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Thread: 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, bumps, deposits, or “slag” piles of calcium carbonate which are formed from material that has been released from the plaster. The small calcium nodules are rough to the touch, hard, and generally gritty. Nodules may form singularly (far apart or sporadically), or many and close together along a crack in the plaster surface.

    Here are some pictures of calcium nodules: The first picture is of black colored plaster with many white calcium nodules forming along cracks in the plaster surface.




    The second picture is of a single large calciuim nodule on old white plaster with copper (turquoise) staining present.




    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…)




    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, sanding, or a direct acid treatment, 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, AND IT IS LIKELY THAT MORE NODULES WILL FORM AGAIN AFTER THE POOL IS FILLED WITH WATER. 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: autogenously 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. That is absolutely incorrect. High water hardness and high pH will cause a uniform layer of calcium scale throughout a pool surface, not in isolated and individual spots (bumps). Service techs and pool owners 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, but actually facilitates the visible growth that exposes the underlying problem.

    Other good pictures of nodules can be found at this post: calcium-deposits-on-pool-wall-t55603.html

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    Re: Calcium Nodules in Pools

    Very interesting and informative, for sure... This one needs to be stickied, or put in pool school or something, so this bit of good tech is not lost in the "pool" of threads in the forum. Bad pun, I know...

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    Re: Calcium Nodules in Pools

    Wow what a great write up! I am so glad I found this post before I started attacking my pools calcium nodules. A few questions on an original plaster pool from the 70's is ok to sand them down while the pools is drained? If it is what should I use (sand paper? what grit? Angel Grinder?) Lastly should I grind down a little below flush with existing plaster and then spackle in some fresh plaster?

    1975, 18000 GL, Plaster never been re-plastered, 2 year Old Hayward Sand Filter, Very Old Pump made out of cast Iron, and Single Speed 1 HP motor. Mesa AZ

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    Re: Calcium Nodules in Pools

    It is better not to drain the pool if possible. Draining will only cause more shrinkage cracking and thus, more nodules will form after the pool is filled with water. If the pool needs draining, then fill as soon as possible.

    If possible, sand them while full of water using 80 or 100 grit wet& dry sandpaper only to the flush point. Look closely for a pin hole or small crack that leads into plaster matrix and fill hole or crack opening with "Epoxy" cement to prevent further nodules forming. A grinder will not be necessary. Nodules are very porous and will not need much sanding to remove.

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    Re: Calcium Nodules in Pools

    Thank you for the info. I do have to drain due to having CYA over 100>, I just thought to take advantage of all the nodules being exposed at that time. But if I can easily sanded while water is covering then that's what I'll do.

    1975, 18000 GL, Plaster never been re-plastered, 2 year Old Hayward Sand Filter, Very Old Pump made out of cast Iron, and Single Speed 1 HP motor. Mesa AZ

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    Re: Calcium Nodules in Pools

    I read about this on your website and it was a very interesting read. Thanks for posting.
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    Re: Calcium Nodules in Pools

    onBalance: Nice summary of what causes efflorescence to form. Many people often mistake this for calcium scaling resulting from water with a high pH and hardness. Though the appearance of efflorescence has some similarities to calcium scaling, the underlying causes are quite different. You post/pics should help clear this up.
    Gold Supporter, TFP Lifetime Supporter, 26,680 gal Plaster IGP 3.5 - 10' depth / Attached Waterfall Spa, Manually Chlorinated, Triton Sand Filter, 1.5 HP * 1.1 SF = 1.65 SFHP 1-speed Pentair WhisperFlo WF-26 Pump, 400K BTU NG Teledyne Laars Series One Heater, Polaris 360, Test Kit Comparison, Chlorine/CYA Chart, SLAMing Your Pool, OCLT
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    Re: Calcium Nodules in Pools

    Good point about mistaking calcium nodules (due to improper plastering materials and workmanship) from calcium scaling due to high pH and hardness. Let's add to the above that high pH and hardness will cause a uniform layer of calcium scale throughout a pool surface, not in isolated or individuals bumps (nodules) scattered around and apart from other nodules. Thank you for your comments.

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    Re: Calcium Nodules in Pools

    Extremely appreciative of this write up!!!

    Looks like we have this problem happening in our pool. Unfortunately, when it originally started over a year ago, all that was found online pointed to calcium scaling. After performing a long pool dive armed with plastic scrapers, it was pretty cleaned up and we followed by draining and refilling the pool to lower the hardness of the water.

    This year the nodules came back WORSE so don't drain your pool to fix this specific problem!!! A month ago, I jumped back in to do my annual scraping. Not even a month later and they are all back. I guess that dive tank purchase was well worth it since I'll be jumping back in and scraping away again.

    I will try to attach a couple picts a little later. These nodules are concentrated on the walls at the deep end (1/6 of the pool). There are about 100 of them now. There was probably only 1/3 of that last year. The rest of the pool is nodule free.

    Couple questions:
    -Since we have a darker bottom pool, I noticed when I did the cleaning last month that there was a little discoloration of the plaster once the nodule was removed. Is that expected?
    -Any more detailed suggestions/instructions on how to plug these buggers with the pool full of water?
    Tustin, CA (Orange County)
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    Re: Calcium Nodules in Pools

    Is the discoloration a lighter or darker color than the surrounding plaster surface?
    I would expect that it is darker and probably close to the original intended color of the plaster.
    So yes, that would be expected. Dark plaster will usually lighten over time.

    Use a knife to scrape off the nodules and to open up the cracks; then fill the cracks with an underwater epoxy, is the simplest way.

