Calcium Crystals vs. Scale

NOTE: This article has been revised from its original version, due to learning more from laboratory results. We still have more unanswered questions about crystals than we have answers. Why do they harden? How many forms are there (we know of at least 3)? Why are some crystals easy to clean up, and others stubborn? What common denominators are involved, beyond cold water and lack of calcium hardness?


Key Takeaways of this article:

  • Calcium Crystals are a consequence of a severely low-LSI.
  • Calcium Crystals almost always form in very cold, still water during the winter.
  • We have no evidence that suggests crystals are the result of–or even related to–flawed plaster materials or workmanship.
  • Crystals are not unique to any one brand or type of surface. Pebble, Quartz, Plaster, etc. are all susceptible.
  • We have abundant evidence that suggests crystals grow out of a surface due to water having a severe need for calcium saturation (in other words, the water is starving for calcium and extracts it from the plaster).
  • We do not yet know why crystals harden instead of simply dissolving into solution.
  • We do not yet know why some pools have winter dust, and others have crystals.
  • We know of at least 3 distinct types of winter crystals.
  • All forms of winter crystals we know of can be prevented by adding enough calcium and alkalinity to prepare the pool for the winter (raise the LSI in expectation of cold water).

Calcium is misunderstood

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 crystallization for carbonate scale. Calcium hydroxide leeching out of curing plaster carbonates and becomes calcium carbonate (plaster dust)...but that's not what we're talking about in this article. Pool plaster surfaces are being damaged in the wintertime, sometimes severely. Cold water is more aggressive, and these calcium crystals are a consequence of under-saturated water that needed more calcium in the winter. There are some other possible consequences too.

Calcium crystals are a common problem, especially in colder climates, so we are sharing 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 crystallization. How to spot the difference, and the do's and don'ts of how to treat each one.


Calcium Crystals are not 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:

We were up in the Northeast and kept hearing of these horror stories of "scale." It seemed epidemic... as in everyone in the pool business has a story to tell about it. As it turns out, the "scale" problems turned out to be calcium crystals, not scale. A few factors didn't add up to being scale, so we knew something was different. Here are three common factors in all of the cases we encountered:

  1. In each of the pools' water chemistry, the LSI was very low. Like -1.20 and below. It was severe.
  2. Each customer was using low pH (acidic) products to combat the problem.
  3. The water was cold. These crystals were being formed over the winter and discovered during spring openings.
  4. These deposits were forming underwater, below the winterization waterline. None of this was present on dark tiles on stairs or anything else. Since scale falls out of solution and lands on any surface, this was evidence that it was not scale.

All these observations told us that this calcium was not falling out of solution and landing on the pool surface (scale)--it was growing out from the plaster. This led us to start doing research. But perhaps it could've been carbonation as well. At first, we thought it was calcium hydroxide, because we know that calcium hydroxide bleeds out of plaster when it's curing.

Calcium Crystals

There are several types of calcium crystals we have identified. The most common forms are both calcite, which is a dense form of calcium carbonate that grows in a "skeletal" pattern, according to the laboratory. Some grow long and thin, with a microscopic hollow tunnel through them. Others grow short and sharp, like gritty sand paper. Some forms of crystals are easy to clean up and dissolve, and others are brutally stubborn.

Screen Shot 2018-12-05 at 9.48.46 AMScreen Shot 2018-12-05 at 11.15.28 AM






calcite crystals underwater, pool crystals, plaster crystals, winter scale, pool scale, winterization, calcium crystals, orenda crystals, remove pool crystals

The top two images show the same selection of calcite crystals that the laboratory concluded "grow from the surface." Growing from the surface is very different than scale, which would deposit out of solution and land on a surface. The third photo with the hand touching the surface shows a more stubborn, and common version of calcium crystals. We do not know why this form is more common, nor why it is so much harder to dissolve and clean up. They are the exact same material. We have more unanswered questions about crystals than we have answers, unfortunately.

What is scale?

Scale is calcium carbonate that deposits 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).

Scale usually forms in the warmest areas first (again, look at the LSI--the warmer the water, the easier it is for scale to form). Think spillways, dark tile, heat exchangers and salt cells. Or for a more practical example, look at scale on your shower head at home. 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). By the same token, it is nearly impossible for a pool to scale during the winter...the water is just too cold.

How to distinguish scale from calcium crystals

Carbonate Scale

  • saltwater scale calcium hardnessComes from the water being over-saturated with calcium (high LSI)
  • Usually forms in the warmest water first. 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 crystals 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. SC-1000 removes it more gently, by slowly dissolving the calcium back into solution.

Calcium Crystals

  • Calcium crystals, calcite, calcite crystals, orenda, SC-1000, pool crystals, how to prevent pool crystals, plaster crystalsOccurs when water is under-saturated with calcium (low LSI)
  • 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

  • orenda calculator, orenda app, lsi calculator, lsi app, LSI, orenda, pool chemistry, water chemistryDO 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. 
  • 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 chelant or sequestering agent to help dissolve the calcium scale back into solution. For scale above the water line, raise the water line so the treated water can soak 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 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 chelant to help dissolve the calcium crystals back into solution and remove them from your walls.
  • 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 crystals into solution, and go from there. If you insist upon draining your pool, do it after the calcium crystals are gone.
  • DON'T expect SC-1000 to remove crystals overnight. It can take several weeks.


In either case--calcium carbonate scale or calcium 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 crystals. In the case of calcium crystals, get the pool's calcium level up (we say 300-400 ppm) to stop the bleeding, then use a chelant to chemically dissolve the crystals into solution. You cannot reverse the damage already done, but you can prevent 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.

Leave a Comment