Calcite Saturation vs. Calcium Level for Plaster Protection

[chem geek]

The pool industry has historically used the Langelier Saturation Index (LSI) for plaster/gunite/grout pools to try and prevent the corrosion of such surfaces that contain calcium. I won't go into why the currently calculated LSI isn't correct nor why the Calcite Saturation Index (CSI) used in my spreadsheet and in Jason's Pool Calculator sticky is better. Instead, I want to write about a more fundamental assumption that "water that is saturated with calcium carbonate will not dissolve pool plaster/gunite/grout surfaces". I believe that this is only approximately true, but isn't the whole picture.

First of all, pool plaster isn't real plaster anyway. "Plaster" usually refers to Plaster of Paris and is calcium sulfate semihydrate. This does not appear to be what is used in pools. Instead, the underlying gunite is mostly a combination of aggregate (stones) and cement. The smoother "plaster" applied over the gunite is also closer to concrete using cement as a binder with far less aggregate. Though some of the aggregate may be limestone (calcium carbonate), the cement is mostly tricalcium silicate that gets cured as described by the following equation:

2(Ca₃O•SiO₄) + 7H₂O --> 3CaO•2SiO₂•4H₂O + 3Ca(OH)₂ + heat

There is also an equilibrium between the calcium oxide and the carbonates in the water via the following equation:

CaO + CO₂ --> Ca²⁺ + CO₃^2- + heat

where the reverse of the above reaction is how the precursor to cement is made in a kiln starting with limestone (CaCO₃) -- and yes, you can see that carbon dioxide is released making cement production a significant contributor to adding this greenhouse gas to the atmosphere. [EDIT] The above reaction is very thermodynamically favorable and in fact pure calcium oxide absorbs carbon dioxide from the air and reacts in water so the above reaction is only inhibited in pool plaster by the calcium oxide being strongly bound to the silicon dioxide in a crystal structure. [END-EDIT]

So, interestingly, having the product on the right hand side be as high as possible will have the dissolving of calcium oxide be as low as possible and this product can't be higher than the solubility product of calcium carbonate or else scaling will occur. So this is one basis for using a calcite saturation index (the other being the presence of limestone, calcium carbonate, in the aggregate). However, notice that on the left hand side of the equation is dissolved carbon dioxide gas. This is higher at higher TA levels (and lower pH levels) so this implies that, all else equal, having calcium carbonate saturation would be better at lower TA levels (and higher pH levels) so as to keep the dissolved carbon dioxide lower. This means that having a higher CH and lower TA (or higher pH) might be better at maintaining cement (calcium oxide) surfaces. This is consistent with anecdotal evidence that higher CH, lower TA works better in preventing dissolving of grout (which also contains cement).

Since higher TA levels tend to have a greater tendency for the pH to rise in pools using pH neutral sources of chlorine (when accounting for chlorine usage -- so bleach, chlorinating liquid, Cal-Hypo, SWG), the above discussion is just another reason why having a low TA with higher CH to compensate makes sense. In practice, one would want a slightly corrosive (perhaps -0.2 or so) saturation index anyway so that the (30F) higher temperature in heaters and the higher pH at one of the SWG electrodes doesn't scale so readily. At a pH of 7.5, in a non-salt (500 ppm salt) pool a TA of 50 (with CYA of 30) would need a CH of 400 to be around -0.2 in saturation index. For an SWG pool (3000 ppm salt), a TA of 75 (with CYA of 80) would need a CH of 500 to be around -0.2 in saturation index. It is not a serious problem to not be at these levels -- these are merely targets.

Richard

 

[Guest]

Richard,
At first glance this does not seem to be inline with what OnBalance is recommending on their website with their 'bicarb startup method' for new plaster. but on second look it appears to since the idea behind this method is to promote the formation of an even deoposit of calcium carbonate right on the surface of the plaster instead of forming calcium hydroxide that leaches in the water and then forms calium carbonate preicpitate in the water (the calcium 'dust' that is commenly seen in new pool start ups.) I also find it interesting that they imply that pH is the main factor (ONLY factor?) in determining scale formation in a pool, which is what I have been saying for some time now.

