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IMPACT OF HARDNESS OR HARD WATER IN SWIMMING POOLS

Calcium carbonate scaling is a type of precipitation and occurs when calcium (Ca2+) and carbonate (CO32-) ions dissolved in a water react to form solid calcium carbonate (CaCO3).
calcium bicarbonateWhen the product of the calcium ([Ca2+]) and carbonate ion ([CO32-]) concentrations is greater than the calcium carbonate solubility product (KS), the water is supersaturated with respect to calcium carbonate and precipitation can occur. Conversely, when the product of the calcium and carbonate ion concentrations is less than the calcium carbonate solubility product, the water is under saturated with respect to calcium carbonate and the water can dissolve solid calcium carbonate; thus scaling will not occur.

[Ca2+][CO32-] > KS, Water Supersaturated,
CaCO3 Can Precipitate

[Ca2+][CO32-] < KS, Water Under Saturated,
CaCO3 Dissolves


The calcium carbonate solubility product KS is not constant, but decreases with increasing temperature. Therefore, a water that is under Saturated at a low temperature may become supersaturated when heated to a higher temperature. In general, a water's tendency to scale increases with increasing temperature. This is why, in areas with hard water, scaling tends to be worst in appliances that heat water, such as solar hot water systems, electric water heaters, coffee makers, etc... Mathematical functions that relate KS to temperature can be found in the literature (AHPA, 1989).

In general, the more a water is supersaturated with respect to calcium carbonate, the faster scaling will occur. Therefore, the most important predictor of a water's tendency to scale is the degree to which it is supersatured with respect to calcium carbonate; estimating the a water's degree of supersaturation requires knowledge of the water's chemistry.

When a water is analyzed, typically the calcium ion concentration, [Ca2+], and/or the total hardness is measured. Total hardness is the sum of the dissolved calcium and magnesium ion concentrations.

Total Hardness = [Ca2+] + [Mg2+]

The total hardness of most domestic waters is predominately due to calcium. A water's tendency to scale increases with increasing calcium concentration and hardness.

When a water is analyzed, the carbonate ion concentration is not measured directly, but must usually be estimated from a pH and total alkalinity measurement. The carbonate ion concentration, and therefore a water's tendency to scale, increases with both pH and total alkalinity.

Finally, due to the competing effects of temperature on pH, alkalinity, and the calcium carbonate solubility product, the tendency of lower pH waters (pH ~7.5) to scale increases much more rapidly with temperature than for higher pHs waters (pH ~ 8.5) (Baker, 2000).

In Summary, Scaling Mainly Varies with Water Chemistry and Temperature:

  • Water chemistry: Scaling increases with increasing Calcium Concentration, Hardness, Alkalinity and pH.
  • Temperature: Scaling increases with increasing temperature. Scaling rates increase much more rapidly with temperature for waters with lower pHs (~7.5) than for those with higher pHs (~8.5).

When describing a water, it is important to make a distinction between the qualitative term hard and the quantitative measure hardness. A water is described as hard if it tends to produce scale, irrespective of its hardness. For a given hardness, a water may or may not produce scale, depending on its alkalinity, pH and temperature. Therefore, knowing a water's hardness in absence of its alkalinity and in particular its pH does not indicate if the water will tend to scale.

For more detailed information on water chemistry and scaling refer to Baker (2000), Snoeyink and Jenkins (1980), and Loewenthal and Marais (1976).

What are calcium nodules? In swimming pools and spas, they are small mounds, or “slag” piles of calcium carbonate which are formed from material that has been released from the plaster.

Here are some pictures of calcium nodules:

calcium nodule on pool plaster  calcium nodule in pool plaster 1  calcium on cool decking  calcium nodule on pool tile

In the cement/concrete industry, this phenomenon would be called a form of “efflorescence” (but in keeping with the spirit of common pool industry practice, we call them “calcium nodules” instead…) 

What causes calcium nodules? Why are they much more likely to appear on re-plastered pools rather than on new plaster? Why can they even show up on tile or fiberglass pools? Why is there usually a crack in the plaster where the nodule is located? What role does water balance play in nodule formation? Is there scientific data for those who want to see it?

The most common type of nodule is the “delamination” nodule. These nodules grow because of a void (usually a bond separation) between plaster and its substrate.

calcium nodule in pools

Under normal conditions, the plaster (white) is bonded to the gunite substrate (speckled), which in turn rests on dirt (tan). When the plaster is new, and the pool is first filled with water, calcium hydroxide bleed-off from the plaster surface dilutes into the pool water (blue) and is converted to non–soluble calcium carbonate (plaster dust).

Continuous “rinsing” of the hydroxide bleed-off by the circulating pool water, as well as normal pool maintenance (such as brushing), prevents a buildup of hardened plaster dust on the new plaster surface.

calcium nodule in pools

Sometimes, an air cavity can be formed between the gunite and the plaster, or between multiple layers of plaster. This is referred to as “delamination” and the cavity is referred to as a “void.” As long as the void is not connected to the surface of the plaster, the fact that the void even exists may not be known. Unless the plaster completely breaks free from the surrounding plaster, creating what is referred to as a “pop–off,” or unless the delamination is extensive, this is not considered by the plaster industry to be a defect.