Is the Saturation Index always Reliable?

[onBalance]

When calculating the Saturation Index (SI), does a single low water parameter such as calcium hardness (CH), or a low carbonate alkalinity (C-ALK), make the water automatically aggressive to pool plaster, even when other water parameters are high enough to balance the SI?

To answer that question, an experiment was conducted. Two quality pool plaster coupons were made and cured in balanced water for 90 days. At that point, plaster coupon #1 was placed into SI balanced water, but with a low CH of 90 ppm. The C-ALK was maintained at 110 ppm and the pH from 7.9 to 8.2, which off-sets the low CH and achieves a balanced SI of -0.1 to +0.2.
Coupon #2 was also placed into SI balanced water that had only 45 ppm of C-ALK. The CH of this water was 360 ppm and the pH was maintained between 7.7 and 8.0, off-setting the low C-ALK and achieving a balanced SI of -0.1 to +0.2.

After six months in the water, the coupons were removed, and the water they were in was tested for the calcium content to determine if any dissolution or etching of plaster surface material occurred. Of course, any increase in calcium from the submersion water starting point would indicate that calcium had been dissolved from the coupon, which was the only available source of additional calcium.

The result? There was no increase of calcium in either water container.

Therefore, these results indicate that if the calcium hardness or the carbonate alkalinity is low, but the water is still determined to be SI balanced, the water is not aggressive. This indicates that the SI is applicable for plaster swimming pools.

This experiment was conducted because some in the pool industry claim that pool water with a low CH or a low C-ALK is automatically aggressive despite what the SI actually is. Often, when a new pool plaster surface has undergone severe gray mottling, white spotting, streaking discolorations, flaking, nodules, or other defects actually attributable to plastering errors, the finger is pointed at aggressive water chemistry instead. Specifically, a CH or C-ALK below the APSP Ideal minimum is blamed, even in cases where the actual LSI is balanced.

It is interesting to note that the current National Plasterers Council (NPC) 7th Edition Technical Manual states that if any isolated, individual water parameter (pH, C-ALK, or CH) is lower than the Ideal range as defined by the APSP, the water is aggressive. Therefore, the NPC is (incorrectly) stating that when the C-ALK is less than 80 ppm (while the total alkalinity may be above 80 ppm), or when the CH is less than 200 ppm, or when the pH is below 7.4, (which are the lower ends of the APSP Ideal Standard), the water is automatically aggressive and considered to be detrimental to the plaster finish (even though the SI may be balanced).

This is a complete departure from the NPC text on this topic in their 5th Edition of the Tech Manual. It stated that pool water needed to be within the acceptable tolerance range as established by the APSP, which set the “minimum” for C-ALK at 60 ppm, CH at 150 ppm, and a pH of 7.2. It is apparent that the water balance requirement by the NPC is more restrictive now. Why did the NPC make that change? Where is the study documenting that a low CH or C-ALK is automatically aggressive when the LSI is 0.0 or higher? Has the National Pool Industry Research Center at Cal Poly (NPIRC) proved that? No.

Now, we are not necessarily promoting the concept that pool water should be maintained on a regular basis with the pH, C-ALK or CH below the minimum APSP targets. We are not even suggesting that the ranges below the ideal are always the optimum. We would recommend that pH and C-ALK levels be maintained above the ideals, not below, whenever possible. However, we understand two things: that there are times when lower pH, CH or C-ALK are unavoidable, and when that chemistry is SI balanced, it means exactly that, it is not a tool for blaming plaster defects on water chemistry.

As the above plaster study demonstrated, pool water with a single low water parameter (even below the APSP Ideal or Minimum standard) can be non-aggressive if the SI is balanced.

For proper plastering practices, see this post: ten-guidelines-for-quality-pool-plaster-t42957.html
onBalance

 

[chem geek]

Thanks for the experiment. It is also important to show that aggressive water, that with a low SI, will deteriorate (pit/etch) plaster coupons regardless of whether the low SI is due to low pH or to low CH or low TA (other parameters at normal levels) though the rate might differ depending on which parameter is low (I suspect that low pH is more aggressive at the same negative SI level). I believe you've done some of those experiments in the past using similarly made and cured plaster coupons and perhaps could share those results here. Otherwise, one might argue that no deterioration occurs after curing at all, even when the SI is low.

I have always believed in the chemistry behind the saturation index since it is solely based on chemical equilibrium and has nothing specific in its derivation regarding boilers or closed systems or anything like that in spite of its history originally from Langelier. It is not a corrosion index for metal corrosion. It is only an indicator whether water is under-saturated, over-saturated, or at equilibrium with calcium carbonate. It does not predict the rate of dissolving plaster or of producing scale, but does predict if either is possible. I know there are those who believe that being within standard ranges is all that matters, but I disagree with that approach, especially if one needs to significantly lower the TA level for saltwater chlorine generators or high aeration scenarios where I believe the CH should be raised to compensate (assuming one doesn't raise the pH very much).

