Aqueous CaCO3 dispersions as reference systems for early-age cementitious materials

Cementitious materials are derived from complex colloidal systems in which different minerals react with water to form a variety of hydration products. Moreover, chemical additives are often incorporated in these systems to optimize the behaviour of the fresh slurries, and ultimately, the physical properties of the hardened material. Hence, the solution and interfacial phenomena, which determine the behaviour of cementitious systems, are inherently very complex. In principle, comparative studies on dispersions of portland cement and appropriate ‘inert’ minerals should help the identification and understanding of processes specific to reactive cementitious components. To this effect, several highly insoluble minerals were selected including TiO2, Al2O3, SiO2, Mg(OH)2 and CaCO3. The state of flocculation of these materials in dilute and concentrated slurries was examined through sedimentation and rheological measurements under various conditions, and the results compared to observations on similar slurries containing cement. The comparison showed that calcium carbonate (CaCO3) exhibits surface and colloidal properties very similar to cement particles at early stages of hydration. CaCO3 pastes can be made to accurately duplicate the kinetic properties of a cement paste, including phase separation (sedimentation, surface bleeding) and most dynamic viscosity parameters. It is therefore proposed that CaCO3 pastes can be used to adequately model ‘physical-type’ effects occurring in cementitious systems at a very early stage of hydration, i.e., in the first hour.