Comparison between methods for determining the yield stress of cement pastes

Abstract

The determination of the yield stress (τ0) of cement-based materials is of great interest for engineering applications, since it accurately describes the flow behavior and assesses empirical properties related to its workability, such as the slump of concretes and the spreading of mortars. In this work, the τ0 of cement pastes was determined by different methods. Specifically, pastes with three different water/cement ratios and two supplementary cementitious materials in Portland cement replacement were produced. The mini slump of the pastes was measured, and its static τ0 and dynamic τ0 were determined by rotational rheometry. In addition, small amplitude oscillatory shear (SAOS) was used to further investigate the rigidification rate of the pastes over time, providing valuable information for the discussion. The results showed that the dynamic τ0 values provided by the different rheological models showed strong correlations. However, these values had weaker correlations with the static τ0. The rest period between the finish of the pre-shear and the test run strongly affected the magnitude of the stress overshoot and therefore the static τ0 value. SAOS indicated that the decrease in the inter-particle distance increased the rigidification rate of the paste within the first minute after mixing, which may affect the mini slump results. Finally, the use of the mini slump as a single test to generally evaluate cement pastes with wide ranges of flowability may not be adequate, in line with the existence of different tests for the evaluation of conventional and self-compacting concretes.