 

[mas985]

It does not predict the rate of dissolving plaster or of producing scale..

It would be nice if it did. I suspect there is a correlation between the magnitude of CSI and the rate at which scaling/dissolving would occur. With sufficient testing, do you think that it would be possible? Also, is there a theoretical solution for predicting the rate or are there too many variables involved?

 

[onBalance]

Richard, I agree with your perspective. Obviously, when using acidic sanitizers on a regular basis, one needs a higher TA for good buffering. But as you stated, for SWG or bleach usage, a lower TA could be advantageous for more a consistent and stable pH. One just needs to be careful about the amount of acid that is added, if it is necessary to add. I also agree that pH is probably the biggest factor, based on past experiments and various cement literature. Plus, the pH largely determines the amount of carbon dioxide/carbonic acid that is present in the water.

My past experiments have shown that maintaining water within a range of -0.6 to -1.1 for the CSI has led to the dissolving of about 7-8 ppm of calcium per month with quality made plaster coupons.

In regards to mas985 comments; yes, the greater the magnitude of the CSI, the greater the potential for etching or scaling. But as you suggested, there are too many variables, such as the age of the plaster surface, the quality of the plastering workmanship (which is often overlooked and probably the most important variable), the cement to aggregate ratio, the water to cement ratio, the quality of materials, the ongoing maintenance, brushing, cleaning, etc., and all this can affect the actual or eventual rate in the dissolution of calcium carbonate.

 

[chem geek]

It would be nice if it did. I suspect there is a correlation between the magnitude of CSI and the rate at which scaling/dissolving would occur. With sufficient testing, do you think that it would be possible? Also, is there a theoretical solution for predicting the rate or are there too many variables involved?

The problem with prediction is the unknown amounts for variables involved. There's a lot to cover, so let me group things by topic.

Calcium Carbonate Equilibrium
Let's start out with the most simplistic equilibrium equation which is the following:

CaCO₃(s) <---> Ca²⁺ + CO₃^2-
Calcium Carbonate solid <---> Calcium Ion + Carbonate Ion

The equilibrium point is described by the solubility product: K_sp = [Ca²⁺]*[CO₃^2-] where I show this as the product of concentrations, but technically it is the product of activities (or of concentrations if one also multiplies by activity coefficients). The activity coefficients account for ionic strength. The ionic strength is calculated from the TDS and assumptions of composition from the pH, CH and TA parameters (and to some degree, temperature). For non-salt pools, the Calcite Saturation Index (CSI) is lowered from ionic strength by around 0.2 units while for salt pools at around 3000 ppm salt, it is lowered by around 0.4 units, so this can't be ignored, but for my discussion purposes I'll ignore it since it's just a factor on the concentrations.

What we are interested in is the reaction rate and there are two primary reactions going on that can be described by rate equations:

CaCO₃(s) ---> Ca²⁺ + CO₃^2-
rate_dissolving = a constant k_d (because the concentration of calcium carbonate solid is a constant related to its density)
Ca²⁺ + CO₃^2- ---> CaCO₃(s)
rate_scaling = k_s*[Ca²⁺]*[CO₃^2-]

Net Dissolving Rate = rate_dissolving - rate_scaling = k_d - k_s*[Ca²⁺]*[CO₃^2-]

Plaster Quality Physical Effects
The rate of dissolving of the solid will be dependent on the quality of the plaster because it is proportional to the surface area exposed to the water (ignoring diffusion rate limits) so smoother plaster will have less exposed surface area. Also, poorly made plaster may have impurities or weak physical bonding of material (i.e. air gaps) that allow for physical removal and/or water penetration. Finally, the calcium carbonate is not in the plaster in isolation, but is rather in a mixture with calcium silicates due to the curing process which is mostly the following (there are other calcium silicates formed as well):

2Ca₃SiO₅ + 7H₂O ---> 3CaO•2SiO₂•4H₂O + 3Ca(OH)₂ + heat
Uncured Pool Plaster (tricalcium silicate) + Water ---> Cured Pool Plaster (calcium silicate hydrate, CSH) + Calcium Hydroxide + heat

The bicarbonate start-up procedure has the TA be higher in order to have more of the calcium hydroxide form calcium carbonate via the following equation:

Ca(OH)₂ + HCO₃^- ---> CaCO₃(s) + H₂O + OH^-
Calcium Hydroxide + Bicarbonate Ion ---> Calcium Carbonate solid + Water + Hydroxyl Ion

Note that with other startup methods (acid, pH neutral, and traditional startups) calcium hydroxide is allowed to dissolve in water which increases pH, TA, and CH (the TA increases by the same amount as CH). With an ideal bicarbonate startup, only the pH rises and there is no change in TA nor CH. Also, with the bicarbonate startup, the pH rises less requiring half the acid compared to not forming any calcium carbonate at all.

