Calcium Nodules and Stains - Fixable or time to re-plaster?


New member
Nov 27, 2015
Sugar Land, TX
Around the beginning of Q2 2015, I started noticing the appearance of small, white deposits on the bottom of my pool (see attached pictures). This is the first time I have encountered this issue in the 4 years I've owned this pool. It was only a few at first, then there was a large increase around mid-summer and the development of new ones seems to be tapering off now in cooler weather. Most of them were and still are located from the middle to shallow end of the pool and are usually surrounded by a brown stain. There are also some slag-like deposits on the walls of the pool, but they are far less numerous.

My research into this issue indicates that these are calcium nodules/slag. Initially, I thought we had a build-up of calcium and a PH problem in the pool from over-shocking, but several checks of the chemical balance by my pool service and a local pool supply store have shown no issues with the pool chemistry. My pool service indicated that this might be due to a form of "efflorescence," whereby the concrete is leaching various minerals through the plaster. My research also suggests this might be the beginning of delamination of the plaster from the concrete (time to re-plaster). The plaster is probably between 10 and 15 years old, but I don't have a good record of it.

The nodules are very stubborn. They have not responded to regular brushing or chemicals (in this case, Scaletec). I'm at a loss for what to do except wait for warmer weather and try to remove them with pumice stone or sandpaper. Based on the pictures, I'd appreciate anyone's insight on the subject. What, if anything, can I do about the nodules and stains? Is it simply a lost cause and time to re-plaster?

My personal belief is that some porosity or damager occurred to the plaster when I drained the pool back in the fall of 2013 in light of a buildup of cyanuric acid. Coincidentally, the nodules are located primarily in the areas where the plaster dried out prior to re-filling (shallow end and walls). That, along with some sort of natural phenomenon in the concrete, is what I believe has led to the current issue. Your thoughts?


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TFP Expert
Platinum Supporter
May 23, 2015
Tucson, AZ
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Hi William, welcome to TFP :wave:

We would all love to help but our advice is predicated on pool owners testing and maintaining their own water chemistry. We can not rely on the "good word" of pool store testing or pool service companies as they are, more often than not, completely wrong. One also can not mix what we teach here on TFP with pool service company maintenance, the two are completely incompatible. If you through enough threads here on TFP you can understand why we are so insistent on this practice of each owner having their own, high quality water testing kit and doing their own chemical balancing of their pool water.

We compare test kits in this thread - Test Kits Compared

Our general pool care philosophy is in this thread - What is TFPC?

This is a basic, introductory thread on pool water chemistry - ABC's of Pool Water Chemistry

There are many experts here on TFP willing to help but all our help starts with you posting water test results. It's the only way we can give you the proper advice. Your calcium nodules may be calcium nodules or scaling or lots of other possible issues but, just like going to a doctor, we can only give you the correct diagnosis by first getting the proper tests done.

Good luck,



Well-known member
Jul 25, 2012
Central Texas
My pool showed similar issues in a few spots and was also diagnosed as calcium nodules. Best explanation was that in a few spots, the plaster had a weakness and leached whatever it could thru a micro fracture. I also had no chemistry issues at the time.

We simply took them off with a knife and once everything was 'eaten' by the water in the void behind, they never came back. Ours were not accompanied by brown staining like you mention.

As Joyful Noise stated, the first step is an accurate water chemistry reading. I can only speak to the fact that the pictures look similar.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
See Calcium Nodules in Pools. It's primarily due to a defect in workmanship where a void was created between the plaster surface and the gunite below -- that is, a bad troweling job in some places. The problem may not show up for years -- in my pool it started to show after around 7 years. Some small defect in the plaster, such as settling of the ground to cause micro-cracking or poor water chemistry that erodes the plaster surface, causes a pin-prick hole connecting the underlying calcium hydroxide paste in the void to the pool water. The calcium hydroxide comes out and forms calcium carbonate which is the hard volcano if on the floor or a drip-line if on a wall. You may notice the pH of the pool rising more than normal and requiring more acid than usual.

If you over-saturate the water with calcium carbonate and use a pumice stone to sand down the calcium carbonate (i.e. sort of like doing a bicarb startup again) you may seal it off at least until there's some other crack that develops. While you can control the water chemistry, you can't control the movement of the earth. In a properly plastered pool such movement doesn't cause problems because any micro-cracks don't expose very much calcium hydroxide and any that is exposed simply reseals in place. It's the larger voids that are a problem since they have a much larger volume of calcium hydroxide so can produce much more extruded calcium carbonate in the pool water.
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