The yield stress of cement pastes as obtained by different rheological approaches

Differentrheological methods for yield stress estimation of cement pastes during initial hydration were used and results were compared. These methods include measuring of the hysteresis loop, flow curves (recalculated to the same time of hydration) and large amplitude oscillating strain (LAOS). Experiments were performed with four Ordinary Portland Cements from one manufacturer, produced at differentfactories and one polycarboxylate acid based superplasticiser(SP). The yield stress values obtained by constructing flow curves is the only method which gives information aboutthe evolution ofthe rheological properties, reflecting structure evolution of cements pastes. It was shown that the yield stress values established by the LAOS method and that calculated from the flow curves are similar while the values found from the downward part ofthe hysteresis loops are much lower. Differences in the yield stress values obtained by various methods are related to the different states of the material corresponding to the kinetics of hydration. The hysteresis loops provide information about thixotropic characteristics of the material including characteristic times of rebuilding and the rate of yield stress evolution of cements. The rheological properties are very sensitive to the chemical and physical differences ofthe cements and could be used for their characterization.PPC Lt