Hydration and structure development of ternary binder system as used in self-levelling compounds

Self-levelling flooring compounds (SLC) are applied in thin layers (less than 1cm) to level floors before further finishing cover such as parquet or plastic cover. They are composed of a ternary binder system based on Portland cement (PC), calcium aluminate cement (CAC) and calcium sulfate to provide fast set. This mixed binder is modified by additives in order to obtain specific properties such as flow properties or defoaming. In total, more than 10 components make up the formulations of SLCs. SLCs are one of the most complex systems in the field of cementitious materials. It is known that ettringite is the main hydration product of mixed binder systems but the hydration development at early ages is little known. The main objective of this work is thus to clarify the mechanisms of the development of the hydration at early ages of ternary binder systems modified by additives. Two formulations were studied: a system in which the mixed binder system is dominated by PC and one in which it is dominated by CAC. Both systems exhibit the formation of ettringite, but the formation of this phase is mainly due to the hydration of monocalcium aluminate CA from the CAC. Thus, the PC formulation is not robust. Parameters such as the minor phases of the PC and the type of the calcium sulfate have a major impact on the hydration development. As CA is a minor constituent of this mixed binder system, the amount of ettringite formed is low. Thus, the PC dominated formulation reacts relatively slowly. The CAC dominated formulations react faster because the CA content is higher. The porosity is lower and internal and external parameters such as the minor phases of the PC and evaporation affect much less the development of hydration. As the CAC dominated system forms more ettringite, the chemical shrinkage is higher but the autogenous expansion measured is lower than that observed for the PC dominated formulations. Calcite is also added in the formulation in large amount as a filler to fill the porosity. The formation of monocarboaluminate from the reaction of calcite is also observed.