You give some target values for maintaining a low TA while staying basically balanced by the LSI. I find the recommened calcium values to be on the high side, particularly for a salt pool, and will most likely lead to scaling under real world conditions. My suggestion would be to run the pH a bit higher (target 7.6 and not let it rise above 7.8 which is an easy window of pH to maintian. Most of my customers have no trouble doing so!) This slight rise in pH will aloow lower CH levels and still maintain calcium saturation balance. I still do not feel that strict adherance to calcium saturation is that critical under real world situations where water parameter can and WILL flutuate. I am uncomfortable with CH above 400 ppm (less for a salt pool) if it is avoidable because I have seen the scaling occur on too many occasions when calcium levels were elevated only becuase pH is the one factor that is predictive of scaling and pH is one water balance parameter that does NOT stay stable for very long in real world conditions.
Your thoughts on this are welcome and, most of all, desired!
Evan

BTW, here is the link to their PDF of the method. I find it interesting that they even mention the use of borax in final water chemistry adjustements! As you may well know OnBalance is an offshoot of the now defunt (to the best of my knowledge) JSPSI.

 

[chem geek]

I agree with you that if the pH is unstable and tending to rise, as is often the case with SWG pools even after lowering the TA a bit to reduce this tendency, then just a 0.2 rise in pH completely wipes out the -0.2 of the index and in the high pH area around the electrode producing hydrogen gas, scaling could occur more readily. So what you are saying is quite practical. Having a pH target of around 7.7 along with the lower TA is even more stable for pH while still providing enough saturation. [EDIT] (deleted incorrect sentence) [END-EDIT]

I also had the sense of the pH wrong (in what I initially wrote and I've now corrected it) with regard to CO₂ concentration so actually a higher pH is better. So the combination of lowering the TA and raising the pH accomplishes the same thing as lowering the TA and raising the CH. So certainly for SWG pools a low TA higher pH makes a lot of sense and still has the theoretical benefit of reduced pool plaster and grout corrosion.

I'm definitely being picky and out on a limb with what I wrote. It's not intended to be an absolute change in recommendation, but rather another factor to consider. I'm thinking that perhaps in an SWG situation where a higher pH is desirable to help mitigate the pH rise problem, that one wouldn't follow the advice I laid out. It might be more doable for those without an SWG, but again it's not a big deal and certainly is just THEORY and not proven. The only reason I started going down this path was to try and understand where the heck this rule came from and also try and understand some anecdotal evidence of grout degradation that some PBs mentioned on other forums where they didn't see the problem in higher CH pools, all else balanced normally.

Richard

 

[JasonLion]

Almost all instances of visible scaling are associated with very high PH values. Just how high the PH gets in those cases is difficult to know since the common tests don't work above 8.0 (or sometimes 8.2). I assume the PH has gone noticeably higher than 8.2 in the clear majority of these cases. The high PH is most commonly a result of either plaster curing or aeration combined with a lack of attention to maintaining proper PH levels. In these cases the LSI/CSI is irrelevant, as the pool owner is not testing the water frequently enough and almost any starting LSI/CSI value will quickly become scaling as the PH rises.

Perhaps the only situations I know of where knowledge of balancing LSI/CSI has proven obviously useful are for people with unavoidably extreme chemistry, typically very high CH levels caused by arid conditions and high CH fill water. In those case knowledge of LSI/CSI gives the pool owner some guidelines on how to manage their water chemistry to avoid scaling.

Quite separately, my assumption has been that keeping the LSI/CSI at least somewhat balanced will influence the lifetime of the surface over many years. This assumption is much more difficult to test, as it is almost always impossible to know how many extreme PH swings actually occurred over a decade or two of surface lifetime.

 

[Guest]

Almost all instances of visible scaling are associated with very high PH values. Just how high the PH gets in those cases is difficult to know since the common tests don't work above 8.0 (or sometimes 8.2). I assume the PH has gone noticeably higher than 8.2 in the clear majority of these cases. The high PH is most commonly a result of either plaster curing or aeration combined with a lack of attention to maintaining proper PH levels. In these cases the LSI/CSI is irrelevant, as the pool owner is not testing the water frequently enough and almost any starting LSI/CSI value will quickly become scaling as the PH rises.

Perhaps the only situations I know of where knowledge of balancing LSI/CSI has proven obviously useful are for people with unavoidably extreme chemistry, typically very high CH levels caused by arid conditions and high CH fill water. In those case knowledge of LSI/CSI gives the pool owner some guidelines on how to manage their water chemistry to avoid scaling.

Quite separately, my assumption has been that keeping the LSI/CSI at least somewhat balanced will influence the lifetime of the surface over many years. This assumption is much more difficult to test, as it is almost always impossible to know how many extreme PH swings actually occurred over a decade or two of surface lifetime.