Even without the above conversion, there is initially calcium carbonate in the mix along with the uncured calcium silicate. Though we call it pool plaster, it's really a type of concrete which is a combination of Portland cement, aggregate (such as sand which is usually marble composed of calcite and/or dolomite or is quartz which is silica) and water. Cement is made by heating limestone (calcium carbonate) to form calcium oxide along with clay which provides silicon dioxide.

In addition to the dissolving of calcium carbonate, it is possible for calcium oxide to dissolve as well, though it it less likely due to it being bound in a hydrated calcium silicate compound:

CaO + CO₂ <---> Ca²⁺ + CO₃^2- + heat
Calcium Oxide + Carbon Dioxide <---> Calcium Ion + Carbonate Ion + heat

The reverse of the above reaction (starting from calcium carbonate solid instead of ions) is what is done in a kiln when heating limestone to make cement. Note that the above reaction implies that even with saturation of calcium carbonate where the rate of the above going to the left is constant, the rate going to the right is a function of the carbon dioxide concentration. This implies that all else equal, a lower TA may be more protective of plaster. This is speculative, however, since we don't know the rate of the above reaction (for hydrated calcium silicates) so it may be a non-issue.

Net Reaction Rates
So let's get back to the calcium carbonate situation. When the saturation index is 0, calcium carbonate is being dissolved at the same rate that it is being formed. Reaction rates increase with temperature so this back-and-forth will very slowly reform the plaster surface even under ideal conditions. Statistically, a calcium or carbonate that is knocked loose from the plaster and dissolves won't necessarily be replaced and instead some additional calcium carbonate growth can occur elsewhere. Nevertheless, there will not be any net gain/loss overall and in practice any microscopic pitting and scaling that occurs will be relatively minor though it does occur and may be why re-plastering (or other surface re-smoothing) is required after some decades.

When water is out-of-balance, then the rate is proportional to the product of concentrations. A saturation index of 0.3 is a factor of 2 difference in that product while 0.6 is a factor of 4 and 0.9 is roughly a factor of 8. So a saturation index of -0.3 will have the plaster dissolving twice as fast as it is reforming so if the equilibrium rate is R, then the net dissolving rate is R-R/2 = R/2. If the saturation index is -0.6, then we have R-R/4 = (3/4)R which is 1.5 times faster. However, you can see that this is limiting in that an infinitely negative saturation index would have the rate of dissolving of plaster be R, the equilibrium back-and-forth rate, but going only in one direction. This is the situation that occurs when there are no catalysts to the reaction, so would be what would happen if there were no calcium or carbonate in the water, but everything else (such as pH even at the surface) were kept constant. If we designate R as the equilibrium reaction rate, then the following table shows the net rate of dissolving plaster or of scaling as a function of saturation index which is (10^CSI - 1)*R:

Saturation Index ....... Net Rate . Effect
....... -inf ....................... -R ......... baseline rate of plaster degradation AT A GIVEN pH (since it's a catalyst)
....... -2.0 ...................... -0.99*R . so roughly 1% longer plaster life compared to no CH
....... -1.0 ...................... -0.90*R . so roughly 11% longer plaster life compared to no CH
....... -0.6 ...................... -0.75*R . so roughly 33% longer plaster life compared to no CH; in practice, this is when onBalance started to see degradation
....... -0.3 ...................... -0.50*R . so roughly 2x longer plaster life compared to no CH
....... -0.2 ...................... -0.37*R . so roughly 3x longer plaster life compared to no CH
....... -0.1 ...................... -0.21*R . so roughly 5x longer plaster life compared to no CH
......... 0.0 ...................... 0 .......... in theory infinite, but in practice the surface gets rough even though its average thickness may not change
....... +0.1 ...................... +0.26*R
....... +0.2 ...................... +0.58*R
....... +0.3 ...................... +1.00*R . in practice, this is when scaling may be seen in hot spas
....... +0.6 ...................... +2.98*R . in practice, this is when scaling may be seen in pools
....... +1.0 ...................... +9.00*R
....... +2.0 ...................... +99.00*R

Remember that the above is for rates at the same pH (at least in the range of pH where hydrogen ion catalytic effects play a role), so varying CH and TA but keeping the pH constant. Note that the above rates for positive CSI are for scaling, not precipitation which can also occur more rapidly (depending on seeding). The above table is why some people emphasize the importance of having the saturation index be as close to zero as possible since a saturation index of -0.3 is already halfway to the maximum dissolving rate and +0.3 is double that rate for scaling at a given pH (this ignores buffering and local effects that I describe later). The above rates ignore effects such as diffusion that limits rates significantly unless there is water circulation, so longer pump run times would be expected to degrade plaster faster in areas of good circulation under conditions of negative CSI.

