Stiffening Behaviors of Cement Pastes Measured by a Vibrational Viscometer

This study explores the stiffening behavior of cement pastes up to 30 min using a new vibrating-fork technique, in which the vibro-viscometer controls the electrical current to maintain a uniform vibrating amplitude of the sensor plate for approximately 0.2 mm at 30 Hz. The applied electrical current is related to the product of the apparent viscosity and the density. The viscosity evolutions of the cement pastes at water–cement ratios of 0.4, 0.5, and 0.6 in the presence or absence of high-range water-reducer admixtures (HRWRA) were compared using a rotational viscometer and a tuning-fork vibration viscometer. The results indicated that the viscosities increased with hydration time and decreased with admixture dosages. The vibration viscometer seems to have applied higher shear rates than the rotational viscometer during the early hydration so the viscosities measured by the vibration viscometer were always lower. However, the increases in the viscosity of the cement pastes, as measured by the vibration viscometer, were more stable than those measured by the rotational viscometer. In addition, the subtle increases in the paste viscosities during very early hydration were detected well by the vibration viscometer. The vibrating-fork technique has been proven to be a useful tool for characterizing microstructure changes in early cement pastes.