After a plastering company has finished the job of applying a premium quality plastering finish to the interior of a swimming pool, it is up to whoever is performing the startup to make sure that the new plaster continues to be the best finish it can possibly be. Let’s talk about that process, its challenges and possible pitfalls, and let’s discuss how to take control of the start-up to avoid those potential problems.
In the normal course of events, as plaster is exposed to alkalinity, the portion of the plaster that is soluble calcium hydroxide will chemically convert to more durable calcium carbonate. So after surface carbonation, the entire water-exposed plaster surface is beneficially carbonated and set to survive and shine in the pool environment for decades.
The challenge, then, is to encourage the chemical conversion of hydroxide to carbonate as quickly as possible. When this challenge is met successfully, no plaster (as calcium hydroxide) is dissolved from the surface, and the surface is therefore more dense, water-impervious, and longer-lasting. If, on the other hand, the start-up challenge is not met, some of the calcium hydroxide is allowed to dissolve, and the result is more porous plaster paste, the formation of plaster “dust,” and a shorter plaster lifespan. Yes, you may have been led to believe that plaster dusting is normal, but it is in fact a curing failure.
A few key facts
The calcium hydroxide portion of the new plaster surface is water soluble even in “balanced” water.
If any of it is allowed to dissolve, it carbonates while in the pool water and converts to “plaster dust,” which is a precipitate version of calcium carbonate.
Solubilized hydroxide also spikes the pH up into the 9 or higher range.
Unless there is a good start-up process, the plaster dust will continue to form for a week or two… until the surface is fully carbonated (i.e., until all calcium hydroxide at the surface
onBalance consulting group: Que Hales, Doug Latta and Kim Skinner. of the cement paste is converted to carbonate).
Anything that accelerates the conversion of hydroxide to carbonate on the plaster surface without causing any other harm is a better process than just letting things happen, which is essentially the traditional start-up method (i.e., add acid and brush and filter until the dust is removed).
A few key needs
Amethod of adjusting the chemistry of the tap water used to fill a pool to stay at or under pH 8.3 It should be able to chemically convert the surface calcium hydroxide “in place” to calcium carbonate… thus, together with the right pH, no plaster dust!
The change should be both good for the plaster and not harmful to it.
It should be good for the water balance.
It should be safe for swimmers. And it should be reversable when the start-up is complete.
Hmmm, what would work? As it turns out, there actually is a chemical that is virtually a perfect match for all of these needs! It is a pH buffer with a pH of 8.3, so if enough is added to the tap/fill water before it enters the pool, it will be very difficult for the water pH to go higher than that.
The chemical is sodium bicarbonate.
Also, sodium bicarbonate will chemically convert calcium hydroxide to calcium carbonate immediately on contact, or in other words, before the hydroxide can dissolve into the water. So it is good for the plaster by stopping the dissolution of part of the surface paste, leaving it more dense and more durable over time.
Adding sodium bicarbonate – even if you drastically overdose it – is not capable of damaging new plaster.
The sodium bicarbonate is good for the water because it helps to establish the pH buffer (alkalinity) that pool water needs.
Sodium bicarbonate is non-toxic – in fact, one of its other names is baking soda.
When the start-up process is complete, all that needs to be done is to add enough acid to the water to balance the alkalinity. And if there is anything swimming pool service techs are intimately familiar with, it is adding chemicals (including baking soda and acid) to balance water.
The Role of LSI in Plaster Curing
Maintaining water with a balanced Langelier Saturation Index of 0.0 is good for well-cured pool plaster. But the LSI was designed for calcium carbonate solubility. So that same
What a beauty! Time to start ‘er up. Photo credit: onBalance LSI 0.0 balanced water is detrimental to brand-new plaster that is less than two to three weeks old because that plaster contains calcium hydroxide. This simple fact comes as a surprise to some who believe that perfectly balanced water (0.0 LSI) is always the goal.
The good news is that filling pools with water that has an LSI of at least +0.5 helps prevent calcium hydroxide from being dissolved from the plaster surface in the first place and allows it to carbonate in place.
The Proper Start-up Steps
For a step-by-step description of the Bicarb Start-up see http://www. poolhelp.com/home/onbalanceresearch/ education/the-bicarb-start/. But here are a few additional thoughts: Be Patient with new plaster: The first important step is to not fill the pool immediately after final troweling. Waiting a few hours (preferably four hours) to begin filling the pool will allow the fresh plaster in the deep-end bowl (which is applied, troweled, and finished last) to be ready to submerge in water. Even positive LSI water will dissolve and erode plaster material if it hasn’t been given enough time to harden, hydrate, and complete certain chemical transformations sufficiently.
