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Ok so from reading on PGN I understand the reasons behind the high alkalinity start ups but what then?  Once the pool has stabilised what does the alkalinity do that the calcium hardness doesn't by way of protecting the surface.  I don't get the buffering idea as bicarbonate of soda buffers at too higher pH so requires acid to drag down the pH which requires more bicarb in an endless cycle. Increasing the calcium carbonate hardness would protect the surface as it's pretty much the same as the plaster or grout so can anyone shed some proper chemical light on this please.

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For cured or older pool plaster, you are correct that CH protects the plaster much in the same way as alkalinity does. But for new fresh plaster, a high alkalinity is needed to do two things.  One, to help convert the calcium hydroxide on the plaster surface into calcium carbonate, which it does within one month. And Two, to help keep the pH from spiking to levels higher than 8.4.  A high TA does that, but a low TA won't.

Even if the CH is high, if the alkalinity is low, then there is not enough buffering (control) of the pH to offset or control when calcium hydroxide dissolves from the plaster surface into the fresh pool water and causes the pH to rise. The bigger problem is dealing with the pH going above 9.0 with a low alkalinity. 

Yes, the downside is having to lower the alkalinity once the plaster start-up process is over after 30 days. But it is not normally true that bicarbonate will need to be added after every dose of acid.

And remember, a high calcium hardness can't be lowered in simple ways, while a high alkalinity can.

Thanks Kim, it was your excellent post and explanation on the other thread that greatly helped my understanding of what was going on.


Interestingly I ran into concrete densifiers and chemical hardeners such as the following:

It's basically sodium silicate and when applied it converts the calcium carbonate into calcium silicate hydrate.  That begs the question of why the initial plaster mixture doesn't contain enough excess silicate to form more CSH and produce a stronger pool plaster surface without relying as much calcium carbonate.  Any thoughts about this?

Hi Richard, sorry to break in to your conversation.  Yes a good thought, I used densifier on a polished concrete kitchen work top I made for a customer a few years back and the surface becomes incredibly hard and stains do not soak in so interesting thought!

Hi Richard,

I am not an expert on this topic, but there are different types of pozzolans (some of which contain sodium silicates) that are added to cement/concrete (and pool plaster) mixes and applications. My understanding is that they essentially convert the excess calcium hydroxide compounds into a harder and less soluble compound such as CSH.

I believe that less than 10 percent is used in cement mixes to react with calcium hydroxide. However, the calcium hydroxide content of cement/plaster is about 20 percent, so not all gets chemically converted. There are probably some limitations involved with adding more.

Also, there are issues of discoloration that can occur with pool plaster.  So it isn't a perfect solution to solving all plaster problems.

But there are plasterers that are using and trying to strengthen their plaster mixes with the addition of pozzolans and/or polymers.

When there aren't other sources of rising pH such as the calcium hydroxide from plaster, then if one is using a hypochlorite source of chlorine (e.g. bleach, chlorinating liquid, Cal-Hypo, lithium hypochlorite) then a lower Total Alkalinity (TA) will lessen the rate of pH rise.  This is because pool's are intentionally over-carbonated to (ironically) provide pH buffering and to protect plaster surfaces.  However, this over-carbonation leads to carbon dioxide outgassing and that causes the pH to rise (with no change in TA).

Generally speaking, you wouldn't want the TA above 80 ppm but the real rule is to lower it to find a sweet spot where the pH doesn't rise too much.  If your TA is on the low side, then you may need to raise the CH to keep the saturation index near 0 to protect plaster surfaces.  Also, there is less carbon dioxide outgassing at higher pH so one can also have a higher pH target and not try and keep the pH at 7.5, but instead target something like 7.7 or so.  This technique is needed even more in saltwater chlorine generator pools since they have increased aeration of the water that may increase carbon dioxide outgassing (they may also have undissolved chlorine gas outgassing, but that's another matter).

Thanks Richard, yes from your other information we exchanged some time ago I have been experimenting lowering the alkalinity levels.  I used an analogy with a customer of treating alkalinity like a rubber band on a scale of 0 to 14 with the rubber band placed at 8.2.  The more alkalinity you add the thicker the rubber band is so more pull is required to get it to 7.2   Using a much lower alkalinity is like using a much thinner rubber band so it's easier to move and keep at 7.2.

I expected to run into trouble with my alkalinity at around 38 (pH 7.3) as I had read a paper describing a cliff and if you go off the edge then the pH would plummet but that would require a chain reaction to release that much hydrogen.  On a customers pool they got chemical additions wrong and with soft water ended up at alkalinity of 22 but still bad things didn't happen.

My understanding is

When added to water, an acid releases hydrogen ions.  Hydrogen ions in water combine with the water molecules to form hydronium ions, thereby decreasing the pH and increasing the acidity of water.

Water exposed to air is mildly acidic because it absorbs small quantities of CO2 from the atmosphere.  The CO2 breaks down to carbonic acid that then dissociates to release hydrogen ions into the water, decreasing the pH.  So at some point it maybe possible to get the wrong side of the dissolved carbonic acid equation and actually have the pH fall.