Hydrogen Catalysis
In practice, there are catalysts to these rate-limited reactions. Specifically hydrogen ion is a catalyst and explains why low pH can more rapidly dissolve plaster. It is due to the following reaction:

CaCO₃(s) + H⁺ <---> Ca²⁺ + HCO₃^-
-or-
CaCO₃(s) + H⁺ <---> Ca²⁺ + CO₃^2- + H⁺

where you can see that the forward (dissolving) rate is proportional to the hydrogen ion concentration while the reverse (scaling) rate is proportional to the product of calcium and bicarbonate concentrations. At higher pH the catalysis is less important and the regular reaction rate in the saturation index table I gave earlier dominates. We know that the above is in equilibrium with calcium carbonate saturation so the hydrogen ion is in effect acting as a catalyst to speed up the dissolving/scaling reactions and I show this more explicitly in the second reaction above. This is why low pH can so quickly dissolve calcium carbonate. Low pH alone is not enough -- the saturation index must also be in the right direction, but it does mean that a negative saturation index will be dissolving plaster more quickly if this is at lower pH. It is possible that a positive saturation index at lower pH would also form scale more quickly though this isn't directly from hydrogen ion but rather from having more bicarbonate ion at lower pH. This may explain people's experiences with seeing scale occur faster at high CH and TA even when the pH is normal or a little low vs. having the CH and TA be normal or low but the pH being high. The following table shows how pH affects the rate due to the hydrogen ion concentration where I arbitrarily define the rate at a pH of 7.5 as 1 and I assume that the rate from catalysis dominates throughout the pH range listed (in practice, the un-catalyzed reaction rate will dominate at higher pH):

pH .. Relative Reaction Rate
9.0 ............... 0.03
8.5 ............... 0.10
8.0 ............... 0.32
7.8 ............... 0.50
7.5 ............... 1.00
7.2 ............... 2.00
7.0 ............... 3.16
6.5 ............. 10.00
6.0 ............. 31.62
5.0 ........... 316.23
4.5 ......... 1000.00 (where TA test turns red immediately)

pH Buffering and Local Effects
In practice, the actual dissolving of plaster or of scaling changes the water chemistry itself, especially locally, and this self-extinguishes or slows down the reaction. When plaster dissolves, it increases the concentration of calcium and carbonate ions and with the carbonate buffer system the net effect is mostly an increase in calcium ions and in pH. This has the saturation index rise so tends to slow down or stop the dissolving of plaster. The exact opposite occurs for scaling which is also self-extinguishing in the same way. What keeps the reaction going is circulation of the pool water to dilute the local effects and return the calcium and pH levels back to their bulk water values (or close to them). Even when there is no circulation, there is still diffusion though that is much slower. This implies, however, that out-of-balance water will be more destructive when there is more circulation.

Ironically, if the water is more highly pH buffered, then the pH will change less so there will be less self-extinguishing of dissolving plaster or scaling. To account for these effects, there is another index called the Calcium Carbonate Precipitation Potential (CCPP) which calculates the amount of calcium carbonate that must dissolve or precipitate (scale) in order to bring the saturation index to 0 (both the CCPP and the CSI are in my PoolEquations spreadsheet). For example, using 50 ppm Borates at a pH of 7.5 would cause over 3x more calcium carbonate to dissolve (from a low 50 ppm TA and 100 ppm CH combination for saturation index -0.8). Of course, the benefit of the pH buffer is that it will tend to keep the pH stable and that's a good thing if the saturation index is already near zero. Changes in pH affect the saturation index more directly than any other factor.

