maxit floor Moisture Environment Concepts  
   
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Background
Risk Factors
Damage Processes
When tp Use Adhesives?
Drying
Drying Phases
Concrete Beams
Calcium Sulphate
Concrete with levelling
Single-side Drying
Optimal Drying
Humidity Measurement
Questions

Calcium Sulphate

Calcium Sulphate-based levelling Compounds and Drying
 

Limit values for moisture


For calcium sulphate screeds there are two limit values for moisture:

  • 0.5% for normal screed
  • 0.3% for screed on floor heating systems

The kind of covering material does not affect the limit values.
Covering the calcium sulphate floor screed shortly after measuring the moisture content is recommended to avoid moisture absorption from humid air.
Calcium sulphate screed on floor heating systems


Several methods for building floor heating systems into floor screeds are used. The floor screed must, however, be heated independently from the system applied. An inspection sheet of the heating should be drawn up.
Note: For picking up samples test areas have to be marked, where no pipeline or wire is underneath.
Influences on the drying behaviour of calcium sulphate floor screeds

To reach the limit values of moisture the floor screed has to be dried. But the drying time depends on several factors:
  • Thickness of the screed: The normal-drying time will be around 3 weeks when the thickness is 4cm or less assuming this thickness is constant. Thicker screeds have to be dried for a longer period because the drying time is then extended in a non-linear way. Double thickness leads to 3 or 4 times the normal-drying time. Because there could be a moisture gradient, the measurement should be representative over the whole thickness.

Uneven groundlayers have to be levelled in advance.
  • Surface structure. A hard and dense layer on top of the screed surface makes the drying process difficult and some times nearly impossible. In such cases the screed surface has to be removed by grinding.
  • Temperature. A higher temperature accelerates the drying process.
  • Air humidity. Air with higher humidity(> 80% rel. humidity) cannot absorb further humidity. The drying process will come to a standstill. Dry air is required.
  • Surface. If a hard, dense layer has been formed at the surface, grinding after a few days improves the drying out. Hard, dense layers are most probably formed when too much water has been used in mixing the product or when products have been applied at low temperatures (< 10° C).
  • Air ventilation. The air has only a certain capacity to absorb humidity depending on the temperature. Therefore the air has to be changed often and thoroughly.
In the first 48 hours the screed must be protected against contamination. On the 3rd day, windows and doors should be left wide-open.
Wetting of calcium sulphate floor screeds


If calcium sulphate screed is exposed to humidity, the degrees of changes inside the screed are dependent upon the water exposure time. A temporary attack of water is harmless, if the water can be dried out rapidly.

If calcium sulphate screed is exposed to water in the long-term a reaction will occur. This can happen when the underlying substrate (for example concrete) is not sealed against rising damp, or the floor is permanently exposed to water.

In this case the strength and the adhesion between screed and floor covering will be weakened and an expansion of the screed may occur. Therefore calcium sulphate floor screeds should not be used in rooms where there is frequent water exposure.

Note: Ensure that there is no water damage from rising damp (i.e. from the sub-floor).