It seems to me that the levels suggested for pools have come from ponds where it's generally accepted that you wish to encourage life and all sorts of plants in the ecosystem.  By contrast we really want pools of zero life so all nutrient levels should be reduced to a minimum where possible.

No, your understanding about this is wrong.  Though it is true that there is an equilibrium between the carbon dioxide in the air and the water and that carbon dioxide in the water forms carbonic acid that then further dissociates into bicarbonate ion and carbonate ion, most pool water is OVER-CARBONATED.  What I mean by that is that there is MORE carbon dioxide in the water than the equilibrium amount based on the amount of carbon dioxide in the air.  That means that unless there are other acid/base additions to the water, the water will tend to RISE in pH.  Technically, the pH won't stop until equilibrium is reached which is at the pH listed below for each carbonate alkalinity level (TA if no CYA or borates are present):

TA ..... pHeq

. 20 .... 7.75

. 40 .... 8.04

. 60 .... 8.21

. 80 .... 8.33

100 .... 8.42

120 .... 8.49

Now in practice, the pH slows down in its rise so much that it appears to settle in below the pH noted above.  Also, there are always some acid or base additions to the pool.  Even hypochlorite sources of chlorine contain a small amount of base -- excess lye in sodium hypochlorite chlorinating liquid or bleach, for example, or some calcium hydroxide in Cal-Hypo.

Why in the world are you trying to keep the pH at 7.3?  That is too low.  The rate of carbon dioxide outgassing is higher at higher TA or at lower pH or with more aeration.  See the following chart that shows how much the water is over-carbonated at various TA and pH:

So in general, you do NOT have carbon dioxide entering the water from air to lower its pH.  Usually it's the other way around with carbon dioxide outgassing causing the pH to rise.

Finally, you shouldn't normally have your TA as low as you have, most especially if you have plaster surfaces that need to be protected by saturating the water with calcium carbonate.  I usually say not to go below 50 ppm, but as you have found so long as you aren't adding any acid or net acidic chemicals accounting for chlorine usage/consumption (e.g. Trichlor or Dichlor or MPS) then the TA does not "crash" and the pH doesn't drop precipitously.

Thank you Richard, I haven't got it wrong, I do understand but I think I placed a red herring in my previous post mentioning the air acidifying the water, please forget that part.  I know from your chart that we have more carbon dioxide in the pool hence off gasing.

The table is very useful and still shows why one of the pools (vinyl) can still behave perfectly with TA at 22ppm.  My own, again vinyl at 38ppm is still slightly climbing in pH requiring a cup of acid or two over a season

My pH is happy at 7.3-7.4 and requires very little acid additions. In reality I have noticed the cheap NaClO has contained a bit more sodium hydroxide this year as the pH did rise marginally more than last season.  Also chlorine is more potent at the lower pH and

Separately why does bicarbonate do more to protect plaster than calcium, as calcium is the bulk of the plaster/grout?

You are right that I honed in on that air acidifying water statement so glad that wasn't your core understanding.

Plaster/grout doesn't have just calcium.  The actual solid is calcium carbonate in between the calcium silicate hydrate (CSH).  The calcium in the CSH is strongly bound and won't generally dissolve in water, but that is not true for the calcium carbonate.  Basically what happens is that uncured plaster is mostly tricalcium silicate and when it cures it absorbs water and converts the tricalcium silicate into calcium silicate hydrate while releasing calcium hydroxide.  In a bicarbonate startup, the calcium hydroxide which is soluble in water is instead converted to calcium carbonate in place (i.e. in the plaster matrix) at least near the surface of the plaster.

The calcium carbonate is susceptible to dissolving in water, especially if the pH is low, and saturating the water with calcium carbonate and maintaining a reasonable pH help prevent this dissolving from occurring.  If you don't do this, then the calcium carbonate can dissolve into the water producing pitting and if it is severe enough it can have the CSH break apart as well.

Thanks Richard, so apart from the new start up curing time the pH is the over riding factor and alkalinity could be seen as 3rd behind calcium hardness?

Most of the pool scripts/info I have read have always stated get your alkalinity up/adjusted before anything else which in most cases except a fresh pool start up is again not correct and the pH control is by far the most important.

Incidentally I had to raise calcium hardness and alkalinity on a pool this year but no calcium chloride available for a hundred miles so I added hydrated lime (calcium carbonate) which did a good job of both at very minimal cost.

No, that's not quite right.  A pool startup is different.  You want to saturate the water with calcium carbonate for a bicarbonate startup, but you also want to have the bicarbonate portion be high if possible.  This is because the conversion of calcium hydroxide to calcium bicarbonate consumes bicarbonate, not calcium.  So for the bicarbonate startup you normally increase TA and don't increase CH unless it is very low (below 150 ppm) as noted here:

After the pool has done the bulk of its curing after 30 days, then both TA and CH are important as are other water parameters including pH and to a lesser extent CYA and temperature.  The combination of these determines the calcite saturation index that should be kept near 0 to saturate the water with calcium carbonate.


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