A common misunderstanding when it comes to flowing screeds is that all types of flowing screeds must be surface ground and primed before floor coverings are installed over the screed. Surface grinding is carried out to remove a weak fine layer of dust called laitance which is present on certain types of flowing screeds. To confuse things further there are two types of laitance which can occur.
The first type is a very weak friable dusty layer that is easily identified. The bond strength between the layer of surface laitance and the rest of the screed is very weak and therefore de-bonding of floor finishes will occur if adhesives are applied directly onto this.
The second type of laitance is a very dense hard layer of fine particles which is difficult to remove and can be difficult to identify. This type of laitance forms a glass like surface with a very low porosity. Due to the very dense closed surface; adhesives cannot penetrate the surface of the screed and therefore will not provide an adequate bond between the floor finish and the screed.
Priming of screed surfaces is carried out to provide a stable surface to which adhesives can be applied. Sometimes they are also necessary to prevent loss of the water from the adhesive into the screed. Priming is also essential for Calcium Sulphate screeds to provide a barrier between the calcium sulphate and the cement based adhesive. When a cement-based adhesive is applied directly onto calcium sulphate, cement can react with the gypsum in the screed resulting in a mineral called ettringite being formed at the interface. This will completely destroy any bond between the two materials.
Cemfloor screed is a proprietary flowing screed based on Ordinary Portland Cement (OPC) and Cemfloor binder. Due to its specific formulation the surface of the screed is strong with no laitance present. Since Cemfloor is a cement based screed there is no risk of a reaction taking place between the screed and the adhesive applied.
After Cemfloor screed is installed, a specially formulated evaporation control agent is applied to the surface of the screed. This has two functions; firstly, it ensures that the screed hydrates adequately in the first 24 hours and secondly it increases the bond strength between the surface of the screed and the adhesive used to install floor coverings.
Numerous tests carried out with various adhesive manufacturers show that a bond strength far in excess of 1.0 N/mm2 (double the normal requirement for tile adhesives) is achieved when the adhesive is applied as per the manufacturer’s instructions to Cemfloor. To achieve this, it is only necessary for the screed to be cleaned and to be free from any contamination or debris.
If you wish to learn more about Cemfloor, talk to us today on +353 94 9553900 if calling from Ireland or 01415 307 587 if calling from the UK