My point exactly. We do not really know in real world conditions the variations that occur in water balance. This is where empirical evidence becomes more important and that evidence points to ph control as one of the most important aspects of water balance as long as CH and TA are in the ballpark. Of these latter 2 TA is probably the more important to keep at a certain range depending on whether the sanitizer source is acidic (stabilized chlorine and bromine) or not (unstabilized chlorine, including SWGs). Even in cases of extreme water balance such as very high calcium levels I do not believe that knowing the CSI is that important. What is important in such a case is to keep the pH down and lower the TA. Both these actions will help prevent scaling conditions. Corrosive conditions (in terms of calcium) are much harder to predict and both the Langalier and Ryznar indices have proven ineffective at that task in real life conditions. The CSI that is used in your calculator and in Chemgeeks spreadsheet is actually unproven as to whether it can predict corrosive conditions (and to the best of my knowledge is just a modified Langelier index. Please correct me if I am wrong on this!)

I am speaking about real world conditions in pool care. Many pool owners, even those that have access to good water testing, often do not do it often enough to correct problems before they happen (as witnessed by the activity on this and the several other pool forums and boards on the net. If testing is carried out at the recommended frequency of daily checks of sanitizer and pH, weekly checks of TA and weekly to monthly checks of CH and CYA then many, if not most, pool problems would be caught and corrected before they even occur!) so there needs to be a margin of safety built into water balancing recommendations and simplification of the procedure for the average pool owner. Hence my recommendation to play closer attention to pH and TA and to not worry about saturation index computations or TDS.

Let's not lose sight that a pool is a recreation activity for most people and not a science experiment. They really want it to be as simple as possible to maintain.

BTW, according to OnBalance, scaling occurs at pH higher than 8.3 because that is the cutoff point of where there is a higher carbonate (and therefore hydroxyl ion) concentration in the water as compared to the bicarbonate concentration. The solubility for calcium bicarbonate is much higher than for some of the other calcium species (in fact, it can only exist in solution, in ionic form!) so to maximize the bicarbonate ion concentration will keep the calcium in solution instead of precipitating out. This is still basically a function of pH!

Actually, the Puckorius Scaling Index might be a better indicator for us than the Langelier, Ryznar, and Calcium Saturation indices since this one takes into account the buffering effects of TA on scale formation and goes along with what we know intuitively, that water can have a high calcium level and yet not exhibit much tendency to form much scale as long as the TA is very low since the precipitation of scale would cause a rapid drop in pH because of the lowered buffering capacity of the water which would then inhibit scale formation.
In salt pools the Stiff-Davis index might prove to be more useful since it takes into account the high ionic strength (or TDS if you prefer) of the water and it's effect on scale formation which is an area that the LSI and CSI both fall short. This particular index was actually designed for brackish waters, which are what we have in a salt pool.

(This is actually getting into some rather advanced and theoretical areas now so it's probably a good place for me to stop before I lose all but a few readers. You know who you are, Richard and Jason! The point that I am trying to make is that the saturation indices applied to pools are really just VERY ROUGH guidelines at best and should NOT be given the importance and attention that they usually are. The fact that the Hamilton index (developed by Jock Hamilton of United chemical to promote high pH pools, which happen to allow all of United's bromine based products to work better than at the lower pH normally recommended in pools) works as well as it does is a testiment to this. This index, on inspection, appears 'to be nothing more than a simplified version of Taylor Technologies' Watergram, which is simply a calculator used to make water balancing based on the LSI easier to apply.)

So many indices, so little time!

This is just one man's opinion.

 

[chem geek]

There are several posts on forums that indicate scaling can occur without extraordinarily high pH, but rather at very high TA or high CH, but it's certainly a wide mix. Generally, the scaling hasn't been seen below a saturation index of +0.7 and it seems that +1.0 is the point when scaling becomes likely on pool surfaces (it occurs at a much lower index point for scaling in SWG cell since the pH is so high near the cathode where hydrogen gas is generated), but there are exceptions. On The Pool Forum, see this post for high TA and unknown but rising pH, this post for scaling at high CH but not a very high saturation index, this post with scaling from high pH over the winter, this post with high calcium, no scaling on walls, but flakes from returns in an SWG system, this post has fairly normal numbers with slightly higher pH and possible scale, but could just be a poor initial plaster job, this post has both high pH and high TA, this post with high CH but other numbers normal, this post with high CH and some scaling, but lower pH, this post with high CH and pH, this post for scaling with normal numbers (though TA was not specified), this post describes scaling with high TA and somewhat high pH, this post with calcium flakes from the SWG with high CH and normal pH, this post with a high saturation index but no scaling or cloudiness.

