Thixotropy of reactive suspensions: The case of cementitious materials

Abstract Concrete, mortars, and grouts in fresh state can be pumped, sprayed and now even printed with additive manufacturing. For this type of materials, the knowledge of flow properties is crucial for adequate control during its applications. The thixotropy of cementitious materials must be taken into account due to the benefits of the change of viscosity especially in self-levelling applications. In this study, a rheological approach is used to characterize the thixotropy of a cement paste and to study both the physical and chemical thixotropies. White Portland cement pastes to cement ratios of 0.25. 0.34, 0.40 and 0.46 were prepared. For all pastes a commercial high range water reducing admixture (HRWRA) was used in concentrations of 0.04%, 0.120% and 0.360% and for samples with 0.360% concentration, the same mixes have been done with the addition of a commercially available viscosity modifying admixture (VMA). Results show that by measuring the viscosity over-time it is possible to uncouple the contribution of the chemical and physical thixotropy. An empirical mathematical model has been proposed in order to identify physical and chemical aging over time, finding that in the first instants the physical aging grows exponentially, followed by a linear growth ascribed to chemical aging. The addition of additives only affects the physical aging but does not interfere with chemical thixotropy. This approach could be suitable to predict and control the thixotropy of cement pastes and mortars.