Calcium Scale ≠ Calcium Hydroxide crystals

June 19, 2017

Have you ever encountered the problem of "calcium scale that keeps coming back"? Has it been impacting your pool business? Are you sure you have diagnosed the problem correctly? Because it's easy to mistake calcium hydroxide crystallization for carbonate scale.


This is a common problem, especially in colder climates, so we decided to share what we know, and hopefully it can help you if you're in a similar situation. This article will cover the distinction between carbonate scale and calcium hydroxide crystallization. How to spot the difference, and the do's and don'ts of how to treat each one.

Calcium Hydroxide ≠ Scale

calcium crystal, crystals, calcium hydroxide, calcium carbonate, scale, pool scale, scale removal, dissolve calcium You're looking at the inside of a pump strainer basket. Those white crystals grew out of the plaster surface. We had it lab tested... and it's NOT calcium carbonate (scale).

For background information, we invite you to do some external research on calcium. Here are some of our articles, if you'd like to do some quick reading before going further:

The "scale" problems we have recently encountered turned out to be calcium hydroxide crystals. Here are three common factors in all of the cases we encountered:

  1. A misunderstanding of, or not paying attention to the Langelier Saturation Index (LSI),
  2. Mistaking calcium hydroxide crystals for calcium carbonate scale, and
  3. Using low pH (acidic) products to combat the problem.

What is calcium hydroxide?

Calcium Hydroxide (Ca(OH)2), aka Lime, is an alkaline substance commonly used for neutralizing acids. It is formed when calcium oxide (CaO) is hydrated; a process which happens to occur when new pool plaster is curing. In the pool business, it is mostly known as a pesky substance referred to as "plaster dust". Some things to know:

  • calcium hydroxide has a very high pH (13+)
  • hardened calcium hydroxide (lime) is not very soluble in water (it's hard to dissolve)
  • using acid products to remove calcium hydroxide is effective in the short term, but in a swimming pool, it actually makes the problem worse due to acid's impact on the LSI.

According to the National Plasterer's Council (NPC), the curing process of plaster involves 'bleeding off' calcium hydroxide, aka plaster dust. That means calcium is being extracted from the plaster itself and being donated to the water's solution. This process is irreversible damage to the plaster surface.

That said, this 'bleed off' can be minimized and controlled.

Quick facts about plaster pool startups

The NPC estimates about 87% percent of plaster failures/problems are the result of at least one of three things:

  1. Calcium hardness in the water is too low
  2. Carbonate alkalinity in the water is too low
  3. Not using a chelant/sequest on startup

The first two directly impact LSI balance, meaning they affect the saturation of the water. The third item directly is about holding minerals in solution while the plaster cures. Remember, any one of those three items can cause big problems for a plaster surface.

What is scale?

Scale is usually calcium carbonate that forms on surfaces as a result of too much calcium in the water. In other words, the water is over-saturated with calcium and cannot hold any more. On the LSI, scale occurs over (+0.3). Unlike etching, however, scale can be treated. Pools can increase calcium hardness one of two ways: 1) someone adds calcium to the water, or 2) the water takes calcium from wherever it can (usually plaster).

It is very rare--if not impossible--for a low-calcium pool to form scale, because the water will only draw enough calcium from the plaster to the point of saturation balance (scale requires over saturation of calcium). Where is the saturation point of water? It depends on where you are, and the other factors that impact the LSI. We have found that water wants to have calcium somewhere between 250 and 400 parts per million. That said, if the operator is regularly monitoring the LSI and balancing it accordingly, pools can hold over 800ppm calcium without scale forming. We have seen pools with 900ppm, but at that point it's difficult to manage.

All pool chemistry starts at the tap


How to distinguish scale from crystallized calcium hydroxide

saltwater scale calcium hardness Scale usually shows up on tile lines and spillways.

Carbonate Scale

  • Comes from the water being over-saturated with calcium (high LSI)
  • Usually forms in the warmest water first (again, high LSI). Look for scale on salt cells, heat exchangers, hot tubs, sunny spots in the pool, shallower water and dark tile lines that absorb heat.
  • Scale can form on the face of tile, spillways, rocks, liners and fiberglass. Calcium hydroxide cannot.
  • Scale can also appear above the water line due to the 'wet/dry effect', where the water splashes on there but dries, leaving behind calcium scale. Calcium hydroxide cannot.
  • Acid can remove it, albeit aggressively.

spot etching, calcium hydroxide, pool Calcium hydroxide crystallizes below the water line as it bleeds out of the plaster.