Temperature Dependence
I don't know the activation energy for the reactions which would tell us the temperature dependence for the reaction rates. I do know that for many of the chlorine reactions, there is a rough doubling of reaction rate every 13ºF (7.2ºC) implying around 17 kcal for activation energy which is somewhat higher than the rule-of-thumb that is that reaction rates double for every 10ºC (18ºF) implying around 12 kcal for activation energy. Using the chlorine reaction rate dependence (just to have something), this implies the following relative reaction rates where 80ºF is arbitrarily set to 1.0:

Temp (ºF) . Relative Rate
.. 100 ............... 2.90
.... 90 ............... 1.70
.... 80 ............... 1.00
.... 70 ............... 0.59
.... 60 ............... 0.34
.... 50 ............... 0.20
.... 40 ............... 0.12

Conclusion
The main takeaway is that pH is the greatest factor determining the rate of dissolving of plaster and of scaling. Low pH is very detrimental to plaster since it directly lowers the saturation index AND increases the reaction rates by acting as a catalyst. High pH is a direct factor for scaling, but since it has less catalytic effect a high CH and/or TA are probably likely to scale more rapidly at the same saturation index compared to a high pH alone (and between CH and TA, a high TA will scale more due to the greater pH buffering). The saturation index is a logarithmic scale so keeping the saturation index near zero is important, especially when other factors such as high temperature, low pH, greater circulation and increased pH buffering are all increasing reaction rates or lessening self-extinction effects.

 

[chem geek]

My past experiments have shown that maintaining water within a range of -0.6 to -1.1 for the CSI has led to the dissolving of about 7-8 ppm of calcium per month with quality made plaster coupons.

Do you recall the pH for those experiments? It would be helpful to have dissolving rates for low CH or low TA where the pH was still at a reasonable 7.5. I suspect that at such a moderate pH but with a low saturation index there would still be plaster deterioration, but at a slower rate.

 

[mas985]

My past experiments have shown that maintaining water within a range of -0.6 to -1.1 for the CSI has led to the dissolving of about 7-8 ppm of calcium per month with quality made plaster coupons.

Also, what was the surface area of the coupon and the volume of the water for this experiment? Or more importantly, do you know how much of the plaster surface depth was lost?


It still sounds like an emperical model might be possible even though it may not capture all of the effects. Having an answer with a large error may still be better than no answer at all. Anyway I think everyone would agree that a pool mairtained at an average CSI of 0 will have a surface that lasts longer than one at -0.6. However, what is not entirely clear is how much longer and is it significant. Also, how does the variance in CSI determine surface life. If two pools had exactly the same average CSI but one has twice the variance as the other, will the lower variance pool's surface last longer and if so, by how much?

 

[onBalance]

Mas985, one experiment had the plaster coupon surface area of 1.5 square feet in 28 gallons of water. The other experiments had 40 square inches in 4.5 gallons of water. That should correlate to typical residential pools. If not, let me know.

Richard, for my "Aggresive water" experiments, the pH was maintained between 7.2 and 7.8 for six months.

Years ago, in my soft tap water service area, I experimented with two new startup pools (in 1974) and allowed both the CH and TA to remain very low (about 50 ppm for both) for one month, and had the pH at 8.6 to 9.0. Those two pools developed significant scale. I know all this is just anecdotal and not empirical and not scientific, but that is what I observed. Of course, new plaster pools are a whole different situation. Perhaps someday I will do an experiment on this for verification.

 

[mas985]

Mas985, one experiment had the plaster coupon surface area of 1.5 square feet in 28 gallons of water. The other experiments had 40 square inches in 4.5 gallons of water. That should correlate to typical residential pools. If not, let me know.

Those are pretty close to a typical pool.

The only missing piece is how much CaCO3 is in typical pool plaster. Anyone know the ratio by weight?

 

[onBalance]

Mark, that may be difficult to pin down.
Typical pool plaster is made up of 1 part cement and 1.5 to 2.0 parts aggregate (sand). The aggregate is usually calcium carbonate (limestone), but sometimes quartz is used instead. So that means about 60 to 70 percent is calcium carbonate.
About 20 percent of hardened cement becomes calcium hydroxide, and at the surface the calcium hydroxide is converted into calcium carbonate. I believe that some calcium hydroxide below surface actually travels to the surface and is also converted. It appears from microscopy pictures that the major portion of the surface is calcium carbonate.

 

[chem geek]

Mas985, one experiment had the plaster coupon surface area of 1.5 square feet in 28 gallons of water. The other experiments had 40 square inches in 4.5 gallons of water. That should correlate to typical residential pools. If not, let me know.

Richard, for my "Aggresive water" experiments, the pH was maintained between 7.2 and 7.8 for six months.

Years ago, in my soft tap water service area, I experimented with two new startup pools (in 1974) and allowed both the CH and TA to remain very low (about 50 ppm for both) for one month, and had the pH at 8.6 to 9.0. Those two pools developed significant scale. I know all this is just anecdotal and not empirical and not scientific, but that is what I observed. Of course, new plaster pools are a whole different situation. Perhaps someday I will do an experiment on this for verification.

