Determination of characteristic rheological parameters in Portland cement pastes

• In rheological studies, the stimulus type may yield different shear strength values depending on the measurement scheme used.
• Similar behaviour: ≈50% decline in shear stress after 30″ of rotation and further loss when the stimulus was repeated.
• Clearly different behaviour: higher shear strength in PC2 paste, greater bond recovery in PC1 paste.
• The explanation lies in the very different N2Oeq. (%) contents in the liquid phase of the two PC pastes.

The standard use of admixtures in Portland cements (with and without mineral additions)-based concrete manufacture today has made the physical–chemical interaction among the various components a highly topical issue.One way to analyse the resulting effects is discussed in this paper, which substantiates the importance of a careful selection of the parameters used to evaluate the rheological behaviour of plain fresh pastes of Portland cements. To this end, the pastes studied in this research contained no mineral additions.The initial analysis explored the variations in rheometer response depending on angular velocity and phase duration. Ramp direction, number of angular velocities and the size of the inter-velocity steps were also studied. Lastly, the importance of allowing the cement paste sample to rest during the measurement sequence was also analysed.These trials were conducted on two types of Portland cements with completely different mineralogical compositions. They were repeated every 20 min prior to the initial setting time, more exactly up to the first nadir on their heat of hydration release curves, determined in earlier conduction calorimetry studies.The results showed that parameter selection in designing a scheme to evaluate the rheological behaviour of fresh Portland cement pastes had a substantial effect on the response obtained. That response was also highly dependent upon the degree and type of hydroxy-induced alkalinity in the liquid phase of the pastes, in turn a result of the mineralogical and chemical composition of the PCs, in particular their N2O and K2O (%) contents, which therefore proved to play a prominent role in rheological behaviour.