Maintaining proper water balance is the key to keeping the pool water clear and surfaces scale-free. It is important to consider all forms of scale, but the focus should be on the one most likely to occur. Years of research, along with basic scientific principles, confirms that calcium carbonate usually is the primary culprit and should be the focus of pool professionals.
If calcium hardness is too high, precipitation can occur, which can cause cloudy water or scale on surfaces. The reason for placing an upper limit on calcium hardness is so that calcium-based minerals do not precipitate from solution.
It is important to distinguish between precipitation (or insolubility) and actual scale formation. Scale is adherence to a surface. Just because something precipitates or is somewhat insoluble does not mean that it will adhere. In addition, different types of scale have different crystal structures and their formation is influenced by different factors. While calcium carbonate is the most common, three types of calcium-based scale will be discussed. These include calcium carbonate, calcium phosphate and calcium sulfate.
Calcium carbonate scale
Calcium carbonate scale is driven by high pH. Other factors that contribute to its formation include high temperature (in part, because the carbon dioxide is more volatile) and high calcium or carbonate concentrations in the water.
An increase in temperature causes increased molecular motion. This simply means that all the molecules move around much faster. Because the calcium and carbonate are moving faster, they are more likely to bump into each other causing them to form a bond.
Another side effect of increased temperature is a decrease on carbon dioxide (CO2) concentration. A decrease in carbon dioxide concentration will cause the reaction to move to the right which also promotes the formation of scale. Another way of looking at this is that higher temperature promotes dehydration.
An increase in pH causes bicarbonate ions (HCO3-) to dissociate. This means the bicarbonate tends to dissociate into hydrogen (H+) and carbonate (CO3-2). The higher availability of the carbonate ion makes scale more likely to form.
Lastly, the more calcium and carbonate that is available in the water, the more likely these are to come into contact with each other and form scale.
Calcium phosphate scale
It is important to remember that there is always a much higher level of carbonate than phosphate in pool water. Even in what some would consider a high phosphate pool (1,000 – 2,000ppb), it is still a very low level compared to carbonate. For example, 2,000ppb is only 2ppm. There is typically well over 100ppm of carbonate in the water. Because reactions are often “competitions” between two species, the one with the higher concentration tends to react faster. Of course, the use of a phosphate remover can drive the level lower, but it starts out very low compared to carbonate.
More importantly though, calcium phosphate scale is not driven by high pH because it is not part of the carbonate equilibrium. Calcium phosphate is not very soluble in water and a precipitate can form quickly. It is important to note again, however, that the formation of a precipitate does not necessarily mean that it will adhere (form scale). Listed below are the possible locations where precipitation, then adherence, could take place.
Main body of water — as mentioned, calcium phosphate is not very soluble and will precipitate fairly quickly. With the filtration system running, this material will usually be captured on the filter bed and removed with routine cleaning. It will be trapped in the filter easier if using a water clarifier. So with proper filtration and filter cleaning, the precipitation in the bulk of the water is not really a problem as it never has a chance to settle out.
Heater — calcium phosphate will tend to deposit on warm surfaces. However, adherence also is affected by shear (or turbulence). Shear significantly reduces the chance for adherence. Because the most efficient heat transfer is with turbulent flow, the likelihood of calcium phosphate depositing on a heating element is low due to the high level of shear inside the heater.
Chlorine generator cell — calcium phosphate scale is not caused by high pH. Therefore, calcium phosphate is no more likely to form on a chlorine generator cell plate than anywhere else in the pool. There may be small amounts of calcium phosphate on cell plates, but analysis of dirty cells has confirmed that the vast majority of scale is, in fact, calcium carbonate.
Calcium sulfate scale
This type of scale is driven by the concentration of calcium and sulfate in the water. Scientists use parameters such as solubility constants for specific compounds to predict which material is likely to stay in solution and which will precipitate. The solubility constant for calcium sulfate indicates that this compound is the least likely type of scale to form.
In the unlikely scenario that the calcium and sulfate levels did reach the high concentration required, the precipitation process is different from that of calcium carbonate and calcium phosphate. In the case of calcium sulfate, the rate of crystal growth is extremely fast. So the material “crashes out” of the water very quickly. Essentially, it’s an all or nothing process. This is known as a super saturated solution, where crystal growth is faster than the initial crystal formation (nucleation). Once the water reaches the saturation point, it all falls out of solution. It is not a slow crystal growth process like other types of scale. For example, a pool operator would not see a slow build on a surface such as a chlorine generator plate, but rather all the material coming out of solution coating multiple surfaces all at once.