Kinetics, shrinkage, microstructures and material properties of self-levelling flooring compounds

A comparison of Portland cement and calcium aluminate cement dominated mixed binders.

Dr. Julien Kighelman (EPFL, Lausanne, CH), Dr. Roger Zurbriggen (Nouryon Chemicals  AG, Sempach Station, CH), Prof. Dr. Karen Scrivener (EPFL, Lausanne, CH)
The purpose of a self-levelling compound (underlayment) is to produce an even and smooth floor surface on which a final cover (e.g. carpet) can be applied. A majority of formulations is based on a mixed-binder system (CAC+PC+C$) with ettringite as the main hydrate phase. There are two subtypes of such mixed-binders, dominated either by calcium aluminate cement (CAC) or Portland cement (PC).
In PC-dominated formulations a temporary inhibition of tri-calcium aluminate (C3A) seems to reduce the reaction intensity. Thus, gypsum is formed as an intermediate hydrate. Later this gypsum is consumed to produce further ettringite. The large reservoir of tri-calcium silicate (C3S) causes hydration to last as long as water is available. These complications result in a poly-episodical hydration. In contrast, CAC-dominated formulations show one intense episode of ettringite formation during the first couple of hours. These differences in kinetics have a strong influence on the shrinkage and ageing behavior.
However, in both systems the redispersible powder is an essential component.
The main reason for this is the mechanism of film formation which acts in a complementary way compared to cement hydration.
The paper describes the different mechanisms, related microstructures and resulting mortar properties.