Lime and scale on pool stone: remove without damaging the surface

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Why does lime appear in the pool and on the edge stone?

Pool water, especially if it is hard (with high calcium and magnesium content), leaves deposits of calcium carbonate (lime) on all surfaces it comes into contact with. The deposits are more intense on the waterline, where the water rises and falls constantly, evaporating and concentrating the salts in that strip. On the edge stone, splashes and overflow water also evaporate, leaving their minerals behind.

Scale is similar but includes biofilm (algae and bacteria) and iron oxides that give that characteristic yellow or brown tone. The chlorine in the water can favor the precipitation of calcium and increase the speed of formation of these crusts.

Identify the type of stone before acting

This step is critical because some products remove lime perfectly but damage certain types of stone:

• Limestone or marble: are calcium carbonate, so the acids that dissolve lime also attack the stone itself. They require products with controlled pH.
• Granite, slate or volcanic stone: more resistant to acids. They can withstand more aggressive treatments without significant damage.
• Terrazzo, concrete or mortar: resistant to mild acids but sensitive to strong acids like concentrated hydrochloric acid.
• Ceramic and tiles: Ceramic resists well to acids, but the mortar joints are more sensitive.

If you don't know what type of stone you have, always test the product on a small and invisible area before treating the entire area.

Method for resistant stones (granite, slate, ceramic)

For non-calcareous stones, you can use acid-based descaling product with more confidence:

1. Wet the area before applying the descaling product to avoid the acid concentrating in a dry spot and attacking the stone.
2. Apply the descaling product specifically for pools (HG, Mapei Antical, or pool store products) and let it act for 5-10 minutes. You will see that it effervesces: the lime reacts with the acid and dissolves.
3. Scrub with a medium-bristle brush (never metallic on colored surfaces).
4. Rinse thoroughly with water before the product dries to avoid acid residues.
5. Repeat if there are remaining traces. Thick crusts may require two or three applications.

Method for calcareous stone, marble and terrazzo

With stones sensitive to acids, the situation is paradoxical: the lime you want to remove (calcium carbonate) is on a surface that is also calcium carbonate. Strong acids dissolve both without distinction.

The solution are pH-controlled descaling products formulated specifically for calcareous stones and marble. Brands like HG Antical for Natural Stone or Faber StoneDefender Antical work with very mild acids (gluconic acid, low-concentration citric acid) that dissolve the lime deposits with a slower action without significantly attacking the underlying stone.

For thick lime crusts on calcareous stone, the mechanical method with a plastic spatula or nylon brush (not metallic) can remove most of the crust dry before applying the mild descaling product. Less acid needed means less risk to the stone.

Prevention: so they don't reappear

Sealing the stone is the best prevention. A penetrating sealer for natural stone applied once a year closes the stone's pores and reduces the adhesion of lime deposits. When lime deposits form on sealed stone, they form on the sealer layer, not on the stone itself, and are much easier to remove.

Maintaining the pool water pH between 7.2 and 7.6 (the optimal range) also reduces calcium precipitation. At high pH (above 7.8), calcium precipitates much more easily and the deposits are faster and more intense.

• Do not use pure hydrochloric acid on calcareous stone or marble: it destroys them in minutes in an irreversible way.
• Do not scrub with a metallic brush on light-colored stone: it leaves scratches and oxidation marks.
• Do not use vinegar as a descaling product on marble even if "it's natural": the acetic acid in vinegar attacks marble exactly like any other acid.
• Do not leave the descaling product in contact for longer than indicated: the excess contact time increases the risk of damage to the stone.
• Do not work in full sun at midday: the product evaporates before it acts and may leave difficult-to-remove residues.

• Green algae on facade and stone: remove and prevent
• Acid stains on marble: neutralize without etching
• Moisture marks on wood: remove without sanding