Calcium sulphate used to be known as gypsum. Gypsum is the common name for plaster, because plaster is mainly composed of calcium sulphate.
When the salt dissolves in water absorbed by the plaster, it moves to the evaporating surface, where it concentrates.
Calcium sulphate salts are benign. They are not toxic, nor are they damaging to the property. They are just unsightly.
In most cases, once dry they can be rubbed off with sandpaper without damage to the plaster wall. Occasionally they cause salt erosion, where the salts have concentrated so much they are pushing behind the surface of the plaster. In these case it may be necessary to skim the plaster wall. A skim coat of plaster is very quick and therefore surprisingly inexpensive.
The combination results in heat loss, increasing the risk of condensation.
Tips for treating calcium sulphate
- Identify the root cause of damp and remediate.
- Typically the root cause of damp with calcium sulphate is either intense condensation, often interstitial condensation, sometimes penetrating damp, or leaks and very rarely rising damp – but be aware it is consistently misdiagnosed, so get a second opinion from a competent and independent damp surveyor (not benefiting from recommendations) – avoid damp surveyors who used to work for contractors (they rarely improve their ways).
- Once dry, most walls with surface calcium sulphate salts can be simple sanded down with sand paper.
- Occasionally, when a wall has been very damp, the surface plaster splits. In which case you may need to fill holes or even skim plaster the wall.