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 responsible mechanisms to quantify the stability are reviewed and a conceptual model is proposed describing the sedimentation progress as bleeding (supernatant), homogeneous zone, 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 that the mainly restricted Stokesian settlement of particles causes changes in solid fraction and the fresh density at a given height and time. The influence of pressure detection direction and position were investigated by changing the inlet design of the glass pipes. The pressure drop due to cement dissolution, chemical shrinkage and paste structural build up is also investigated but found to be negligible compared to the effect of density variation due to sedimentation before setting. The sedimentation rate by HYSPT measurements were found to coincide with the volume fraction variation detected by in situ measurements. The solid fraction and bleeding results were in alignment with the conceptual model.
Key words: hydrostatic pressure, stability, sedimentation, bleeding, cement paste