A 16' x 32' x 4.5' (avg. depth) pool will have roughly a surface area of 944 square feet, ignoring rounded corners, openings for returns, skimmers and drains, and tile. The volume is 2304 cubic feet (17,235 gallons). So the surface area to volume ratio is 0.41 (and as noted, it's actually somewhat less than this) while your plaster coupon ratios are 1.5/3.74 = 0.44 and (40/144)/0.60 = 0.46 so are very reasonable. A 14' x 24' x 4.5' pool with 11,311 gallons has a surface area to volume ratio of 678/1512 = 0.44, for example.

Thanks for confirming that the pH was within normal range in your low saturation index tests. That shows that the saturation index is indeed important and that simply keeping the pH, TA, and CH somewhere in typical recommended ranges is not sufficient if the saturation index is too far away from zero. I know that there are those who disagree with me on this, but I really do believe that with all the various errors in measurement, one really should try and target a saturation index closer to zero when possible, if one wants their plaster to be in great shape for many years. Just because one doesn't notice deterioration in a year or two doesn't mean that it isn't degrading more than could be prevented over a decade or more.

The soft tap water situation is interesting. The saturation index at the pH of 8.6 at a temp of 80ºF is +0.18 while at a pH of 9.0 it is +0.52. At cooler water temps one would presume for fill water before heating (say, 65ºF), the saturation index is +0.05 to +0.41. However, with curing at the plaster surface, the pH will be much higher as will the CH so I can certainly see how scaling could occur. Since the pH is so high, it's probably rate-limited by the over-saturation amount, but unlike the plaster dissolving case which has a rate limit (at a given pH) due to the fixed concentration of solid calcium carbonate, the rate for scaling has no such limit since one can be way over-saturated (remember that +0.3 is a factor of 2 in concentration). I've corrected the saturation index to reaction rate table in my post above to reflect this. For example, if curing in a 10,000 gallon pool produced 0.83 pounds of calcium carbonate, then this would raise the pH in your conditions from 8.6 to 9.55 and the TA and CH each raised by 10 ppm. The saturation index would go from +0.05 to +0.95. The low TA has less pH buffering so plaster curing creates a greater rise in pH and therefore a greater likelihood for scaling compared to the bicarbonate startup where even though the saturation index may start out > 0 (for the case of CH 200 where the TA is raised to 300 so at pH 7.8 the CSI is around +0.5), the high TA helps to buffer the pH preventing it from rising as much and in my example it would only rise to 8.2. At a CSI of around +0.9 the water is over-saturated by a factor of 8 so the net rate of scaling (accounting for the dissolving rate) is about 7 times the "natural" back-and-forth rate of dissolving and scaling for balanced zero CSI.

What your anecdotal story may show is that though low pH catalyzes the reactions to potentially have faster dissolving of plaster and, at very high TA and/or CH, faster scaling, while at higher pH the catalysis is a slower effect than the main reaction. The key question, of course, is "at what pH are these effects roughly equal?" Also, figuring out the rates of scaling and dissolving can let us roughly model this as Mark is desiring, but it would only be a rough model due to effects such as circulation that I've described earlier. Nevertheless, even that can be modeled in a tank with pumps running on a schedule, such as 8 hours per day. Also note that it may be possible to significantly accelerate your experiments by using higher CSI (at a given pH and temperature and circulation) and noting the rate of scaling. Of course, doing the longer dissolving tests is still important for confirmation of the theory, but once confirmed doing faster scaling tests could check for various pH, temperature and circulation conditions much more quickly, taking hours to days instead of months, so long as we don't go so far as to produce precipitation or cloudiness.

 

[mas985]

Assuming I did this correctly, the test would imply a life span of around 138 years for a 1/4" thick plaster coating. I guessed at the plaster density. Does anyone have a better number?

CSI Range: -0.6 to -1.1
Coupon Size (sq-ft): 1.5
Volume (gal): 28
PPM rise: 7.5
Test Duration (days): 30
Plaster Density (lbs/cu-ft): 145
Initial Plaster Depth (in): 0.25
Plaster CACO3 %: 65%

CACO3 Lost (lbs): 0.0018
Plaster (lbs): 0.0027
Plaster Lost (cu-ft): 1.86E-05
Plaster Surface Lost (in): 1.49E-04
Plaster Surface Lost (%): 0.059%
Plaster Life (years): 138.22

So either that CSI range is insignificant in terms of plaster loss or circulation is a much bigger factor. BTW, did the test tanks have circulation?

 

[chem geek]

Assuming I did this correctly, the test would imply a life span of around 138 years for a 1/4" thick plaster coating. I guessed at the plaster density. Does anyone have a better number?

