Sulphates (SO₄²⁻) are substances that slowly build up in your pool. You rarely hear about them because pool maintenance usually focuses on chlorine, pH, and alkalinity. But sulphates affect both the pool’s concrete or plaster surfaces and how well your disinfectants work.
Important: sulphates do not evaporate, are not removed by filters, and are not broken down by chlorine. If they enter the water, they will accumulate over time.
Important: sulphates do not evaporate, are not removed by filters, and are not broken down by chlorine. If they enter the water, they will accumulate over time.
Where sulphates come from
1.Dry acid (sodium bisulphate)
- The most common granular pH reducer.
- Lowers pH and adds sulphates to the water at the same time.
- Regular use over a season can significantly raise sulphate levels.
- Often used in automatic dosing systems.
- Works like dry acid: lowers pH and leaves sulphates in the water.
- Automatic dosing accelerates accumulation.
- An oxidizer often used in SPAs or as a gentle shock treatment.
- Leaves sulphate residues after reaction.
- Frequent use increases water’s total dissolved solids (TDS).
- Tap water, well water, or even seawater may already contain sulphates.
- Adding makeup water contributes to gradual accumulation.
- Certain aluminum-based or industrial products contain sulphate salts.
Why sulphates matter
Sulphates don’t cause instant damage, but over time they wear down cement-based surfaces like plaster, marble finish, and grout:
- Surfaces become rough or lose smoothness
- Micro-cracks form and accelerate wear
- Plaster becomes more porous → water chemistry becomes harder to control
How sulphates affect disinfection
High sulphate levels create two major problems at once:
1.Faster surface wear increases chlorine consumption
2.High TDS reduces chlorine efficiency
In other words, surface degradation and high mineral content feed off each other: sulphates damage the surface, and that same damaged water requires more chlorine to stay clean.
1.Faster surface wear increases chlorine consumption
- Micro-cracks and porous plaster absorb more chemicals
- Chlorine is used up faster
- More chemicals are needed to maintain proper disinfection
2.High TDS reduces chlorine efficiency
- Chlorine breaks down more quickly
- Doses must be higher
- ORP (oxidation-reduction potential) and disinfection stability drop
In other words, surface degradation and high mineral content feed off each other: sulphates damage the surface, and that same damaged water requires more chlorine to stay clean.
What recent studies say (2024–2025)
- Sulphates penetrate deeply into cement, not just the surface.
- High temperatures speed up damage.
- Wet-dry cycles amplify degradation.
- High mineral content increases chemical consumption for disinfection.
Bottom line: heat + high sulphate levels + regular use of dry acid, sulfuric acid, or MPS = faster surface wear and higher chlorine demand.
When to Be Concerned
Up to 300 ppm — safe
300–500 ppm — monitor levels
Over 500 ppm — risk for plaster and grout damage
Over 1000 ppm — accelerated degradation likely
What you can do
- Use hydrochloric acid (HCl) instead of dry acid or sulphuric acid to avoid adding sulphates
- Avoid overusing MPS
- Monitor TDS and sulphate levels
- Partially replace pool water periodically
- Check plaster and grout condition regularly