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Calcium Nodules in Pools

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Calcium Nodules in Pools

What are calcium nodules? In swimming pools and spas, they are small mounds, or “slag” piles of calcium carbonate which are formed from material that has been released from the plaster. Here are some pictures of calcium nodules:

In the cement/concrete industry, this phenomenon would be called a form of “efflorescence” (but in keeping with the spirit of common pool industry practice, we call them “calcium nodules” instead…)

The most common type of nodule is the “delamination” nodule. These nodules grow because of a void (usually a bond separation) between plaster and its substrate. Here is the sequence:

Under normal conditions, the plaster (white) is exposed to the water (blue), is bonded to the gunite substrate (speckled), which in turn rests on dirt (tan). (Picture not to scale…)

When the plaster is new, and the pool is first filled with water, calcium hydroxide bleed-off from the plaster surface dilutes into the pool water (blue) and is converted to non–soluble calcium carbonate (plaster dust).

Continuous “rinsing” of the hydroxide bleed-off by the circulating pool water, as well as normal pool maintenance (such as brushing), prevents a buildup of hardened plaster dust on the new plaster surface.

Sometimes, an air cavity (a bond failure) can form between the gunite and the plaster, or between multiple layers of plaster. This is referred to as “delamination” and the cavity is referred to as a “void.” As long as the void is not connected to the surface of the plaster, the fact that the void even exists may not be known. Unless the plaster completely breaks free from the surrounding plaster, creating what is referred to as a “pop–off,” or unless the delamination is extensive, this is not considered by the plaster industry to be a defect.

In some cases, however, the void is connected to the surface by a small pinhole or hairline crack. Pinholes and cracks are often created from structural flexing of the delaminated plaster.

Gradually, water from the pool penetrates the void via the hole or crack, and creates a localized chemistry environment completely separate from the water balance in the pool. As calcium hydroxide bleeds into the void water, it creates a calcium–rich, high pH solution like a little “calcium/pH factory” beneath the plaster surface. The pinhole or crack is not of sufficient size to create a rinsing effect like that which occurs at a new plaster surface.

Calcium–rich void water and pool water containing bicarbonate alkalinity, in contact with each other at the plaster surface, react with each other, which results in the production of an insoluble calcium carbonate by-product.

If the conditions are right, this insoluble calcium carbonate can build up a “slag pile” around the exit point, thus forming a nodule.

Attempts to remove a nodule through scraping or sanding can be successful – but the node may reform on the same site once or twice, until all of the source hydroxide is used up. Then it shouldn’t come back.

Acid washing nodules off usually isn’t the best response, since the calcium carbonate it is made of is the same stuff much of the plaster is made of. So to dissolve the one, you also dissolve some of the other.

Plugging the hole, by drilling and epoxying, has been successfully used to stop nodule formation, since water can then no longer travel in either direction. But remember, draining the pool may cause more delaminations to occur, so this repair should be performed under water.

It is important to keep in mind that nodules are a symptom, rather than the disease itself. The disease is bond failure, cracking or some other imperfection that allows water “to boldly go where no water has gone before” (sorry, Captain Kirk).

Nodules that form on horizontal surfaces like pool floors end up round shaped:
While those that form on walls drip downward, reminiscent of some stalactites:

Although the calcium carbonate is white, and thus pure calcium nodules are white, when the crystalline matrix incorporates or absorbs dirt, metals, or other contaminants the nodule takes on a color:

Another type of nodule is associated with crazing (small cracks in the plaster) rather than with delamination. Craze cracking can form from high heat, excessive wind, low humidity, or from shrinkage due to excess water or calcium chloride in the mix. Plaster is capable, to a certain degree, of “backfilling” craze cracks with calcium hydroxide, which later carbonates. (Technical term: autogenous healing)

Some of the hydroxide releases into the pool water, and is converted to calcium carbonate “plaster dust.” However, if conditions are right, especially if an extensive network of shrinkage micro fissures is present, nodules can form at these initiation sites

If the surface is sanded or acid washed, the existing nodules may be removed. However, more of the craze network is exposed, and many more nodules may then be formed.

Nodules can even grow on tile grout! This is actually two nodes – a large one to the upper left and a smaller one to the lower right. Notice the “volcano hole” where the continuing process happens!

But the mechanism is the same. In this instance, the source of the calcium- and hydroxide-rich solution is a delamination in the grout/plaster/gunite interfaces:

Nodules can also form on fiberglass surfaces! However, research has shown that in these instances, pinholes existed in the fiberglass, and the source of the node material (calcium) was the plaster/gunite beneath the fiberglass coating:

For decades, it was understood among plasterers that calcium nodules primarily formed due to bonding failure (delamination) of a new coat of plaster, and occasionally from severe craze cracking, and that they were responsible for this plaster defect. Plasterers learned that bonding failure rarely occurs on fresh gunite substrates (new pools); however, bonding new plaster (replaster) to old plaster surfaces can occasionally be difficult to achieve. This is why most calcium nodules occur in replaster jobs.

Unfortunately today, some in our industry (primarily plasterers) will claim that faulty water chemistry maintenance is the direct cause of nodules. The National Plaster Council’s 2009 Tech manual also suggests that improper water chemistry maintenance causes calcium nodules to form on cracks in the plaster surface. That is absolutely incorrect. Service techs should not accept the blame that their chemical treatment caused this plaster defect. Aggressive water would actually prevent nodules from forming, and properly balanced water will not prevent nodules from forming, and actually facilitates the visible growth that exposes the underlying problem.

