond coat, delamination becomes more of a risk, and therefore nodules become more of a risk too.

How to fix calcium nodules

We have good and bad news. The good news is most nodules will eventually run their course, and run out of 'lava' oozing out of the volcano. This can take a few years, but you could just keep sanding them down by hand or grinding them off in the mean time. SC-1000 can also help soften and dissolve the nodules, but it will not solve the root cause, so the nodules will continue to come back until the source 'dries up' so to speak.

The other good news is many of these nodules can be spot treated by core drilling to remove the spot, and patching it in. No, it won't look perfect, but it can save you from having to replaster the pool entirely. These have shown to be effective most of the time, but not 100% of the time, as we have heard of nodules coming back years later...but this time around the perimeter of where the core was drilled out. Yikes.

The bad news is nodules are a structural issue that cannot simply be cured by good water chemistry. Simply balancing the LSI prevents etching and scale, but nodules are still possible because they grow out of the plaster surface. Many of our customers hope that good water chemistry can prevent calcium nodules from coming back, but we have no evidence to support that. In fact, we have abundant evidence that water chemistry alone does not stop nodules from coming back. To us, they appear to be unrelated issues. And that's really saying something, because we at Orenda preach the LSI and best-known water chemistry practices. We really wish water chemistry could solve the problem. It would be great for Orenda's business if it did. It's just not the case, and we're here to tell you the truth, even if it stings. Sorry.

tiny nodules on pool floor

This pool had thousands of tiny nodules on the walls and floor. It was not waterproofed, and was located in a coastal area with a high water table.

The last resort to ultimately solve a nodule problem–particularly if there are a lot of them in your pool–is to chip out and replaster the pool, and include waterproofing in the process. By the way, we said chip out and replaster, not bond coat and plaster over the old plaster. If you already have nodules, simply bond coating over the old surface does not get to the root cause of the problem.  You need to get it all out and start over. Don't treat the symptom; treat the fundamental problem.

How to prevent calcium nodules

We wrote this article to share the facts about nodules, and two opposing opinions on why they form. We did not write it to tell you our opinion on what causes them, because we have not done experiments ourselves. While we interpreted both opinions from our perspective, the truth is, we are more interested in helping you solve the problem than pointing fingers. So to us, prevention is a matter of a few best practices that need to be done by the pool builder, plaster applicator, and whoever is starting up/filling the swimming pool.

  • First, the builder needs to make sure the concrete shell is shot and cured correctly. This means discarding shotcrete rebound and shooting to the proper PSI. It means taking necessary precautions to avoid issues from cold joints and any other weaknesses in the shell. Then it means hydrating the shell properly for enough days so that it cures to its optimal strength and has minimal shrinkage cracks.  It also means waterproofing, especially in areas with a high water table (like coastal areas).
  • Second, the plaster applicator needs to mix the plaster with the right ratios and follow best practices for applying the material in the pool. If it's a replaster, this means applying a bond coat correctly if not completely removing the previous surface.. Avoid letting a pool sit for too long without water in it, because shrinkage cracks are inevitable, especially in hot climates.
  • Third, whoever is filling the pool should do so with the LSI in mind. Feed the water the saturation it needs so it is physically unable to steal calcium hydroxide from the fresh plaster surface. Doing so will minimize the risk of open voids/pinholes/cracks, and therefore minimize the risk of nodules.
  • Finally, communicate with all parties involved. If you're a homeowner reading this, take ownership of your pool project! Whether it's a remodel or new construction, you have the right to demand that certain steps–like waterproofing–be taken with your pool. Sure, you'll pay more for them, but the risk of nodules goes down substantially. The cost of something like waterproofing is a fraction of a replacement plaster job.

We hope this article has helped you understand calcium nodules, and what your options are if you have them. If you have questions, contact us or request a virtual training session with us.


1  National Plasterers Council, Technical Manual, §9.0 - Terminology. 9th Edition, pp. 25-26

2  We use the term "migrate" to the surface because experts disagree on whether or not these compounds are drawn toward the pool by aggressive water, or forced from within the surface due to pressure. We have seen hundreds of pools with nodules, and we have enough evidence to suggest both may be occurring. That being said, in our experience, more often than not, nodules appear to be forced out of the surface with pressure.

3  onBalance, http://www.poolhelp.com/home/onbalance-research/education/calcium-nodules/


All information provided is intended for educational purposes and is not implied to replace consultation with a qualified pool professional. It is recommended that all information from this or any other source is to be performed assuming individuals performing these functions will consult local state and federal requirements before you act upon it in any way. While this site attempts to provide information that may be relevant to you, no guarantees are made that some relevant information will not be missed. We recommend you consult a local pool professional before acting. 

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