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    Re: Calcium Nodules in Pools

    Thanks for the help onBalance!

    I was expecting to jump in the pool this weekend but have had difficulty finding an underwater epoxy that is clear. Does something like that exist? We have a darker bottom pool. Can't wrap my head around scraping all those white nodules off just to seal them with a white, off white or tan epoxy.

    I found a few epoxies that may be the closest I can get:
    http://www.alcolin.com/diy-products/epoxy/insta-crete
    http://bluemagicusa.com/index.php/bl.../products/164/
    http://bluemagicusa.com/index.php/bl.../products/171/

    But ideally if there's a clear option, it will blend the best and not make the plaster look stained. Otherwise looks like I'm buying my 3 options and trying to color match as best as possible.
    Tustin, CA (Orange County)
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    Re: Calcium Nodules in Pools

    I have one more week before jumping in the pool and performing the fixes... any news on whether there is a clear underwater epoxy available that my web searches just aren't finding?
    Tustin, CA (Orange County)
    Curacao Quartz Finish (grey plaster with grey, black, blue & white quartz)
    29k gallon IG
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    Hayward SwimPure T15 SWG

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    Re: Calcium Nodules in Pools

    Three years later, this thread is still helping people. Jason pointed me to it. Thanks so much for an informative and picture filled write up. Thanks to Jason for providing the reference.
    Central TX / 12K GAL / IG / SWG / DE filter / Pentair SVRS & 3 swim jet pumps / Spa bench and jets / Single body of water / No heater.

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    Re: Calcium Nodules in Pools

    For those that don't know, the guys at On Balance are the real deal. I took an advanced chemistry class from Que a couple years ago and was blown away by his depth of knowledge and professional and truly scientific approach along with stringent research methods on par with other researchers on the cutting edge of science.
    Co owner of Swimming Pool Science. My pool is green and full of dirt but all of my customer's pools sparkle. Check out my YouTube channel and Facebook page by searching SwimmingpoolAZ.

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    Re: Calcium Nodules in Pools

    Great thread five years later!

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    Re: Calcium Nodules in Pools

    Still a great thread. Let me caution that you can turn a nodule into a crater pretty easily. I got a hard pumice stone yesterday, as it would attach to one of my poles, since my pool is about 50 degrees right now. Ground gently on a couple of smaller nodules, came right off. Ground a little more firmly on a couple of larger nodules and all looked well. Just went out in the dark and threw on the pool light, and can now easily see a couple of craters where I apparently "knocked" off nodules, taking plaster with them, instead of "grinding" off the nodules.

    I wanted them gone, as they were stain magnets, and I have dark rings around them. Now I kind of wish I would have left them alone.. I don't think there is massive damage to the plaster, but I don't like the looks of the divets I made. Guess I will be doing some swimming with some epoxy when it warms up. Oh well, screw up and learn, I guess.

    Thanks for letting me know what they were.. I appreciate the thread!
    21K bent-rectangle plaster, 330 gal spa with spillway, Pentair CCP320 cartridge filter, Pentair WF-3 (011512) 3/4HP single-speed pump, Hayward H Series heater, "Herbie" the Polaris P93 bot, TF-100 test kit
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    Re: Calcium Nodules in Pools

    I have new pool. Finished end of last June. Pool company opened up a few days ago and there were thousands of little white things, primarily on the walls of the pool. Pool company says they are calcium deposits and says it is bc the calcium was low in the pool when closed. Huh. We spend a fortune on this new pool and it looks like cr#p and the walls are gritty and not smooth. I did my own research and it seems there might have been a problem with plaster bonding to the gunite? Has anyone encountered this in a new pool? any pool? seems like the only was to SOLVE the problem is remove the plaster and start again.
    IG Gunite/Plaster 35,000 gallons
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    Re: Calcium Nodules in pools

    Wow I have never seen this writeup before. I had that problem before I replastered.




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    Re: Calcium Nodules in pools

    Brokenskier,
    Your pool may have two separate problems. First, a uniform gritty and sandpaper like plaster surface (that you described) may be due to calcium scaling caused by a high pH and/or high calcium hardness of the pool water and occurred over the winter.

    Second, individual and random calcium "bumps" (that are also sharp to the touch) is either from the new plaster coat not bonding to the gunite or whatever substrate it is, or from severe shrinkage cracks. This is what this post/thread describes and explains about "calcium nodules." Yes, this is what can happen in new pools, but is far more rare than when it is a new re-plaster job over old plaster.

    If your plaster problem is calcium scaling only; that can be remedied by an acid wash. But calcium nodules are a more serious problem as explained in the original post above.

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    Calcium Nodules in pools

    Thanks to everyone for contributing, this is extremely informative. We started to notice calcium nodules building up on top of bumps in the Krystalkrete surface. Interestingly, the bumps and nodules appeared mostly, if not exclusively, in the deep end of our pool. When I contacted the contractor asking for a warranty fix of the problem, the responsibility was immediately deflected to the maintenance of the water chemistry. They could not answer the question about the isolated locations in the deep end. Makes little sense, since the water chemistry is the same all over the pool. They reluctantly agreed to an acid wash to remove the nodules, which was only accomplished after the bigger nodules were physically scraped off with a pumice. They refuse to address the remaining bumps and white pockmarks where the nodules used to be. Some my question to this group would be: is there a way of telling the difference between calcium scaling, which results from a high positive Saturation Index with scaling tendencies, and calcium nodules as a result of separation issues with the new plaster or failing bonding to the old plaster or simple bad workmanship when applying the new plaster ?

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