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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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Thank you Richard, indebted as always.  I don't fully understand the chemistry but from what I have read the LSI and CSI were devised for boilers and heating systems where metal holding and delivery systems were subject to possible corrosion.  I have tools which get coated in tile grout and plaster finish and once these have hardened throwing them in a bucket of water doesn't really do anything to soften them and allow me to clean up the tools. The tools do rust though.  Lowering the pH would begin to soften the grout/plaster but the relatively low TA in the tap water and soft water do not do very much and that can be seen from the ppm figures and relatively small reductions in LSI & CSI figures whereas pH adjustments have far bigger effects on the LSI & CSI figures which is where I drew my assumptions from.  Generally and in our area the tap water is slightly lower in pH than we would normally have in a pool water.

Maybe Kim could come back and add their comments as to the solubility of hardened plaster and grout in tap water?

You are right that the process of dissolving of calcium carbonate in water that is low in calcium or carbonate or both is relatively slow unless the pH is lower, but slow in terms of years is still a problem since we want to keep our pool plaster in good shape for at least a decade or more.  So while you do not see a fast dissolving for your tools, if you were to have water with a negative saturation index over years then you risk dissolving the calcium carbonate from plaster.  Yes, it would occur faster at lower pH and at higher temperatures (if the saturation index were still as negative -- normally higher temperature has a higher index so other parameters would need to be lower), but it still occurs, albeit slowly, at normal pH and normal temperatures.

Kim has done experiments to show that well-made plaster, such as that done with the bicarbonate startup, tends to hold up better in the presence of negative saturation index water than poorly-made plaster that either uses a more aggressive (i.e. acid) startup or where there is too much calcium chloride in the mix.  This may be because a poor plaster surface is already either thin or pitted in some areas so the process of dissolution may either be faster or more noticeable compared to a more solid surface.

A high alkalinity will more quickly and effectively "carbonate" (converting calcium hydroxide into calcium carbonate) a new plaster surface creating a more dense and harder surface.  Once that is accomplished (generally 30 days), the alkalinity should be lowered to normal operating levels, and the plaster surface will be more resistant to aggressive water, and also balanced pool water. 

As Richard points out, if the plaster has not been properly mixed and applied, it (a porous surface) can be eventually broken down by balanced pool water, and of course, aggressive water will do it much faster. So the quality and durability of a plaster finish plays a big part in the overall lasting ability of pool plaster.   

Once plaster has cured, both calcium hardness and alkalinity are equally important to protect plaster, with, of course, the pH being the most dominant factor.  

Thanks Kim,  do you think maintaining the higher TA will also work long term on cured plaster by continuing to convert calcium hydroxide into calcium carbonate? For instance once the surface is brushed as part of the regular on going house keeping side of the pool.

Any thoughts on Richards suggestion on applying densifier (sodium silicate)  to the surface?

Once the new plaster has cured for about 30 days, I don't see as much of an advantage to continually maintain a very high alkalinity.  The surface is carbonated and the layer of calcium carbonate is very thin, but also very dense.  So the carbonation process is mostly over, and a normal TA will still have the ability to continue carbonating when it comes into contact with calcium hydroxide.

Generally, a sodium silicate densifier is added to the plaster mix, not to the surface after the fact. My guess is that adding it to the plaster surface will cause a non-uniform discoloration. Whereas, a Bicarb start-up will do a good job of carbonating the surface without any discoloration occurring.  

Thanks Kim,  It's what is required as a TA level that is the question. We know the standard 80-120ppm but as Richard showed:

TA ..... pHeq

. 20 .... 7.75

. 40 .... 8.04

. 60 .... 8.21

. 80 .... 8.33

100 .... 8.42

120 .... 8.49

The TA is buffering too high to really regulate the pH level so acid is added to drag the pH down to our normal levels which is costly and potentially is beginning to soften the surface compared to 20-40ppm which would require minimal effort to maintain the pH.

Richard have you any figures for the TA/pH with the effects of CYA at say 20, 30, 40, 50 ppm as that will also be buffering the pH down a bit from the bicarbonate?

You are mixing up two different effects.  Just because one uses acid to lower the pH below the carbon dioxide outgassing equilibrium, that does not mean it is softening the plaster surface.  One usually has the Calcium Hardness and other parameters make the calcite saturation index near zero so that the plaster surface will not be affected at normal pool pH.

Also, though the pHeq numbers I show are when carbon dioxide outgassing would stop completely at various TA levels, in practice such outgassing slows down considerably before reaching that pH. It depends on the amount of aeration in the pool.  The use of waterfalls, spillovers, fountains, etc. increases the rate of carbon dioxide outgassing and therefore the rate of rise in pH.

As for the effect of CYA or of borates that are both pH buffers, I show their buffer capacity (and that of the carbonates) in the thread pH Buffer Capacity.  Note that increased pH buffering does not directly change the rate of carbon dioxide outgassing.  It slows down the rate of pH rise, but it still takes the same amount of acid per time to maintain the pH.  This is because the pH buffering works both ways.  While it lessens the rise of pH from carbon dioxide outgassing, it also requires more acid to lower the pH by the same amount.  Basically, the amount of acid needed is dependent on the amount of carbon dioxide outgassed, not on the amount the pH moved.

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