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

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) that dissolve lime without damaging the stone. These products are less aggressive and more suitable for sensitive surfaces.

FAQ

Can I use vinegar to remove lime from pool stone? Vinegar is a mild acid and can be used on some types of stone, but it is not recommended for calcareous stones like marble or limestone. It can cause etching and damage the surface. Always test on a small area first.

What should I do if the lime is very thick? Thick lime deposits may require multiple applications of descaling product. Apply the product, let it sit, scrub, rinse, and repeat until the stain is removed. For particularly stubborn cases, consider using a pressure washer with a wide spray nozzle to help loosen the deposits.

Can I use a pressure washer to remove lime? Yes, a pressure washer can be effective, but it should be used with caution. Use a wide spray nozzle and keep the pressure low to avoid damaging the stone surface. Always test on a small area first to ensure it doesn't cause any harm.