This post from Ben talks about when corrosion is seen and his take is that it isn't from low calcium, but low pH and low TA. The equation in my first post on this thread would be shifted to the right (i.e. corrosion) most from low pH since (at constant total carbonates in the water) that both increases the amount of carbon dioxide in the water and lowers the carbonate ion concentration as well. A lower TA lowers both the carbon dioxide and the carbonate ion so would theoretically be worse for calcium carbonate than calcium silcate. A lower calcium would theoretically be bad for both but as Ben points out it may need to be quite low before becoming a problem. This post is from Ben about the Langelier Saturation Index with his main points being that 1) errors in measurement mean one shouldn't sweat over being accurate with it, 2) it should be slightly corrosive, if anything, to protect heaters, 3) experience and some calculations show pH and TA being more important than CH (I can see this for pH, but not for TA vs. CH).

The Calcite Saturation Index that I derived has absolutely nothing to do with boilers or heat exchangers in terms of its derivation and it is pretty clear that the Langelier Saturation Index (LSI) was based off of a very similar derivation with the only differences being using logarithmic scales for temperature and TDS that technically should not use such a scale (but it makes for a simpler formula which may be what pool people wanted so perhaps they were the ones who modified his formula). The derivation is shown near the bottom of the Pool Equations spreadsheet (here) and is completely and solely a computation of the solubility product of calcium carbonate. The computations for calcium ion concentration and carbonate ion concentration are a function of pH, TA, CH, CYA (for adjusting TA to get carbonate alkalinity), temperature and TDS, with the TDS used to determine ionic strength (you are incorrect that CSI doesn't take that into account -- it does and even LSI attempts to but does so incorrectly). I even account for ion pairs (ion association model) though that is a minor effect (unless sulfates are very high, such as 400 ppm or more). It has nothing to do with open systems or closed systems. It is a pure solubility solution. The only issue with the open system of a pool is that the outgassing of carbon dioxide and the additions of chemicals (including those from bathers) will affect the chemistry over time, but those are relatively slow processes compared to the chemical reactions themselves and even compared to normal circulation mixing. Finally, I have compared my calculations against the Taylor watergram and they match almost exactly except at very high temperatures well above spa temps, so I suspect the Taylor watergram also used a full model and not the logarithmic approximation of the pool industry LSI. And you are correct that it has nothing to do with metal corrosion but rather predicts the thermodynamic tendency for dissolving of calcium carbonate which may be related to corrosion of pool plaster.

I can tell from chemical cloudiness and from rate of dissolving of calcium carbonate (from adding pH Up too quickly in the past so that calcium carbonate nuggets formed) in my own pool that the index can't be very far off. If I'm over the 0 point, then calcium carbonate just sits on the bottom while under the 0 point it dissolves slowly. This is not the same thing as predicting scaling or corrosion, however, as that involves chemistry at pool plaster surfaces that can be very complex and rate limited, but as a predictor of the dissolving of calcium carbonate, I believe it to be reasonable, at least in the "normal" chemical ranges I've seen in my own pool. Unless we have something more concrete to go by, it seems to be a reasonable index to use at least approximately (i.e. for ballpark).

By the way, I've had both Ryznar and Puckorius indices in my spreadsheet (near line 390). Both of these indices actually use the saturation index as a starting point via the pH_s (the pH at saturation which is just pH minus LSI). Ryznar is 2*pH_s - pH while the Puckorius Scaling Index (PSI aka Practical Scaling Index) uses pH_eq instead of actual pH in the Ryznar formula. See this link for more about various indices (the formula for Puckorius is reversed in the two pH parameters on the website, but their description is correct). This link also shows formulas for the various indices, all of which are based on the pH of saturation and all of which use the same flawed log formula with tables for handling temperature and TDS (ionic strength). This link also talks about the various indices and also has the Puckorius index calculation reversed. Also note that in the discussion about the Puckorius index the low buffer situation does not say that saturated water won't scale but rather that it's capacity is low since the formation of scale lowers the pH (due to low buffering). The LSI or CSI indices never said they predicted the capacity of scaling nor the rate of scaling but rather the (thermodynamic) tendency. Also, I've noticed that the Puckorius index (and even the reversed index that is incorrectly documented) is relatively stable with even large swings in pH which doesn't make sense. It essentially predicts scaling from high CH or TA independent of pH whereas LSI, CSI and Ryzner all have a strong pH correlation.

