Spot etching and CSI

[JamesW]

Low CSI might contribute to spot etching, but I don't think that it is the primary cause. Spot etching is where there are small spots of damaged, discolored or etched plaster where the plaster is noticeably different in color, composition, porosity or texture.

I think that the term is somewhat controversial and might be caused by several different things. Some definitions don't include actual etching or texture differences. The increased porosity can allow algae to dig into the plaster. The porous spots also will tend to absorb metals, and you will see metal staining in those areas.

I don't think that aggressive water alone causes spot etching, because if it were just the water, then the etching would be more uniform. There has to be some sort of defect in the materials or application to cause spot etching. I think that the spots have to be weaker than the surrounding plaster and are therefore more vulnerable to aggressive water. Due to the defect, it is likely that the spots would become etched even with balanced water.
Spot leaching might be a better term. I think that aggressive water accelerates the etching (or leaching). I think that some of the causes could be the following:

1) Excess calcium chloride used to accelerate the cure rate of the plaster.
2) Calcium chloride being added in a way that leaves lumps of calcium chloride instead of fully dissolved and distributed calcium chloride
3) Lumps in the cement or aggregate that do not get fully mixed and hydrated.
4) Workers throwing or spraying water on plaster that is setting up to make it more workable. Where the water hits might cause those spots to be weaker than the surrounding plaster (possibly by washing out the cement and leaving the aggregate). Workers are more likely to need to use water to rewet the plaster when excess calcium chloride is used and the plaster sets up too fast.

For plaster to last as long as possible, it is important to keep the water fully saturated with calcium carbonate. I always keep the CSI in plaster pools on the 0.0 to +0.3 range, I keep the pH in the 7.7 to 8.0 range, and I do not get scaling. I have pools with the same plaster for 20 years and the plaster is still in good shape.

 

[PoolGuyNJ]

Additional potentials:

Inconsistent mix times.
Different batch numbers mixed.
Poor QA by the manufacturer.
Poor application, over troweling or under troweling.
Work time exceeded.
Rest time not reached.
Circulation of water. Stagnant spots in a pool will have different chemistry.

Scott

 

[JasonLion]

For plaster to last as long as possible, it is important to keep the water fully saturated with calcium carbonate. I always keep the CSI in plaster pools on the 0.0 to +0.3 range, I keep the pH in the 7.7 to 8.0 range, and I do not get scaling. I have pools with the same plaster for 20 years and the plaster is still in good shape.

Just as you don't see any scaling when CSI is mildly positive, I never see any etching or loss of plaster life when CSI is mildly negative. It takes fairly large swings in CSI before anything obvious happens.

 

[chem geek]

At least for scaling, we rarely see it until the saturation index gets to around 0.7 or so, especially at +1.0 or more. Obviously, one can see scaling in an SWG cell below these levels, but that's because the pH is very high at the plate where hydrogen gas is generated. One can also see some scale on surfaces where water has evaporated. In spas with their hotter water, we sometimes see some scaling effects at around +0.3, but mostly it's in the heater that is at hotter temperatures where scaling is an issue.

Likewise, the dissolving of plaster is not only a function of a lower saturation index, but of specific water parameters such as lower pH that can accelerate such degradation. The saturation index is a thermodynamic quantity saying what is possible vs. not possible, not how fast it occurs. The quality of the plaster makes a big difference as well since a well-formed solid surface degrades more slowly than one that has been improperly cured, contains contaminants, is very uneven, etc.

 

[onBalance]

I agree with JamesW that when "spotting" shows up in relatively new plaster pools (usually less than two years), the white spots are "porous and soft" which allows water to penetrate and dissolve away calcium compounds. I also prefer to use the term "leach" as opposed to "etch" because typical balanced pool water can dissolve calcium hydroxide that is within the cement matrix. Calcium hydroxide is more soluble than is calcium carbonate and calcium silicate. As James mentioned, adding calcium chloride to the plaster mix and adding water while troweling are two contributors to this spotting problem. The result of doing these two things causes more shrinkage and porosity than normal, and thereby creating a weak, porous, and soft spot or areas. Lately, I have began using the term "white soft spots" to describe this problem.

I would also add that filling the pool immediately after final troweling, very hot and dry weather, can contribute to the possibility of spotting developing depending on the degree of abuse of the other factors. And yes, aggressive water can accelerate and exacerbate the spotting problem if the underlying problem is there, but improper water chemistry doesn't cause this inherent plaster defect. I am sure that we have all seen how aggressive water will cause general and fairly uniform etching on good plaster.

How many service techs have performed "no drain" and "zero alkalinity" programs, along with regular acid washes and don't see white spotting show up afterwards? If this is not the case, let me know. I have also placed large plaster coupons in aggressive water for over a year, which did not result with any spotting problem. Professional cement laboratories have examined actual pool plaster with spots, and determined that the plaster had not been subjected to aggressive water at any time.