On salt, plaster and waiting 30 days....

[JoyfulNoise]

Perhaps this idea has been answered and explained more clearly in a another thread but, in case it has not, I thought I would make this quick post.

I had the opportunity to run across this thread from user onBalance back in 2012 about the reasons for waiting 30 days before adding salt to a newly plastered (or replastered) pool - When Should Salt Be Added. The empirical data in onBalance's experiments show that water tanks with curing plaster and salt added immediately showed higher pH levels consistently as opposed to those without salt. Obviously the salt has an effect on the calcium hydroxide in the plaster but no definitive explanation was given.

While I have not purchased it, this paper (J. Am. Chem. Soc., 1931, 53 (11), pp 3976–3991, DOI: 10.1021/ja01362a009) seems to indicate that chloride salts (with various cations) increases the solubility of calcium hydroxide. So perhaps the reason for not adding salt right away to a pool with curing plaster is that the salt increases the solubility of the calcium hydroxide in the pool water and thus leads to an inferior plaster surface finish.

Again, my apologies if there is a thread covering this. I could not find one but that doesn't mean it hasn't been discussed.

Matt

 

[chem geek]

In Table I of that paper the difference in solubility of calcium hydroxide doesn't change very much at the salt level found in SWCG pools. It's 19.76 mmole calcium hydroxide/kg water with no salt, and 20.65 with 0.012 moles salt/kg water which is about 700 ppm salt, and 22.68 with 0.064 moles salt/kg water which is about 3700 ppm. The solubility peaks at 29.66 mmole calcium hydroxide/kg water with 1.21 moles salt/kg water which is about 70,000 ppm so about double the salt level of sea water. The paper is old and attributes changes in solubility to both ionic strength increases and to "the further deformation, in these solutions, of the already highly unsymmetrical hydroxyl ion." However, they apparently did not know about the calcium chloride ion pair, CaCl⁺, which along with ionic strength would account for the difference (there is also an ion pair for calcium hydroxide, CaOH⁺).

The problem with the salt in the water may have less to do with actual calcium hydroxide solubility and more to do with calcium carbonate being more soluble due to the higher ionic strength so that it is slower to form even with the bicarbonate start-up. One could probably overcome this effect by increasing the calcium and/or bicarbonate levels. With a bicarbonate startup at pH 7.8, TA 250 ppm, CH 250 ppm, and around low salt in fill water, the CSI is +0.65 (at 80ºF), but with 3000 ppm salt the CSI drops to +0.44 and would need the TA and/or CH raised to compensate such as each raised to 320 ppm. However, the experiments by onBalanced showed the pH rise higher than 0.2 units so beyond what would compensate for the lower CSI.

There may also be some more direct inhibiting effect of calcium carbonate crystal formation at higher chloride levels (i.e. a rate effect, not thermodynamics) similar to the effect of some scale inhibitors, but that's just a guess and may not be true. I did find this paper that describes the effect of ionic strength on calcium carbonate growth formation in terms of the crystal types that are formed (i.e. polymorphism) and it states the following:

This study reveals that ionic strength in the growth solutions of CaCO₃ is able to influence the polymorphism of CaCO₃ precipitation. The major experimental results follow a trend that more vaterite is found among the precipitation as the ionic concentration increases in the solutions; especially the runs added NH₄⁺. However, for the runs with divalent additives Na2SO4, there is no clear linear relation of the concentration of additives with the percentage of vaterite, a less stable polymorph of CaCO₃.

They only tested up to 0.01 M NaCl which is 584 ppm whereas 3000 ppm would be closer to 0.05 M. Also, the X-Ray diffraction graph of Figure 2B only shows small amounts of the weaker vaterite form of calcium carbonate. Nevertheless, this might be the effect that plasterers are trying to avoid -- they want calcite formation and want to avoid the formation of weaker aragonite and the weakest (least stable) vaterite. The fact that the study showed this trend when using sodium chloride and not with sodium sulfate may mean that it is higher chloride levels that affect the type of crystal formation. Of course, we already know that higher sulfate levels lead to other problems due to it reacting with calcium hydroxide to form gypsum that can react with monosulfoaluminate to form ettringite (see this study for details).

