You may find the following links helpful in explaining nodules:

Calcium Nodules on swimming pool plaster Pt 1

Calcium Nodules on swimming pool plaster Pt 2

 

Calcium nodules in swimming pools are small, unsightly bumps composed of calcium carbonate that can form on plaster, fiberglass, or tile pool linings. They typically result from a poor plastering job or normal aging in older plaster pools. When air bubbles become trapped in dried plaster, it leads to the delamination of the pool lining, allowing elements of it to become exposed to the pool water. Loose or Drummy Surface: If the pool surface becomes loose or develops voids, water can seep behind the pebbles or other materials. Calcium hydroxide may then bleed into the water, leading to the creation of calcium carbonate nodules along the surface. Calcium hydroxide in the pool lining may combine with alkaline elements in the water, forming calcium nodules

To remove calcium nodules, you can scrape them off with a stiff trowel, but this is only a temporary fix as they will likely reform. The only true solution is to repair or replace the pool’s lining. Formation of Calcium Nodules in Pool Water: When the pool water has an elevated pH level, it can contribute to the formation of calcium nodules. Maintaining the correct pH balance is crucial to preventing this issue.

Very High Calcium Hardness: Proper calcium hardness levels are essential for protecting the pool surface from calcium-related problems. For pebble, quartz, and glass bead pools, the suggested calcium hardness level is 200–250 ppm. Fiberglass pools vary by manufacturer, but most recommend a minimum of 150 ppm. side of the surface: Sometimes, calcium nodules form within the pool surface itself. These stubborn calcium deposits often appear as hard white scales on the pool surface.
To prevent calcium buildup, focus on maintaining the right pH and alkalinity and ensuring proper water balance. While some treatments temporarily reduce the effects of high calcium, they don’t permanently remove calcium buildup. Keeping the water chemistry balanced is key to avoiding calcium-related issues in swimming pools.

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Chemical Treatment:

 

1. Establish the water chemistry and, if possible, the water chemistry history of the pool.

2. The parameters to consider are pH and T.A. (total alkalinity), as they are the most important in proceeding with a treatment.

3. A target of pH 7.0 and a T.A. under 100 ppm but no less than 60 ppm.  Consider an acid bath with water in place at this stage if you feel the severity of the scaling justifies it, at a rate of 10 liters of undiluted acid per 50,000 liters (be aware that this will drastically reduce both pH and T.A. and should only be used for a short period of time, 24 hours to a maximum of 48 hours, only to soften the deposits, and after that period a less aggressive pH and T.A. level should be obtained).

4. The calcium hardness level should be established to provide a base point, recorded, and referenced after treatment has been performed.

5. To soften the scaling, products like Lo-Chlor Stain and Scale Attack Parts 1 & 2 have proved effective in softening and subsequently removing light to moderate scale.

6. Heavy scaling may require multiple treatments and more aggressively low pH levels for prolonged periods of time—weeks, possibly months.

7. Manual Removal: Scrub the scaling using a pumice stone, stain eraser sticks, or metal algae brush to help loosen the softened scale.

8. Testing of calcium hardness needs to be performed at this stage to establish if the calcium level has increased from the baseline established before the treatment started. This will indicate the removal of calcium from the surface and the success or failure of the treatment.

9. A sequestering agent like Lo-Chlor Stain & Scale Defense, Lo-Chlor Aquaguard, or Riversands CalStop is used to retain the removed calcium in solution.

10. Filtration should be addressed at this time, and a regular filer-clean schedule of every 2 days should be in place at least a week after treatment.

11. If a cartridge filter is being used, the condition and effectiveness of the filter need to be assessed, and a replacement may need to be considered.

12. If a sand filter is in use, you should consider using Lo-Chlor Filter Aid & Metal Remover.

13. Partially draining and refilling the pool water to reduce calcium hardness may be required at extreme levels of calcium hardness. Re-balance the pool, continue to monitor calcium hardness levels, and continue removal protocols until it reaches between 150 ppm and 250 ppm.

14. * Use a pumice stone or a tile brush to gently remove the scaling from pool walls, tiles, and other surfaces.Be cautious not to damage the pool finish while scrubbing.