Calcium Hydroxide crystals

  • Occurs when water is under-saturated with calcium (low LSI)
  • Initially appears as plaster dust, which is brushed during the first several days after a startup.
  • Normally occurs over the winter in colder water, when most etching damage is done to pools (again, low LSI).
  • Forms crystals that can be sharp. We have heard of crystals being two inches thick!
  • Will only appear on plaster surfaces and tile grout...but never on the face of tile or other surfaces.
  • Only appears below the water line, because the water is extracting it out from the plaster.
  • Acid products remove it in the short term, but the problem will come back again because the water still craves calcium. That, and the low pH of the acid lowered the LSI again, yielding more etching.
  • Common in pools that winterize with a normal or lower pH (< 7.4).

Treating each: Do's and Don'ts

Carbonate Scale

  • DO test your pool water (AND tap water!) for calcium hardness, alkalinity and pH. Log your results, either on the Orenda App and email them, or by hand.
  • DO adjust your pH and alkalinity to balance your LSI according to the dosing instructions in the Orenda App.

    LSI calculator Our Orenda App has two separate LSI values: one for the current readings, and one for the desired (future) readings.

  • If you use cal hypo, DO consult a service professional about potentially switching to a different type of chlorine. It may not be necessary, but never make a switch without a trained professional.
  • DO use a calcium sequest/chelant to help dissolve the calcium scale back into solution. For scale above the water line, raise the water line so the sequest can reach it.


  • DON'T use acid products if you're not a trained pool professional, with the proper safety gear. Acid products are okay to use for scale removal, but be aware of its impact on overall pH of the pool and how that affects the LSI.
  • DON'T treat for scale without being sure it's actually scale.

Calcium Hydroxide crystals

  • DO test your pool water (AND tap water!) for calcium hardness, alkalinity and pH. Log your results, either on the Orenda App and email them, or by hand.
  • DO adjust your pH and alkalinity to balance your LSI according to the dosing instructions in the Orenda App.
  • ADD CALCIUM. The dosing calculator in the Orenda App will tell you exactly how much you need. If the pool is one that gets winterized, we recommend 350-400ppm calcium hardness. And we recommend using flakes, not granular (prill). Apply to the deep end of the pool and/or pre-dissolve the calcium, so it does not get to the bottom and burn the surface. As you will find out, when calcium dissolves, it gives off a lot of heat!
  • DO use a calcium sequest/chelant (like SC-1000) to help dissolve the calcium hydroxide crystals back into solution and remove them from your walls.

    See the clean spot on the top step? Acid will show results quickly, but is an aggressive treatment for calcium hydroxide, and can actually further the problem in the long term.


  • DON'T use acidic products as a strategy, because your problem is with a low LSI (your plaster is bleeding out calcium). Adding acid will make the problem worse in the long term, despite short term visible results.
  • DON'T drain the pool and acid wash. The better strategy is to use a sequest/chelant (like SC-1000) while the pool is full, chemically break down the calcium hydroxide into solution, and go from there. If you insist upon draining your pool, do it after the calcium hydroxide crystals are gone.
  • DON'T expect SC-1000 to remove crystals overnight. It takes several weeks.


In either case--calcium carbonate scale or calcium hydroxide crystals--the Langelier Saturation Index is your measurement of success. If your LSI is balanced, neither problem can occur in the first place. Preventative care, anyone?

Removing existing scale or crystals is where strategies differ. Here's a video we found online of a customer testing our SC-1000 product on calcium scale scum line. To us, it looks like the wet/dry effect where some calcium has dried on the tile, and isn't necessarily pure scale. But either way, SC-1000 will gently dissolve the calcium back into solution in a matter of weeks.

Beware of the trap: acid products work before your eyes against both problems! That said, if acid is to be used at all, it should only be on carbonate scale, not calcium hydroxide crystals. In the case of calcium hydroxide, get the pool's calcium level up (we say 300-400 ppm) to stop the bleeding, then use a sequest/chelant to chemically dissolve the crystals into solution. You cannot reverse the damage within the plaster that has already done, but you can stop further damage from occurring.

If you are in a climate requiring winterization of pools, management of the LSI becomes even more critical, because colder temperature yields a lower LSI score. Play around with our calculator and watch how the LSI changes in real time.

For more questions or general advice on how to manage these problems, we are available to speak with you in confidence. Just contact us.

Thank you for taking the time to read all this. If you know someone who would find this article valuable, please share it with them.

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