Calcium Carbonate Or Chalking?
If you’ve noticed your fiberglass pool starting to appear white, hazy or faded, don’t assume it’s simply a cosmetic issue — the cause could be a water chemistry issue that needs serious attention. In fiberglass-shell pools, two distinct phenomena can lead to this appearance: calcium carbonate scale (commonly thought of in traditional pools) and the less-understood phenomenon known as chalking. According to Orenda Technologies, understanding the difference between these will help you diagnose the problem correctly and take appropriate action.
What is Calcium Carbonate Scale?
When water becomes oversaturated with calcium carbonate (CaCO₃), the excess precipitates and deposits on surfaces, often as white, chalky buildup or flakes. Orenda explains that scale occurs when the Langelier Saturation Index (LSI) goes above approximately +0.30 (on the Orenda calculator, this appears purple) and calcium carbonate begins to come out of solution and adhere to surfaces. Typically, this kind of scaling is seen in tile lines, heat exchangers or pool equipment, but can also manifest on pool shells. When scale builds up, it’s often dissolvable with acid or chemical treatment, and at least removable in many cases.
What is Chalking? (Especially in Fiberglass Pools)
In the context of fiberglass pools, Orenda identifies a different issue: chalking. While it may look like scale (white haze, chalky film), chalking is not the same as calcium carbonate scale. Their lab results suggest chalking is caused by calcium oxide (CaO) bonding within the gel coat of the fiberglass shell rather than a surface deposit of carbonate.
Key distinguishing features:
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The white film often cannot be wiped off easily with acid — the acid test that typically dissolves CaCO₃ scale fails.
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Chalking tends to occur more readily on flatter surfaces of the shell (less-densely crosslinked areas of gel coat) rather than edges or highly radiused surfaces.
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Once chalking has penetrated the gel coat, surface treatment may not fully restore original appearance; resurfacing may be the only reliable remedy.
Why Does My Pool Shell Fade or Turn White?
Several chemistry factors contribute:
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With calcium carbonate scale: high calcium hardness, high pH, high alkalinity, high temperature or elevated LSI can force calcium carbonate out of solution and onto surfaces.
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With chalking: Orenda’s research suggests conditions of higher pH and elevated free chlorine may correlate with worse risk of chalking in fiberglass shells. Also, calcium hardness exceeding about 120 ppm (with no sequestration) appears a common threshold in lab tests where chalking began.
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Lack of calcium is also risky: for fiberglass or vinyl pools (which cannot supply calcium to water as plaster can), if the water is undersaturated (low calcium hardness + negative LSI) the water may become aggressive, promoting deterioration of gel coat or surface fading.
How to Tell If It’s Scale or Chalking
Here’s a simplified comparison:
| Condition | Wipe Test with Acid | Likely Cause |
|---|---|---|
| White film wipes off easily | ✅ Acid dissolves it | Calcium carbonate scale |
| White film doesn’t wipe off (or etches into surface) | ❌ Acid ineffective | Likely chalking in gel coat |
Orenda specifically recommends this test:
“If it’s chalking, the damage is done … we do not know of a way to clean it up.”
What You Can Do: Prevention & Correction
For Calcium Carbonate Scale
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Use the Orenda LSI calculator to maintain LSI in a safe zone (–0.30 to –0.01) when treating scale.
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Lower pH to 7.2-7.4, bring down calcium hardness, add sequestering agents such as SC‑1000 to help pull calcium back into solution.
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Brush surfaces, increase circulation, and physically remove scale after softening.
For Chalking (Fiberglass Pools)
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Keep pH from climbing too high; Orenda recommends a “pH ceiling” of ~8.09 or less.
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Keep calcium hardness controlled; ideally around or below ~120 ppm if no sequestration is applied in fiberglass pools.
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Use a high-quality calcium-sequestering agent (SC-1000 or equivalent) to bind free calcium and prevent bonding inside the gel coat. Orenda’s lab data showed no chalking when SC-1000 was used even at very high calcium hardness (1000 ppm) in test samples.
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Maintain overall water balance (LSI), limit oxidizer/ chlorine levels to reasonable ranges, and monitor for early signs of hazing.
If You Already See Fading or White Film
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Perform the acid-wipe test to check for scale vs chalking.
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If scale: treat accordingly with scale-removal procedure.
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If chalking: unfortunately, the damage may require resurfacing/refinishing the pool shell because the gel coat has chemically changed.
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Consult your pool manufacturer and your pool service technician for shell-specific options and warranties.
Why It Matters for Fiberglass Pools
While fiberglass pools are often marketed as being “low maintenance” and having "lifetime warranties", Orenda’s insights remind us that chemistry still matters. Even though fiberglass shells don’t supply calcium (unlike plaster), they can still suffer from chemistry-driven surface degradation (like chalking) when water is aggressive or imbalanced. Maintaining balanced water isn’t just for clarity or algae prevention—it’s essential for protecting the integrity and appearance of your pool shell.
