Newbie Fighting Green Aglae Cc 5.2, Tc 0

[quantumchromodynamics]

__________________________________________________

"Where do you get that the most susceptible element in pool plaster is calcium hydroxide?" - Richard

"The curing of pool plaster is mostly the following and the hydroxide portion comes from the water used in hydration: 2(Ca3O•SiO4) + 7H2O --> 3CaO•SiO2•4H2O + 3Ca(OH)2 + heat" – Richard

You show calcium hydroxide as one of the reaction products.

"These products contain chemicals known as "pozzolons" that essentially take the weakest element in plaster (calcium hydroxide) and "lock it up" so that it is less susceptible to chemical attacks. Pozzolons - based products have been proven to reduce etching, cracking, mottling and other problems, as well. "

http://www.npconline.org/mc/page.do?sitePa...7&orgId=npc

Calcium hydroxide is a primary component of Portland cement.

"Portlandite is a mineral formed during the curing of concrete, also known as Portland cement. It is a calcium hydroxide mineral, Ca(OH)2," http://virtual-museum.soils.wisc.edu/portl...te/content.html

______________________________________________________

"The pool plaster that is dissolving is calcium carbonate and what results in the water is calcium carbonate"

What is dissolving can be calcium hydroxide, calcium carbonate or magnesium carbonate. What ends up in the water is calcium ions, hydroxide ions, carbonate ions and magnesium ions. There is no calcium carbonate in the water unless it is in suspension and the water is cloudy.

____________________________________________________

"As for outgassing, that would only increase the pH and would remove carbon dioxide and not calcium -- calcium will not outgas"

I said that the carbonates could outgas by way of carbon dioxide. I didn't say that calcium could outgas. That's why I said that you would count the calcium part of the calcium carbonate.

The carbonates are in equilibrium with the bicarbonates, which are in equilibrium with the carbonic acid. As the carbonic acid outgases as carbon dioxide, the bicarbonates convert into carbonic acid and the carbonates convert into bicarbonates so that the equilibrium will be restored.

The carbonates take up hydrogen ions to become bicarbonates and the bicarbonates take up hydrogen ions to become carbonic acid. The end result of aerating is lower amounts of carbonates, bicarbonates and carbonic acid. This is why the pH is raised and the Carbonate and Bicarbonate Alkalinity is lowered.

______________________________________________________

"The TA tends to slowly rise over a full season in spite of the acid addition"

Which could be the result of the addition of carbonates from the calcium carbonate or magnesium carbonate.

____________________________________________________

"According to this link more dissolved carbon dioxide in the water leads to faster dissolving of limestone which at first didn't make sense but I suspect it is due to the lower pH"

No, it is not only lower pH; it is because the calcium carbonate combines with the carbon dioxide to form calcium and bicarbonate. Calcium bicarbonate is very soluble compared to calcium carbonate.

CaCO3 + H2O + CO2 <> Ca(HCO3)2

_____________________________________________________

"As for what is used in my pool surface, I don't know whether it is calcium carbonate or calcium magnesium carbonate"

If you're only measuring calcium carbonate, you may be missing half of the hardness rise that you may be getting if your plaster aggregate is half magnesium carbonate.

_____________________________________________________

"Do you have any estimate of the expected CH rise over time from dissolving plaster due to a low saturation index where one would notice a plaster surface being unacceptable?"

No, due to dilution and such, I wouldn't expect that calcium rise would be a very helpful metric except under very controlled conditions. I'm not exactly sure how much actual mass has to be lost, but it doesn't take much to have a noticeable impact.

______________________________________________________

"I think your table would be very reasonable if low pH were the reason for the low saturation index, but I'm not so sure you'd find the same results if the pH were near 7.5 and the low saturation index were due to low CH or TA. What are your thoughts on that?"

I think that pH is most important, then alkalinity and then calcium, but I really don't know how much difference it actually makes; that's why I always make sure that the water is fully saturated.

______________________________________________________

"The downside is that the risk of scaling in gas heaters and saltwater chlorine generator cells is increased."

You are usually not going to get scaling in a heater unless the water temperature is allowed to go very high. The temperature of water in a heat exchanger should not exceed 10 degrees above the pool water temperature. At 10 degrees, the SI increases by less than 0.1. You should not be concerned about scaling of heaters at CSIs below 0.4.

SWCGs are at risk for scaling, that's why I caution plaster pool owners about using a SWCG.

__________________________________________________

[demfourlife]

I had so many problems trying to always get the water just right. I just decided to buy a Salt Pool Chlorine Generator. Here is some information about it:

http://saltpoolchlorinegenerator.blogspot.com/