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Thread: Perfect Calcium Saturation Index, never re-surface?

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    Perfect Calcium Saturation Index, never re-surface?

    If the perfect Calcium Saturation Index were maintained, would a pool surface last forever in perfect condition?

    If one had to choose one, would it be better to have a constant low Calcium Saturation Index, or constant high?

    Thanks

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    Patrick_B's Avatar
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    Re: Perfect Calcium Saturation Index, never re-surface?

    No, eventually the plaster or tile grout would decay enough it would have to be replaced.

    If you could coat the inside of a pool with molten glass, maybe then it would last a long time.

    Neither, as close to right down the middle as you could get. That would be ideal.
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    Re: Perfect Calcium Saturation Index, never re-surface?

    Even in water saturated with calcium carbonate, there is a constant exchange of solid material with ions so the surface is constantly reforming. This is similar to the effect one finds with ice in a freezer -- eventually the ice surface goes from smooth to jagged even though the total amount of ice may remain the same. However, with well-made plaster this is a very slow effect which is why such plaster lasts for decades. pH is probably a more important factor after saturation of calcium carbonate; temperature is also an important factor. Higher pH and lower temperature both slow down the rate of this surface exchange.

    If the saturation index were consistently high, there would be a slow buildup of scale while if it were consistently low the plaster would slowly dissolve, but for well-made plaster these effects are slow. Ideally, one wants the saturation index at 0, but onBalance has shown that for well-made plaster no degradation is seen until the saturation index gets into the -0.7 to -1.0 range so having it be in the -0.3 to +0.3 range should have the plaster last for quite some time. However, for poorly-made plaster, the degradation rate is faster if the saturation index isn't at 0 so if you are unsure of the quality of the plaster then it would be best to err on the side of saturating or slightly over-saturating with calcium carbonate.
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    Re: Perfect Calcium Saturation Index, never re-surface?

    It seems that a Calcium Saturation Index of 0 to 0.3 is best for the plaster, but less than 0 for the plumbing, as long as the pH is 7.0 or higher.

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    Re: Perfect Calcium Saturation Index, never re-surface?

    The difference in practice between -0.3 and 0.0 is so small that no one has been able to measure it. 0.0 is better for the plaster in theory, but not in a way that makes any difference.
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    Re: Perfect Calcium Saturation Index, never re-surface?

    In this link you can read more about why the Langelier Saturation Index isn't a good predictor for metal corrosion. Trying to saturate piping with a thin layer of calcium carbonate is tricky, especially what water flow rates vary. For pools, we don't worry about the saturation index for corrosion and only consider it for protecting plaster surfaces (or grout in between tile).
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    Re: Perfect Calcium Saturation Index, never re-surface?

    I agree with Jason and Chemgeek.

    The range of -0.3 to +0.3 is perfect for pool plaster (including quartz and pebble finishes). The occasional (slight) negative CSI is normally beneficial.
    Maintaining a constant positive CSI could eventually (in theory) lead to visible calcium scaling. Of course, that could be one way to make the plaster finish last longer than 25 years. The calcium scale covering the plaster surface would probably prevent any natural erosion or aging. At some point in time, one could sand or acid wash the plaster surface and expose the original (smooth) surface again. .

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    Re: Perfect Calcium Saturation Index, never re-surface?

    Thanks for the explanation.

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