I get the same results you do using your density number which seems reasonable since calcium carbonate as calcite is 2.71 g/cm³ or 169 pounds per cubic foot -- if it were half as dense, then the thickness deterioration rate would be doubled. So if you were looking at an even rate of dissolving with the surface simply diminishing, then you would be right that it would take a long time to erode 1/4" (6.35 mm) in thickness. However, in practice, the dissolving results in pitting and an uneven surface which probably becomes objectionable long before 1/4" of material is removed. A pitted surface can also harbor algae more readily (if any spores survive long enough to get to such pits). Also, I suspect that once pitting starts that there are physical erosion effects that occur in the uneven surface that is far less rigid and stable. The surface area also greatly increases with pits so the dissolving may accelerate over time (though we should be able to measure that as an increasing CH rise rate). Finally, the experiments used well-made plaster coupons. The rate of deterioration may be much faster with poorly made plaster that is softer to begin with so more rapidly dissolving via physical removal (i.e. more calcium hydroxide remaining so less rigidity of the overall structure and when pitted much greater surface area).

Nevertheless, based on this experiment, even 1 mm of deterioration (which I would think would feel rough, if uneven) would take a little over 20 years if the rate were constant. I'm guessing that the conditions used in the experiment over an extended period of time might reduce plaster life to the 10-15 year range instead of lasting up to 20-30 years if ideally maintained (again, just a guess), at least for well-made plaster. It may be that an acid wash and/or sanding would be sufficient and that a full re-plastering would not be necessary. I can tell you from personal experience that a small low-profile scale nodule only 1/2 mm high was very objectionable on the feet and I needed to use a sanding stone tool to carefully smooth it down. It formed on our pool's ramp platform that has only 5-10 mm of water over it so tends to have poor circulation and high pH from outgassing.

 

[onBalance]

No, the test tanks did not have circulation. I did however, brush each coupon with a regular plastic brush once or twice a week while submerge in the water.
One more thing, I reviewed my notes and realized that the LSI was maintained between -0.6 and -1.1 for the first month, but for the next five months, the LSI averaged about -0.4 to -0.9. The 8 ppm monthly loss of calcium should be based on those figures instead.

 

[chem geek]

In the "What am I missing? Everything seems good except CSI" thread the issue has come up regarding the low CSI levels that occur when following the recommended ranges in SWG pools, especially in colder water. I am less concerned with what happens when the water is colder since all reaction rates are slowed down and normally the pH is higher when the water is colder (though people need to be reminded of that and not artificially lower their pH as the water gets colder -- just keep it from getting above 7.8 mostly to prevent metal staining).

The SWG plaster pool recommendations have a lower TA range, a higher CYA range (which lowers the carbonate alkalinity range), and has a higher salt level. All three of these lower the CSI. So let's take a look at the CSI level for the low, mid and high ends of the current Recommended Levels (low/mid/high with respect to CSI effect, that is). I will use a temperature of 84ºF.

Bleach (non-SWG) plaster pool has a CSI from -0.23 to +0.35 with a mid-point of +0.07
SWG plaster pool has a CSI from -0.66 to -0.01 with a mid-point of -0.31

Though we do want to have the CSI be more on the negative side in SWG pools to help prevent scaling in the salt cell (though 50 ppm Borates really helps prevent that as well), it makes no sense to have the pH and especially CH recommendation be the same in an SWG pool given how three other parameters have changed to lower the CSI. If we change the pH recommendation to be 7.6 to 7.8 since the pH tends to rise in SWG pools anyway and higher pH tends to be more stable and if we change the CH range to 350-450 then we would have the following

SWG plaster pool has a CSI from -0.43 to +0.09 with a mid-point of -0.15

This seems more rational and reasonable to me and would still allow us to just say "if possible, keep the water parameters in the recommended range" without having to worry about the CSI so much. Note that the use of 50 ppm Borates lowers the CSI by about 0.1 as well and is not shown in the numbers above.

 

[mas985]

I find it hard to keep CSI on the negative range because of PH rise but if I get my acid dosing dialed in I can usually keep it fairly stable. But then when the I reduce run time, have water temp changes, or change SWG setting for winter, I have to reset the dosing so there tends to be some swings and my average is usually on the plus side.

What would interest me more is to know with some more precision, how sensitive a pool is to CSI swings. If a plaster pool has an average monthly CSI of 0 but still has swings of +-1 during the month, is that still ok or over the long term would that still cause issues? Also, if I do a measurement and I get a -1 CSI, is it better to adjust for a +1 CSI to average out for a while or just go with 0 CSI to prevent any further issues?

