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.
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.
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.
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, 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 in the plaster from the pour.