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.
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.
2.Liquid pH reducer (sulphuric acid)
Often used in automatic dosing systems.
Works like dry acid: lowers pH and leaves sulphates in the water.
Automatic dosing accelerates accumulation.
3.MPS (non-chlorine shock)
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).
4.Source water
Tap water, well water, or even seawater may already contain sulphates.
Adding makeup water contributes to gradual accumulation.
5.Some chemicals and coagulants
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:
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
Sulphates are not an instant crisis. But if they accumulate:
Surfaces wear down faster
More chlorine is needed to maintain disinfection
Maintenance costs in Paphos increase
Plaster and grout age quicker
Controlling sulphate sources is a long-term strategy to keep your pool surfaces intact and your water chemistry stable.