Posted by Kim Skinner on September 23, 2009 at 4:59pm
In a nutshell, the “Bicarb Start-up” is a method which helps eliminate the formation of “plaster dust” in new plaster pools and which results in denser, smoother, more durable plaster than other techniques.When a new plaster pool is filled with typical tap water, it is normal for a small percentage of cement paste at the surface to dissolve into the water, and form a solid calcium carbonate precipitate – which we in the pool industry call “plaster dust.” This plaster dust is then brushed and filtered and brushed and filtered and so on, until it is gone.In a traditionally started pool, with a pH around 7.4 to 8.0 and a TA of about 100 ppm, the form of calcium that “bleeds off” into the water from the plaster surface is calcium hydroxide, which has such a high pH (in the 10+ range) that under normal filling procedures (i.e., when the water is not pre-treated), the pH of the pool water jumps immediately and almost always exceeds 8.3 and sometimes as high as 11.0 – which begins to convert the calcium hydroxide in the water into scale. This is the “plaster dust” that we see for the first 3 weeks.The bicarb start-up method begins by adding bicarb (sodium bicarbonate) to the fill water before it enters the pool. This type of fill water helps “fix” or convert “in place” most of the calcium hydroxide in the surface by converting it to the more durable calcium carbonate. As a result, less calcium is released into the water, and the surface retains more of its original constituents resulting in a denser finish.Now, to understand start-up chemistry, there is a pH value to keep in mind – pH 8.3. Above 8.3, calcium carbonate, or scale is likely to form, and the higher the pH rises above 8.3, the more carbonate scale is formed. So we call high pH levels (over 8.3) “scaling” pH levels. Below pH 8.3, calcium tends to stay in a soluble “bicarbonate” form.Saturating fill water with baking soda not only helps keep the pH at 8.3 or below, but it also provides enough bicarbonate to impede and reduce the dissolution of calcium hydroxide from the plaster surface. It does this by reacting with and converting calcium hydroxide on the plaster surface into a protective layer of durable calcium carbonate, thereby creating a denser and harder surface.The primary, immediate difference noticed when doing a bicarb start instead of a traditional start is that there is virtually no plaster dust. Over the long term, there is also better, more durable and smoother plaster.While the acid start will also eliminate plaster dust, help colored plaster pools to retain their color, and dissolve white deposits; it also has a serious drawback to its method. Acid start-ups, where a minimum of four gallons of acid is added soon after the pool is filled with water, will usually dissolve all plaster dust formed, but unfortunately also dissolves and uniformly etches (as determined by magnification) the entire plaster surface at the same time. This process leaves the surface rougher and porous (as seen under magnification), which in turn makes it less durable, easier to stain, and harder to clean in the long run.Furthermore, once the acid start program is over, it can be tricky to raise the pH and alkalinity and balance the water without creating scale. In addition, it is often the case that the person who performs the acid start realizes the short-term benefit (a clear non-scaled pool), but the person who provides the subsequent maintenance suffers the long-term deficit (rougher plaster and worse staining).We know of several pool plasterers who had difficulties creating and maintaining consistent color on colored plaster, quartz aggregate pools and with hydrazzo pools. These plasterers were using the acid start-up method trying to stop white scale from developing on these special pools.When these plastering companies began Bicarb starting all of their colored cement pools, the color was darker and more consistent, and the white film deposits no longer formed! In addition, over time, these bicarb started pools retained their superior finish much longer due to the smoothness and the additional density of the protective, carbonated surface. Years later, the process of removing dirt and other stains by acid washing is vastly improved.The Bicarb start-up is always the best start-up method to use on plaster pools. However, it is also important to have quality and durable pool plaster to start with. Improper plastering practices can reduce the effectiveness of a bicarb start-up and all other start-ups methods or programs.If plaster has too much water in the mix, too much calcium chloride added, and too much water added while troweling, and is filled with water too soon after completion, it will result in a less durable and lower quality plaster finish. The above improper cement practices create greater porosity and more soluble plaster components which results in more plaster dust forming on the plaster surface during the first few weeks.A greater porosity can only result in a surface that is more susceptible to dirt and mineral staining months later. Acid washing will be less effective in removing stains and more likely to cause more “wear and tear” to the plaster surface.But as mentioned before, a bicarb start-up will always help in creating a denser, smoother, and more durable plaster surface despite these other factors.Next, part 2. The step by step process.
A bicarb startup chemistry range is a pH between 7.4 and 8.0, a combined alkalinity and calcium hardness content of 500 ppm, but not any higher than that. Look for the more recent blog post "Bicarb Startups #2, which provides more information.
You are correct that the hardness plays a role in determining the appropriate amount of bicarb to add, and that Acid is sometimes needed. Yes, pozzolans help somewhat, but not entirely. The Bicarb program will seem very counter-intuitive, but it works.
Doesn't the hardness of the fill water, and in particular the Calcium Hardness (CH) level, play a role in determining the appropriate TA and pH levels for a bicarb start-up? If the CH is 50 ppm, pH is 8.3 and the TA is 100 ppm, then at 70ºF water temp, the saturation index is near zero. A higher CH level of, say, 150 ppm, would have a saturation index near 0.5 so a lower pH or TA would be needed to prevent scale..
Also, even if the calcium hydroxide that is formed during curing resulted in formation of calcium carbonate, there would still be a net pH rise (though not as much as when there isn't any formation of calcium carbonate at the surface) with a requirement for some acid addition and also a small amount of baking soda as the TA would drop somewhat as well (after the acid addition).
I also thought that pozzolanic additives can be used in the plaster to react with the calcium hydroxide reducing the amount that is released or that remains behind and the resulting compounds strengthen the cement.
I look forward to part 2 as the step by step process may answer some of my questions.
Comments
Also, even if the calcium hydroxide that is formed during curing resulted in formation of calcium carbonate, there would still be a net pH rise (though not as much as when there isn't any formation of calcium carbonate at the surface) with a requirement for some acid addition and also a small amount of baking soda as the TA would drop somewhat as well (after the acid addition).
I also thought that pozzolanic additives can be used in the plaster to react with the calcium hydroxide reducing the amount that is released or that remains behind and the resulting compounds strengthen the cement.
I look forward to part 2 as the step by step process may answer some of my questions.