 

[JoyfulNoise]

I missed your updates to the original post. I wish Tapatalk would tell a user when a post has been updated. I will read through the additional links you have provided.

As well, I was thinking about your comment on a kinetic-based explanation and found this thesis with lots of details on cement curing (the context is in geological storage of spent nuclear fuel). In it, there's a lot of discussion of the diffusivity of CO2 being a key factor in the carbonation of calcium hydroxide. Of course, this is air-based curing so relative humidity and gas diffusion are limiting steps. But that got me thinking about the diffusion rates of CO2 in water with and without salt and, sure enough, this paper (sorry, once again I have not purchased it...) seems to indicate that CO2 diffusivities are decreased by salt concentration. As I said, having not looked at the paper in detail, I can not say that this is a significant contributing factor, but it would be interesting to study further.

 

[chem geek]

Here again the effect is very small since the diffusion coefficient changes only from around 1.7x10^-9 m²s^-1 to 1.6x10^-9 m²s^-1 going from 0 salinity to 1 mole NaCl per kilogram water which is around 58,000 ppm.

It seems most likely given the paper on calcium carbonate polymorphism as a function of ionic strength that the effect is from chloride ions that somewhat inhibits calcite formation compared to weaker vaterite though the effect still seemed fairly small. There may also be some effect from the more direct effect of ionic strength on calcium carbonate solubility since it is known that the 0.2 decrease in CSI means it takes 58% more calcium or carbonate (or their product of concentrations) to be at the same level of saturation. However, the experiments done by onBalance do indicate a faster dissolving of calcium hydroxide even when the pH has risen above the 0.2 units to bring the CSI back to non-salt levels so this pH rise may be due to a slower "sealing" effect so slower formation of harder calcite. As I noted in this post the pH rose by more than the 0.2 units needed to overcome the CSI effect.

So the polymorphism effect does seem to be the leading candidate explanation unless my guess in my post about calcium chloride ion pair formation inhibiting calcium carbonate crystal growth is valid. Chloride ions are known to inhibit the rate of formation of the chromium oxide (Cr₂O₃) passivity layer in stainless steel so it is plausible that the already known ion pairs formed may inhibit calcium carbonate crystal growth by having some weak bonding strength of calcium and chloride that makes it take longer to create calcite (calcium carbonate). It seems plausible that a hydroxide to chloride exchange would be easier than two hydroxides to one carbonate exchange. Again, this isn't about thermodynamics of what is ultimately produced (i.e. lowest free energy) but rather about reaction rates where the activation energies or certainly number of reaction steps may change due to the formation of intermediates (e.g. calcium chloride). Essentially, the presence of chloride in a calcium hydroxide/chloride mix may slow down the calcite formation enough to have the calcium hydroxide continue to migrate into the bulk water. This isn't about calcium hydroxide solubility, but rather about the slowing down of formation of calcite that would otherwise seal off the surface of the plaster preventing further calcium hydroxide from diffusing into the water.

This paper describes some possible mechanisms for chloride's inhibition of the regrowth of the passivity layer in stainless steel where they state:

The activity of Cl- ions determines the stability of the passive film and it is explained that it plays its role in two ways [47]. First, it may replace OH intermediate compounds within the film and result in the formation of a soluble Cl^- complex. This replacement results in the self-repairing action of the film and inhibits the breakdown of the passivity in terms of a reduction of the metal dissolution. It has a small effect in the passive and transpassive potential regions [48-50], and its presence increases the efficiency of the passive film formation. Secondly, Cl^- may provide a rapid selective dissolution of metal ion in the bulk when both ions meet at a weak site. The high Cl^- concentration around the preferred weak sites accelerate the dissolution of the metal ion and result in the full development of pitting. Consequently, the effects of its incorporation in pit initiation is limited and is not a necessary condition to thinning or a break in the passive film; but rather it simply interferes with the reformation of the film.