As for this thread, all I really wanted to get to was a better understanding of what may be happening with pool plaster for both corrosion and scaling and I had recently seen the cement compositions of pool plaster. Also, I wanted to make sure that the lower TA recommendation for lessening pH rise wasn't going to cause any problems. I totally get the practical side of other indices where lower TA buffering would "stop" reactions sooner due to the pH effect (and an example of LeChatlier's principle), but that would be the opposite of what Ben was saying where low TA was more of a problem than low CH.

I looked up the thermodynamic values for calcium oxide conversion to calcium carbonate and it is so strong that pure calcium oxide absorbs carbon dioxide in the air and reacts quickly in water. So it is only the strong binding with the crystal structure of silicon dioxide that prevents the calcium oxide from reacting quickly. Therefore, the calcium carbonate saturation and the CH vs. TA discussion probably play a very minor role in the dissolving of pool plaster. Instead, the reaction is rate-limited by kinetics, not by thermodynamic equilibrium, so the role of catalysts would be far more important and hydrogen ion (higher at lower pH) may play such a role. That would mean that for dissolving of pool plaster, pH is the most important factor. It is only for scaling where the TA and CH play a significant role. To the extent that there is some calcium carbonate leftover in pool plaster (even as limestone aggregate), then saturation with calcium carbonate would be important, but to the extent that it is mostly calcium silicate, then pH is far more important.

Richard

 

[JasonLion]

Even in cases of extreme water balance such as very high calcium levels I do not believe that knowing the CSI is that important. What is important in such a case is to keep the pH down and lower the TA. Both these actions will help prevent scaling conditions.

In these extreme CH level cases it often becomes a question of clarifying just how much to keep the PH and TA down. Simply saying to keep PH and TA down is vague and doesn't always result in people taking appropriate actions. Most people would like some guidelines about how far down they need to keep PH & TA for their particular CH level. Playing with various levels and seeing what CSI they give is a handy way to get an idea of how far down PH and/or TA need to be brought.

The chemistry of plaster is complex and it isn't clear that the CSI tracks all of the relevant chemistry exactly in all of the various cases. Still, there is evidence, for examples see some of Chem Geek's references, that it tracks it well enough to use the CSI calculation to figure out some rough PH & TA targets in these cases.

 

[JasonLion]

Marcite, pool plaster, appears to typically contain portland cement and aggregate. The aggregate consisting of one or more of calcium carbonate, calcium magnesium carbonate, and quartz. A couple of pool plaster suppliers claim to use marble dust exclusively as the aggregate component which supposedly gives the whitest surface, while others mention other ingredients.

I suspect that once curing is complete the primary reactions are between the water and the carbonates. It looks to me like using marble dust vs fine quartz sand would give very different chemistry, with the marble dust being rather more reactive.

 

[chem geek]

After reading the recent Low CSI thread, I want to write some comments about low CSI as well as an additional index called the Calcium Carbonate Precipitation Potential (CCPP) that accounts for the pH buffering from TA as well as the extreme situations in pH, TA and CH that led to the various other indices (Ryznar, Puckorius). Essentially, what these people were trying to capture was the fact that when the calcium or carbonate levels are low, any precipitation proportionately lowers the calcium or carbonate level by quite a bit so is self-extinguishing. Also, if the pH buffering is low due to a low TA (and no CYA, for example), then the pH rise from precipitation will also be self-limiting.

So while the Calcite Saturation Index (CSI) can indicate whether scaling or dissolving COULD occur, it does not indicate how much could occur nor does it indicate how quickly it will occur. The CCPP indicates how much can occur (how much calcium carbonate scale can form or how much calcium carbonate can dissolve) though it still does not indicate how quickly. The CCPP could have been calculated in my spreadsheet by changing values in the "Dissolving CaCO₃" section until one gets the CSI to 0. I have since made this search automatic by adding a "Calculate CCPP" button to my spreadsheet. Different sources have different recommendations for CCPP, but many generally consider < -5 to be mildly corrosive (meaning pipe surfaces are no longer protected with calcium carbonate) with < -10 being more aggressively corrosive and +3 to +5 needed to form a protective layer without too much scale while > +10 tends to form more scale, at least in water distribution systems.

