Sedimentation and Bleeding of Cement Paste

Abstract

The lower viscosity and high matrix volume of Self-Compacting Concrete (SCC) compared to ordinary concrete can lead to instability in the form of particle segregation, sedimentation and bleeding. This is a problem for the exploitation of all the benefits from the use of SCC for the ready mix industry and contractors. This research started from the investigation on the main factors influencing the stability of the paste as well as the applicability of generally used theories on particle sedimentation and bleeding of modern cement paste such as Stokes’ law, the Richardson-Zaki (R-Z) equation and the Kozeny-Carman (K-Z) equation. A conceptual model for sedimentation and bleeding of cement paste was proposed as a 4-stage process with transformation from homogeneous suspension to various zones. A sensitive HYdroStatic Pressure Test (HYSPT) was developed to detect the density differences over time and depth in fresh cement paste due to sedimentation and bleeding. By comparing with dp/dt plots from HYSPT and the measurements on in-situ solid fraction and bleeding, it was validated that HYSPT can be applied to evaluate the sedimentation process and flocculation state of cement paste. It was also found that the traditional bleeding measurement methods were not applicable for the turbid bleeding layer of the modern cement paste due to high dosage of chemical admixtures and filler replacement. However, the bleeding rate calculated with dp/dt from HYSPT measurements was found to coincide with bleeding measurements and the calculated bleeding rate from the Kozeny-Carman equation. By the use of HYSPT, the stabilizing effects of various powders and admixtures such as superplasticizer and viscosity modifying agent were also analyzed and compared. This PhD work contributes to understand the basic mechanism of the stability and the effect of different materials, such as chemical admixtures and powder additives, on sedimentation and bleeding of cement paste.