Since there is no passive film formation in the case of plaster, the second effect they describe is what may occur in plaster (though one might consider calcium carbonate to be like a passive film keeping calcium hydroxide out of the bulk pool water). Essentially, the calcium hydroxide dissolution into the bulk water may be normally somewhat slow (it tends to be gel-like) and replacement of hydroxide with chloride accelerates that since the resulting calcium chloride is more rapidly dissolved. Again, this is not about maximum solubility, but about bonding strengths (or activation energies for dissolution) and reaction rates. Whether the effect from the other paper on the formation of weaker vaterite plays a role is an open question that may depend on whether the crystal geometry of vaterite is more readily formed when carbonate replaces chloride rather than hydroxide.

 

[JoyfulNoise]

Since there is no passive film formation in the case of plaster, the second effect they describe is what may occur in plaster (though one might consider calcium carbonate to be like a passive film keeping calcium hydroxide out of the bulk pool water). Essentially, the calcium hydroxide dissolution into the bulk water may be normally somewhat slow (it tends to be gel-like) and replacement of hydroxide with chloride accelerates that since the resulting calcium chloride is more rapidly dissolved. Again, this is not about maximum solubility, but about bonding strengths (or activation energies for dissolution) and reaction rates. Whether the effect from the other paper on the formation of weaker vaterite plays a role is an open question that may depend on whether the crystal geometry of vaterite is more readily formed when carbonate replaces chloride rather than hydroxide.

This I can certainly understand. It brings back old memories as an undergraduate working in a corrosion studies lab and helping the grad students look at the pitting-corrosion effect of microbial-induced corrosion using sulfate-reducing bacteria. The Navy was very interested in studies of different types of steels in salt-water environments and with bacterial biofilms.

In terms of cement, it would make sense that anything modifying or interfering with the proper formation of the calcium carbonate layer (I think it makes sense to describe it as a passive film) would allow for greater dissolution of the calcium hydroxide and increased pH and CH in the pool water regardless of any possible modifications to solubility. Unfortunately, it's not a description that lends itself well to the plaster industry or the general public BUT the fact remains that there is no good reason to add salt right away to a newly plastered pool. Waiting 30 days before adding salt is only a minor inconvenience to the pool owner and the PB but can result in much better quality plaster finish and pool water chemistry.

Thanks for the links and the discussion.

 

[pinguy]

Unfortunately, it's not a description that lends itself well to the plaster industry or the general public BUT the fact remains that there is no good reason to add salt right away to a newly plastered pool. Waiting 30 days before adding salt is only a minor inconvenience to the pool owner and the PB but can result in much better quality plaster finish and pool water chemistry.

My PB said that it is a guideline from the National Plaster Council, but was not able to give me a technical reason. Maybe if someone here has the NPC technical guide ($100), they can see if it cites any further scientific reasoning.

 

[onBalance]

There is nothing in the NPC Technical manual that discusses Salt Pools or when to add salt.

 

[chem geek]

According to this link:

How long do I have to wait before I can add salt to my pool?

"NO SALT SHOULD BE ADDED FOR 28 DAYS" National Plasterers Council, NPC Start Up Card, September 2012 edition

This is seen from this link on the NPC website to this PDF file of their start-up card where on the 2nd page in the 3rd Day section in Step 2 it says "NO SALT SHOULD BE ADDED FOR 28 DAYS."

In any event, onBalance has done experiments demonstrating the need to wait for adding salt. As for the exact technical reasons, we just have been speculating based on the research links discussed in this thread.

 

[JoyfulNoise]

It is rather curious, and a bit sad, that four guys on a pool forum can sit around and figure this stuff out (even if some of the science is merely conjecture at this point) and, yet, there are still reports of people having their newly-plastered pools salted right away by PBs and plasterers that just seem to ignore the literature from their own technical/trade associations!?!? "Buyer beware!" indeed ....