This paper gives a good summary of the principles of calcium carbonate saturation protecting metal from corrosion though as I have noted elsewhere linking to this discussion, this is somewhat controversial. I think this can be somewhat reconciled by considering that getting a more even protective film requires water flow and that doesn't happen in water heaters or even in parts of water distribution systems (i.e. flow is uneven and sometimes stop if there is no demand in one part of the system). Specifically, that paper indicates that three conditions are needed for a consistent protective layer to develop: (1) the water must have a Langelier index close to zero, (2) it must have a significant bicarbonate ion concentration and (3) it must be flowing over the metal surface. The LSI/CSI calculates the first item, the CCPP accounts for the second and since LSI/CSI and CCPP are related in that when one is negative/positive/zero the other is negative/positive/zero, one can just use CCPP. At typical pool conditions, a CSI of +0.1 has a CCPP of around +3.

Of course, none of this so far has considered how quickly plaster will dissolve if exposed to water that is not saturated with calcium carbonate. Such dissolving has competing rates -- one of calcium carbonate leaving the plaster to form ions and another of calcium carbonate ions forming scale on the plaster. All that we know is that when there is calcium carbonate saturation, these two rates are equal. We also know that when the saturation index is -0.3 that this means that the calcium carbonate product is half that of saturation; at -0.6 it is 1/4th that of saturation; at -0.9 it is 1/8th that of saturation, etc. If the rate in each direction at saturation is R and assuming the precipitation rate is proportional to the calcium carbonate product, then at a saturation index of 0 the net rate is R-R=0 while with a saturation index of -0.3 it is R-R/2 = 0.5*R; with a saturation index of -0.6 it is R-R/4 = 0.75*R; with a saturation index of -0.9 it is R-R/8 = 0.88*R. So at a minimum, this implies that the dissolving of plaster at -0.6 saturation index is 50% faster than at -0.3 while -0.9 is 75% faster than at -0.3, ignoring local effects limited by diffusion. However, the negative saturation index could accelerate "R" itself through catalyzing the dissolution process, say by low pH.

In practice in pools we have seen on this forum, scaling usually doesn't appear until the saturation index is at least +0.7 and usually +1.0 or more. I've seen in spas on another forum where scaling would sometimes appear at around +0.3 or more (though this was usually scaling in the heater where the saturation index would be higher due to the higher temperature at the heat exchanger surface). What we don't know is at what level a low saturation index results in significant plaster dissolving, possibly noticed by pitting or rougher/uneven surfaces. There are anecdotal reports from some people that give long-term lives of plaster with rules like -0.1 lasts 30 years, -0.2 lasts 20 years, -0.3 lasts 10 years, but others dispute this and the timeframes are so long as to make predictions difficult. Kim Skinner of OnBalance has written in an industry forum that his tests with plaster coupons show that the saturation of calcium carbonate is more important during the early curing stages of plaster and that once high-quality plaster has largely cured, it tends to be more resistant to dissolving later on, even with water that is under-saturated in calcium carbonate. Since curing of plaster has the pH rise, it is tricky to keep the water near saturation without forming plaster dust or scale, but it's not so much about being fully saturated as not being very under-saturated. I'll see if I can get more specific info from him.

Richard

 

[chem geek]

I received a response from Kim and in his tests on plaster coupons in tanks with the same volume to surface area as in a typical residential in-ground pool, he found that there was some slow degradation of the plaster coupons when the saturation index was -0.6 to -1.0. The Calcium Hardness (CH) in his experiments rose at a rate of around 5 ppm per month (these were closed experiments so no water dilution and no evaporation with refill). I calculated that this translates to an average loss of plaster of an average of around 0.015 mm per year or around 0.15 mm in 10 years. However, that's an average and in practice softer areas with more calcium carbonate and less calcium silicates would pit more so if 10% of the plaster area etches significantly, that could be 0.15 mm per year or 1.5 mm in 10 years in those areas which would certainly be noticeable as a roughed or pitted surface.

So while having the saturation index at -0.6 or somewhat lower for a month or two or even over one season may not be a big deal, it's not a target one would want to have over many seasons if one wants to protect their plaster surface to last as long as possible.

I suspect that having a low pH is more damaging more quickly than simply having the CH and/or TA be low, but more experiments would need to be done to see if this is truly the case. The CCPP probably also plays a role as the etching can be self-extinguishing if near the plaster the pH, TA and CH rise from the calcium carbonate though with good circulation such an area will be quite thin.

 

[Beez]

Thanks for taking the time to post this, it is most helpful.