As far as I am concerned, I don't have a definitive answer for you on whether calcium helps a pool without plaster/gunite/grout. Sources I have looked at regarding water without calcium being more corrosive to metal, even at normal pH above 7.0, are not at all clear about this. I simply cannot find anything that definitively says that the presence of calcium in water will inhibit corrosion EXCEPT that a saturation level of calcium carbonate MAY form an inhibiting layer on metal. But other sources say forming such a layer is virtually impossible since it is uneven and if saturation is enough to start significant formation (probably a saturation index above +1.0), then it is enough to continue buildup (which is not good). Clearly in a heater, you don't want a thick layer of calcium carbonate since that reduces heat transfer (calcium carbonate does not conduct heat well) and would eventually burn out the heater. This is why Ben generally recommends being slightly corrosive in water balance since the higher temperature of the heater will shift that balance closer to neutral (neither corrosive nor scaling). The Langelier index was originally developed for boilers so apparently it did help in that environment so in theory should help with pool heaters, but that is at saturation levels.
In an SWCG pool there is a higher level of salt, though that can obviously occur in a non-SWCG pool where additional salt has been added or where a lot of liquid chlorine has been used without splash-out or backwash. Here again it is not at all clear whether calcium inhibits the greater corrosion rates from the higher salt levels and it's also not known exactly how much higher the corrosion rates from higher salt levels actually are. There is no question that the higher salt means higher conductivity so greater potential for electrolytic corrosion (including galvanic corrosion from dissimilar metals touching or electrically connected), but I can't find anything that says that calcium levels inhibit this (except the general discussion I gave above about saturated calcium carbonate levels).
In other words, I don't know, but let's look at an interesting situation. It used to be that water systems used chlorine for sanitation and the water flowing through homes was (and still is) typically in copper pipes. Typical numbers for Total Hardness are around 70 ppm in our metropolitan area (and a Calcium Hardness of around 50 ppm). I don't recall hearing about or reading about horrible corrosion issues in many copper piping systems in people's homes. The residual chlorine levels were relatively low, but there was no CYA so in actual fact the levels were higher than found in pools in terms of disinfecting chlorine level. Today, monochloramine is used and though at higher levels, it's a far weaker oxidizer so far less corrosive to metal.
The reality is that the chlorine in pools is what corrodes metal faster than anything else -- it has a much higher oxidation potential than oxygen in the water, even accounting for actual disinfecting chlorine concentration. We know that stainless steel is also susceptible to higher chloride levels (e.g. in SWG pools) for faster corrosion, but don't know how much faster at various chloride levels.
Your antacid is calcium carbonate because this is alkaline and helps counteract the excess acid. It's not just a buffer, but a base. There is no such thing as calcium bicarbonate (except in solution) and you don't take lots of sodium bicarbonate because the extra sodium isn't good for you (it's the sodium that is of concern in limiting one's "salt" intake). Nevertheless, there are antacids on the market that contain any of the following, sometimes in combination (see this link for more details):
Calcium Carbonate -- a base (alkaline) and a pH buffer
Magnesium Carbonate -- a base (alkaline) and a pH buffer
Magnesium Hydroxide -- a base (alkaline)
Magnesium Oxide -- forms Magnesium Hydroxide in water -- a base (alkaline)
Sodium Bicarbonate -- a pH buffer
Aluminum Hydroxide -- a base (alkaline)
Dihydroxyaluminum Sodium Carbonate -- a base (alkaline) and a pH buffer
Dihydroxyaluminum Aminoacetate -- a base (alkaline)
Additional items may be added for other purposes such as reducing gas (e.g. simethicone) but are not strictly antacids.
Richard
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