It is naturally vital to minimise risks of breakdown, and for this reason limits are set on how damp a substrate may be when an adhesive-coated covering is applied. The damp of interest is that prevailing under the floor-covering in the long term. This is not the same as that present in the surface at the time the adhesive is applied to the covering, and it must be estimated by measurement at a greater depth in the substrate. For concrete constructions with single-sided drying the measurement depth is 40% of the thickness of the construction. For double-sided drying the comparable figure is 20%.

 * With special glue e.g. MS-polymer glue, the RH level can be 80%. Always see the specification from the glue/adhesive producer.
 

 
maxit P-marked levelling compounds, introduced in 2000, are low alkaline because the main binder is aluminous cement. Comprehensive emission measurements have been carried out on composite designs consisting of concrete, primer, levelling compound, and floor-coverings coated with adhesive. Measurements have been made both after six months' exposure and also after a considerably longer exposure of four years. This is important, for the protective function provided by a low alkaline substrate may be lost if alkalinity from the concrete substrate eventually penetrates through the levelling layer and attacks adhesives or floor-coverings. In this context the thickness of the levelling compound and the humidity of the substrate are both of crucial importance.

maxit's comprehensive research into emissions (see Alexandersson J, AMA Nytt 1/2000 and Concrete Sept 2001, LTH 2004) has shown that the protective function against alkaline breakdown of adhesives and floor-coverings is permanent provided that RH is no higher than 90% and the thickness of the levelling layer is at least 5mm.

Different kinds of calcium sulphate binders can be used alone or better still in combination. In any case the principle reaction with water is the formation of calcium sulphate dihydrate (CaSO₄ x 2H₂O) whether the reaction starts from natural anhydrite, thermal anhydrite or alpha-hemihydrate.

On calcium sulphate compounds all kinds of floor-coverings can be applied, provided that the screed is dried out and the surface is prepared for application of adhesives.

This means that the surface is ground. If a cement-based product, e.g. tile adhesive is to be applied, a dispersion primer is applied first, in order to prevent uncontrolled ettringite formation at the borderline.
In contrast to cement-based levelling compounds, calcium sulphate screeds achieve their maximum strength only when they have completely dried out.

Self-levelling cement-based compounds like calcium sulphate compounds need water in the mixing process for several reasons. First water is needed for the reaction of different calcium sulphate phases. It is needed to achieve good flow behaviour and a good workability. Before any floor-covering can be applied, the remaining water needs to dry out to avoid later damage to the floor-covering.

Moisture, that has not reacted with the binder in the screed, has the tendency to migrate to the screed surface. If this surface is left uncovered, the moisture is harmless. In other cases the moisture can cause saponification of the adhesive, softening the surface, or the formation of an undesired new mineral, that can destroy the adhesion of the covering. The risk of saponification however is much less than for products based on ordinary cement or a high alumina cement.

The strength of calcium sulphate screeds depends on the drying conditions of the screed. Every additional percent of humidity results in lower strength values.

Therefore it is absolutely essential to dry out the floor compound before applying a floor covering!