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Cement or Calcium Sulphate Compounds – what is the technically correct solution?

All well-known manufacturers of smoothing compounds and underlayments offer products based on both cement and calcium sulphate. What is the technically correct solution for using calcium sulphate-based or the cement-based versions?

Based on the Product Data Sheets of the manufacturers, with these two product groups the whole spectrum of flooring can be covered. With regards to substrate preparation the relevant differences are as follows:

- cement levelling compounds shrink during setting and can 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 but 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 strength through water absorption.

As a rule, both are determined 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.

Substrates:

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 which can occur, e.g. planning changes in the course of renovation. 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 using fibre reinforced gypsum-based compounds 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 can stop the evaporation of residual moisture from the levelling compounds and 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 preferable. However, the type and size of the wood also plays a great role so that manufacturer-specific recommendations that differ from this may be used.

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.

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. In practice, there are many crossover cases in which both types of levelling compound can be seen as fundamentally suitable. It is therefore best to consult further technical advice.

This article provides a quick guide. 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 support. Please e-mail info@ uzin.co.uk or call 01788 530080.

Author:
Dr. Norbert Arnold - Technical Product Services Manager