Effect of oscillatory pre-shear on the behavior of a cement paste sheared within a parallel plate geometry

Abstract

Cement powder mixed with water forms a paste which, after hardening, constitutes the glue of concrete, essential to its strength. The paste is a complex suspension of micronsize cement particles (ground clinker) and nano-size C-S-H particles reaction product). It exhibits a colloidal behavior due to interactions between charged particles through an interstitial solution of high ionic strength, giving rise to a macroscopic yield stress that increases with time at rest. This study aims at observing the behavior of cement pastes under large amplitude oscillatory shear (LAOS) within a parallel plate geometry. While the rheometer imposes an oscillatory shear, a high speed camera records images of the paste at the edge of the plates. After the LAOS pre-shear stage, small amplitude oscillatory shear (SAOS) is imposed, to follow the structuration of the paste. Oscillatory pre-shear is commonly used to reset the paste history after mixing and loading into the geometry, and before a measurement. It is assumed that the application of the large amplitude oscillatory shear resets the microstructure of the paste to a homogenous state. Our observations show that the evolution of the elastic modulus during the SAOS phase is highly dependent upon the LAOS-pre-shearing time. By analyzing the images using particle image velocimetry (PIV), we explain this result by the emergence of shear localization during LAOS, compromising the sought-after homogeneous microstructural state.