The text and images in this blog post are credited to onBalance and www.poolhelp.com. Many people over the years have asked me where calcium nodules come from? High water hardness and high pH will cause a uniform layer of calcium scale throughout a pool surface, not isolated and individual spots (bumps) on the floor of the pool, or a vein on the wall. Pool owners or pool professionals servicing the pool are are not the cause. Balanced water will not prevent nodules from forming, but actually facilitates the visible growth that exposes the underlying problem.
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, are generally gritty and can cut your feet if you walk on them. Nodules may form singularly (far apart or sporadically), or many and close together along a crack in the plaster surface.
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, are generally gritty and can cut your feet if you walk on them. Nodules may form singularly (far apart or sporadically), or many and close together along a crack in the plaster surface.
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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), which is bonded to the gunite substrate, or concrete (speckled), which in turn rests on dirt (tan). |
 | 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, from the plaster itself, 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 the pool waters natural alkalinity, come into contact with each other at the plaster surface, then react with each other, which results in the production of an insoluble calcium carbonate by-product. This insoluble calcium carbonate can build up a “slag pile” around the exit point, thus forming a nodule. |
| 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 in the plaster from the pour, so acid washing them does not fix the problem. I have removed the build up to eliminate the chance of someone cutting their foot on the sharp slag, but the spot where the the top layer of plaster delaminated remains. |  |
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