Influence of water/cement ratio, admixtures and filler on sedimentation and bleeding of cement paste

• A conceptual model is proposed to describe sedimentation for fresh cement paste.
• Sedimentation describes stability of cement paste better than bleeding.
• Hydrostatic pressure test method can be applied to detect sedimentation of paste.
• Richardson–Zaki equation is too simplified considering solid fraction and flocculation.
• w/c ratio and SP affect sedimentation more than filler and VMA for cement paste.

Stability of cement paste was investigated by solid fraction (Φ) measurements and HYdroStatic Pressure Test (HYSPT). Φ over time was constant in the middle (homogeneous zone), increasing at the bottom (sediment) and with a transition layer between bleeding in pastes with SP. HYSPT indicates sedimentation and flocculation of fresh paste by the pressure gradient dp/dt. Increasing Φ in limestone slurries and pastes without superplasticizer (SP) causes faster sedimentation. SP slows down particle movement by dispersion and producing less clear plateau pressure. Low SP dosage makes limestone slurries of varying Φ behave closer to Richardson–Zaki equation than pastes which need more SP for dispersion. Bio-polymer type VMA prolongs sedimentation only when combined with SP but induces more bleeding. At 40% limestone replacement (w/c increases from 0.5 to 0.835, w/p from 0.5 to 0.526 but Φ constant) SP stabilized slightly better at higher total SP content and w/c.