Effect of materials on sedimentation of cement paste

Abstract

A conceptual model for the stability of paste was proposed and an experimental system, HYdroStatic Pressure Test (HYSPT), was developed for detecting the sedimentation of fresh cement paste. By accurate measurements of the hydrostatic pressure, p, at a depth h and time t, both sedimentation rate and state of flocculation are assessed from pressure drop vs. time curves (dp/dt). Material parameters like solid fraction, water reducer, VMA and limestone replacement were studied with HYSPT. For normal cement paste with high solid fraction (Φ), as Φ increases (w/c = 0.7 – 0.4), the sedimentation is speeded up. Dispersion of the particles by water reducers has a clear effect of slowing down the movement and producing less clear sedimentation plateau pressures. This effect is clearer for limestone slurries of varying Φ than for cement pastes, so cement needs more SP than limestone filler to be dispersed and behave according to the Richardson-Zaki equation. At 40 % limestone replacement (causing increase of w/c from 0.5 to 0.835 to keep  constant) 0.6 % SP stabilized slightly better when added as mass fraction of powder than when added as mass fraction of cement. Thus, limestone gave improved stability at higher total SP content and w/c. VMA enhanced stabilization only when combined with water reducer. Finally we propose that time of set can be defined at the onset of the plateau as particle movement stops.