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Comment by Kim Skinner on April 2, 2010 at 2:59pm
I know you asked David, but I thought I might respond to your question. I believe that if you sanded the calcium nodules off, and were to look extremely close at those spots, you would see a very small hole or thin crack where the nodules formed. If possible, perhaps you could forward some pictures to me. I would like to have a chance and verify what you are describing. Also, if this pool has been drained temporarily in the past, that could have led to some shrinkage (cracking) and resulting in the cause of nodules to form between the tiles and plaster. Just a thought.
Comment by CYNDEE RAIMONDO on March 13, 2010 at 4:47pm
My diamondbrite pool finish started to form nodules at approx 7 years old, on the finish between tiles on vertical and horizontal surfaces and the pool finish itself. My chemistry is always in correct balance. There are no cracks or delaminations to cause this. Using a pumice block takes most of it off but some remains as a almost quartz like substance. What else would cause the nodules to form on a new (7 year old ) finish?
Comment by Kim Skinner on January 19, 2010 at 8:45am
David, something just came to my mind as I read your post. I remember doing replasters 40 years ago. We would put down a very thin scratch coat of cement and glue only. We would let that dry for about 30 to 60 minutes, and then put our thicker main coat of plaster (half an inch) over that. If we let the scratch coat dry too long, we wouldn't achieve a good bond, so that means that the main plaster coat breaks away and leaves the thin scrach coat exposed. Sometimes, plasterers use a thin multi-coat (or glue) formula of some type as a bond and let that dry for several days or more. I know that there are seveal other ways that plasterers perform replasters, but I wonder if what is happening in your area has to do with the thick main coat of plaster not sticking to the underlying scratch coat that is put down first to bond with the old gunite?

Your thoughts on small air bubbles or chambers is certainly a possibility that creates a small delamination site, but I wouldn't think it would expand unless there is an already existing bond weakness surrounding that area.
Comment by David Penton on January 18, 2010 at 9:24pm
In my area there are very few pools that are not stripped to the gunite, yet I see craters very regularly, and they appear to be just through the top layer of plaster.

During acid washes, pool drainings, etc. I have looked closely at them, and they are definitely delaminations, and often times have nodules on them, but VERY, VERY rarely are they all the way through to the gunite. I would say 1 in 200 is all the way through.

One theory that I have heard discussed (shop talk at the pool store) is that it could have something to do with the spiked shoes that the plaster crew uses. Possibly somehow a small air bubble, or something gets trapped under the plaster, and then you end up with a tiny "chamber" which eventaully grows to a full blown delamination. Also I have heard the question asked if it could have anything to do with calcium (flake, prills) not getting fully dissolved in the mix which causes a problem.

I will read the other post on spalling, but it sure seems to be related to delamination in my opinion...?
Comment by Kim Skinner on January 18, 2010 at 2:30pm
You raise some good questions that I need to address and clarify better.

When a plaster layer actually breaks away from the surface, leaving a crater, then generally no nodules will develop.

If a layer of plaster flakes off and leaves a crater only 1/16 to 1/8 of an inch deep, then that is probably “spalling” which is a delamination or separation (of a thin layer) from the main plaster coat. (See Que’s blog “Plaster Flaking and Spalling”)

If a plaster coat pops off and leaves a crater of about 3/8 to ½ inch deep, then that probably means that the entire main plaster coat has debonded from the substrate. If the underlying base is old plaster, then that usually means that this pool was replastered over old plaster. In this case, the plasterer probably either sand blasted or acid etched the older plaster (making it very rough) to achieve a bond for the new plaster. And that would be why the surface below the newer plaster (that has broke or flaked off) is still plaster (but older plaster).

Sometimes, a plasterer will “jack hammer” all of the old plaster off and remove down to the original gunite surface to achieve a rough surface for a good bond. If a bond failure occurs on this pool, it will generally expose the old grey gunite surface.

The reason that bond failure rarely occurs on original gunite pools, is because the fresh gunite is still curing (hydrating) when it is plastered. In this case, besides achieving a “mechanical” bond to the rough gunite surface, there is also a CHEMICAL bond with the fresh plaster (and gunite). If an old pool has the entire original plaster coat removed down to the old gunite (that is no longer new or fresh), there usually is not a chemical bond achieved and may not provide a sufficiently good bond.

Also, as you indicated, sometimes the original gunite surface (or any substrate, plaster or gunite) is not properly cleaned, the dirt removed, etc, and prepared adequately, then a bond failure may result. And if the plaster coat rises upward or away from the wall or floor, (where it has loss its bond), and merely cracks (and does not break away), then calcium nodules are likely to occur.

I hope this addresses your questions.
Comment by Kim Skinner on January 18, 2010 at 2:18pm
Thank you for your comment, and yes, filling in the cracks or pinholes would prevent water flow (or exchange), and should prevent nodules from forming.
Comment by David Penton on January 16, 2010 at 8:48pm
I have a question.

If this is the result of a bond failure between the gunite and the plaster, than how come most delaminations that actually rupture and create "moon craters" the delaminated plaster is VERY rarely all the way through to the gunite, but seems to be between the multiple layers of plaster instead. You rarely see a nodule/delamination break apart, and reveal gunite, usually it is the lower layer of plaster that becomes exposed.

I have long thought that nodules were the result (in remodels) of incomplete washing of the pool shell, or incomplete removal of all the partially broken off gunite fragments. A couple years ago I began pressure washing my shells after they are stripped, and before they are plastered. I carry a hammer with me to break off all the loose gunite fragments as well. The process takes 2 - 4 hours, but I can "eat lunch" off the shell when I am done. Plus it acheives the SSD condition that the Structural Engineers call out.
Comment by Lester Eric Brehm on January 16, 2010 at 12:44pm
Kim, Very good information,this is something I have always wondered about. I've seen this on block and tile and would like to know how to prevent it, I suppose stopping the water from entering the substrate is the key.

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