Fill with positive LSI water: Ensure that the fill water has an LSI of at least +0.5 (and with a minimum alkalinity of 120 ppm to ensure sufficient pH buffering and carbonation). If the tap or fill water doesn’t meet this standard, it will be too late to balance the water after it has filled the pool. The unsuitable fill water will have already done damage by dissolving many pounds of calcium from the plaster surface, creating a more porous surface. One can see the damage by simply observing the amount of plaster dust present in the pool. No start-up program will undo that damage after the fact.
Over 8.3 pH is bad: The best results are achieved when the pool water pH is always below 8.3. A good tool for ensuring this target is met is the liquid reagent phenolphthalein. It remains clear when added to water below 8,3, but turns pink in water above 8.3. We use it as a quality control measure – if a few drops in the water filling the bowl of the pool show pink, we add a few tablespoons of acid to the water in the bowl and a half-gallon to the fill barrel. If the reagent stays clear, everything is good!
After waiting, don’t hesitate: After waiting a few hours, and once the positive LSI tap water begins to fill the pool, fill as quickly as safely possible.
Any start-up program that doesn’t require the above conditions doesn’t provide the best care and protection for new plaster. Up to 40 pounds of calcium hydroxide can be dissolved and removed from plaster surfaces when the above steps are not followed, and this is why many plaster finishes, including quartz and pebble, often become unsightly in the months that follow because of a porous surface.
Research by the onBalance team indicates that the fill water should have a “combined” total alkalinity
Follow the step-by-step process and visit: http://www.poolhelp. com/home/onbalance-research/education/the-bicarb-start/. plus calcium hardness of at least 400 ppm minimum (which essentially achieves the required LSI of about +0.5) to prevent the dissolution of calcium hydroxide from the plaster and the formation of plaster dust. (A combined total of 500 ppm is the ideal amount; 600 ppm is generally the maximum required). For dosing, see the website.
When the above start-up program is followed, there will be little or no plaster dust formation, and the pH will be easier to keep and maintain in the beneficial 7.5-to-8.2 range during the start-up period. This program helps retain a durable, dense, and hard finish that will preserve the vivid color even in pigmented pool plaster. Ensuring that the alkalinity is adequately high enough is why the onBalance team recommends the Bicarb Start-up program (using sodium bicarbonate to pre-condition the water). This start-up has been used successfully for more than 40 years by many pool companies.
Advice for special situations
If the fill water has low calcium hardness (less than 100 ppm) and the alkalinity is high (above 120 ppm), it may make sense to use the “Calcium Start-up: instead of the Bicarb Startup. On the other hand, when the alkalinity is low and the calcium level is moderate to high, use the Bicarb start-up and raise the alkalinity level. The bottom line is that you pre-test the tap water, and adjust its chemistry before you use it to fill a new plaster pool.
Sometimes, the tap water has both the alkalinity and calcium levels being very low (both under 100 ppm). When confronted with that situation, we suggest that the Bicarb program should be used for the primary process since it protects the plaster but also buffers the pH, which the calcium version does not. The calcium level can then be adjusted after the pool is full.
A note about plaster quality: While it is true that a poor pool start-up can ruin a high-quality plaster finish, especially a “hot” or acid start-up, it is equally true that poor quality plaster and workmanship will negate a good start-up program. In other words, a good start-up process cannot fix bad plastering.
Poor quality plaster and workmanship can result in the unpreventable loss of some calcium hydroxide and calcium chloride from the plaster regardless of the start-up program. Any loss of plaster material results in greater porosity of the plaster surface. It also creates more difficulty in balancing and maintaining the pool water during the start-up period.
In addition, poor quality plaster can result in excessive craze cracking, graying, mottling of white plaster, plaster turning whitish in streaks, spots and blotchiness, color fading, calcium nodules, and even some plaster dust. Plastering mistakes that lead to these problems involve excessive water-to-cement ratio, excessive calcium chloride additive, excessive water troweling, and too early filling. A good start-up program cannot prevent or alleviate those plaster defects, nor does aggressive water cause any of the above issues.
Poor chemical start-ups and poor long-term maintenance cause problems such as uniform calcium scaling or uniform etching throughout a pool, and localized damage where non-diluted chemicals come into direct contact with the plaster surface.
Good Tap water
In some areas of the country, the tap water already has a positive LSI with sufficient levels of alkalinity and calcium (combined total of at least 400+ ppm) and can be simply used, as is, to fill new plaster pools without having to do any chemical additions with bicarb or calcium.
If that is the case, thank your lucky stars. There will be no need to do a special water compounding program.
However, the only way to know is to, as we constantly emphasize, pre-test the tap water before filling the pool. onBalance is a consulting group that includes Que Hales, Doug Latta and Kim Skinner. According to their website, their “day jobs” involve owning and running swimming pool chemical, cleaning and repair services, as well as some remodel contracting. Their three primary interests: Research into swimming pool water chemistry and plaster issues, teaching about their research at trade shows and association meetings, and providing consulting and expert witnessing services as needed