K. De Weerdt

Model and Test Methods for Stability of Fresh Cement Paste

The stability of fresh cement paste can be described as its ability to remain homogeneous over time. Instability in the form of sedimentation and bleeding occurs for various reasons related to the material composition. In this study, the relevant mechanisms for quantifying stability are reviewed, and a conceptual model is proposed describing the sedimentation progress in terms of bleeding (supernatant), the homogeneous zone, the variable concentration zone, and the compressible sediment zone. A sensitive hydrostatic pressure test (HYSPT) with good repeatability and without influence from evaporation was developed to describe the sedimentation process of fresh cement paste. HYSPT relies on the fact that the mainly restricted Stokesian settlement of particles causes changes in the solid fraction and the fresh density at a given height and time. The influence of the pressure detection direction and position were investigated by changing the inlet design of glass pipes. The pressure drop due to cement dissolution, chemical shrinkage, and paste structural buildup also was investigated but was found to be negligible compared to the effect of density variation due to sedimentation before setting. The sedimentation rates obtained from HYSPT measurements were found to coincide with the volume fraction variation detected via in situ measurements. The solid fraction and bleeding results were in alignment with the conceptual model.