 

[chem geek]

As Kim's experiments have shown, with good plaster the rate of deterioration from a low CSI is still slow so short-term CSI swings aren't a big deal. Low pH would seem to accelerate the rate of damage from a negative CSI so one would want to minimize the combination of low pH with low CSI. This is at least part of the reason why the lowering TA procedure doesn't have one go below around 7.0 in pH, though that's also because most test kits don't measure lower pH. Still, we've seen some Trichlor pools with zero TA and an implied pH of < 4.5 where there was, in some cases, plaster deterioration but it isn't in a matter of days, but rather over months. We've also seen degradation when people pour acid in one place too quickly on a regular basis, not in a return flow, and without brushing, but such pooling of acid can have far lower pH since even a 100:1 dilution of Muriatic Acid is a pH of 1.0. So when we talk about the more typical CSI ranges especially with pH that isn't low, we are really talking about very long-term multiple-year (even decade) effects and a target to prolong life of plaster over the very long-haul.

If one goes negative, then I don't think it makes much sense to go positive to try and balance it out since any scaling wouldn't just "fill in" where any degradation occurred. The dissolving and reforming of plaster is a random event and you can't repair the damage by overshooting. You'll just end up with more peaks and valleys in the plaster surface. If the CSI is too low or too high, the proper prescription is to get it closer to zero and keep it there. If one is dealing with unavoidable swings, then the best thing to do would be to try and center such swings roughly around the zero point to minimize the plus and minus extremes.

Because many pool's are dynamic with swings in pH and since the test kits have limited accuracy, this is why I believe one's target parameters should be at or near a zero CSI. This gives one leeway in case test parameters are biased and to minimize the effects of pH swings. It's not so much that a -0.3 is going to be bad, but rather that the true saturation index may be lower due to measurement errors and parameter swings.

 

[MattM]

I'm wondering how to translate this to situations where the pool owner has more control of ph (e.g. intellichem) and the pool is covered with an SWG that is only activated as needed - ph will stay within a +- 0.1 range with perhaps 0.05 accuracy while the pump runs and CYA is limited to 30. SWG runs at 100% for perhaps 1hr/day. CSI gets calculated by the controller every few minutes.

In this specific case, especially with pebble/plaster warranty (CH>380 is tough sell) and fill water has high CH, I'm considering just using TA to "center" the CSI...increasing/lowering TA as needed to get closer to 0 csi.

 

[bobodaclown]

I hope I'm not out of base here although I may be over my head. I've got a SWCG on my pool my TA is around 80 and I fight to get it down to 70 or so. I have added borates, and finally got my CYA down to 75 ish. I constantly fight the PH rise (major contributor to CSI). I bit the bullet and bumped my CH to 525 to get my CSI into a more acceptable range not less than -.6. My water/pool looks good and I haven't seen any scale on the SWCG plates (it does polarity reversal). The SWCG has been installed 8 months and hasn't required a cleaning.

I hope this is useful to the conversation. If not, I won't take offense if it's removed.

 

[chem geek]

I'm wondering how to translate this to situations where the pool owner has more control of ph (e.g. intellichem) and the pool is covered with an SWG that is only activated as needed - ph will stay within a +- 0.1 range with perhaps 0.05 accuracy while the pump runs and CYA is limited to 30. SWG runs at 100% for perhaps 1hr/day. CSI gets calculated by the controller every few minutes.

In this specific case, especially with pebble/plaster warranty (CH>380 is tough sell) and fill water has high CH, I'm considering just using TA to "center" the CSI...increasing/lowering TA as needed to get closer to 0 csi.

I'm a little confused here. For an outdoor residential pool exposed to sunlight, with an SWG you would normally have the CYA closer to 80 ppm so that you would lose less chlorine to sunlight (even with a proportionately higher FC level) so that you can turn down your SWG on-time saving on electricity, cell life, and lessening the rate of pH rise. If I assume that you are talking about a pool not exposed to sunlight or a commercial/public pool where the SWG is sized up large enough, then you can more readily dial in a set of parameters that keeps the CSI close to zero (perhaps only slightly negative -- such as -0.2). If you want to minimize the amount of acid you need to add to combat pH rise, then you'll want the TA lower and that implies that something else will need to be higher -- either the pH target or the CH level or both. Note that the acid addition over time will have the TA drop (so will need baking soda to replenish it) unless you've got a lot of evaporation/refill with fill water that is high in TA.

I can understand a pebble/plaster warranty requiring a minimum CH level, but they actually void the warranty if it's too high? That's odd since I would think they would be more concerned about dissolving plaster and less concerned about possible scaling. At any rate, you can only do what you are allowed to do if you want to stay within warranty.

The thing that most of these plaster folks don't seem to get is that the higher salt levels (which is higher TDS) increases ionic strength which lowers the saturation index by around 0.2 units compared to non-salt pools. This shows up in every saturation index formula, including the one used by APSP, yet they don't seem to change their ranges or recommendations to accommodate this.