These days, all well-known manufacturers of floor levelling compounds offer products based on both cement and calcium sulphate. Throughout Germany the share of calcium sulphate (gypsum) based levelling compounds is approx. 10%. However, there is a considerable East / West difference. In the newly-formed German states, the share of gypsum levelling compounds is approx. 40 – 50% whilst, in the West, it is only approx. 5 – 10%. With this background, the question suggests itself: when is the calcium sulphate-based or the cement-based version the technically correct solution?
Based on the Product Data Sheets of the manufacturers, with these two product groups, practically the whole spectrum of flooring applications can be covered. The significant, material-specific differences, and the consequences derived from them for practical application, are opened up to the user only at second glance, if at all.
With regard to substrate preparation for floor coverings and wood flooring, the relevant differences are itemised as follows:
- cement levelling compounds shrink during setting and can, therefore, dependent on the respective product quality, build up relatively high stresses combined with a pronounced tendency to cracking. Gypsum-based compounds are practically stress-free.
- cement levelling compounds bind the mixing water during a long-lasting process and, therefore, at greater thicknesses (> 3 mm), are faster drying. Gypsum-based compounds bind the mixing water very quickly and then dry out purely physically which, at greater thicknesses, leads to relatively long drying times. High-speed construction systems that dry ready for covering in only one hour are only possible with cement-based materials.
- cement levelling compounds, in standard formulations, reach high strength. High quality products have a compressive strength of 30 to 40 N/mm² whereas corresponding calcium sulphate-based products have 25 to 35 N/mm².
- short-term ingress of moisture, e.g. through the joints in tiled flooring, affect the strength of cement-based levelling compounds very little. However, gypsum-based compounds lose significantly in strength through water absorption.
As a rule, for practical usage, both are determined through the prescribed parameters according to project requirements, firstly substrate and secondly proposed covering / intended use. From the above-described differences in combination with the prescribed project requirements, there are certain cases of usage in which one of the two material versions should preferably be used.
Mastic asphalt: due to its high coefficient of thermal expansion and its visco-elastic properties, mastic asphalt is very sensitive to temperature changes and influences from external stresses; the corresponding potential for damages is legion. Old mastic asphalt should, therefore, be prepared where possible with calcium sulphate-based compounds. New, well-gritted mastic asphalt can also be prepared in limited thickness (max. 3 mm) using “low stress” cement compounds.
Mixed substrates comprising different material compositions: they often occur if, e.g. planning changes in the course of renovation work are made. These also are clearly the domain of gypsum-based compounds.
Chipboard / dry screed materials: even when chipboard or pre-fabricated screed materials are installed in accordance with regulations, greater deformation under loading must be taken into account than with cement- or calcium sulphate- screeds. By the use of gypsum-based compounds, including fibre-reinforced as applicable, additional stress-related deformation is avoided.
Surface coverings / mechanical loading:
Of the resilient materials, rubber coverings place the highest demands on the surface strength of the levelling compound. Their high vapour diffusion resistance very effectively stops the evaporation of residual moisture from the levelling compounds and dispersion adhesives. Therefore, under these coverings, primarily high-strength cement-based compounds should be used.
As the result of dimensional changes due to moisture, wood flooring builds up high shear forces between the flooring and the substrate. Therefore, in these cases, the higher strength cement-based products are preferably used. However, the type and size of the wood also plays a great role so that manufacturer-specific recommendations that differ from this may be given.
In wet areas and with ceramic tiles / natural stone, in both installation and usage, high exposure to moisture must be taken into account. This practically excludes the use of gypsum-based compounds.
If coverings are exposed to high mechanical loading from heavy vehicle traffic (e.g. fork-lift trucks), high-strength cement-based levelling compounds should be used as far as is possible.
The above description clarifies that the preference for the gypsum-based compounds is primarily triggered by the substrate, whereas the cement preference stems mainly from high-demand coverings and loadings. Between these, in practice, there are many crossover cases in which both types of levelling compound can be seen as fundamentally suitable. In the attached diagram, this is put together as an overview. Immediately recognisable are also the substrate / covering combinations that do not give a spontaneous material recommendation, e.g. old mastic asphalt / rubber coverings or chipboard / wood flooring. Even for these exacting demands, there are reliable system constructions, e.g. with the aid of special, suitable products or stress-relieving, separating systems. In such cases, it is also recommended to obtain technical advice.
Above all, this article should give a quick orientation through the diversity of product-related information. Ultimately, the individual project conditions and also the user preferences must determine the selection of materials. The UZIN Technical Division is always pleased to provide any support required.
|Substrate||Covering / Load Effect|
|old mastic asphalt||wood flooring|
|mixed substrate composition||rubber|
|dry screed materials||moisture|
|heavy loading / high-speed construction|
Calcium Sulphate Levelling Compound
Cement Levelling Compound
Dr. Norbert Arnold - Technical Product Services Manager