Causes of instability of water separation of cement paste

Cement paste is a concentrated polydisperse suspension of cement grains in water. The presence of particles with the size less than 5-10 microns (about 30-50 %) and submicron particles (5-10 %) gives the cement paste the properties of a structured (Bingham) fluid, which are due to the van der Waals forces acting between the particles. Resistance of cement paste to water separation is provided when the structure formed due to these forces is strong enough to counteract the influence of gravitation on the particles, including the largest ones, and prevent their sedimentation. The intensity of water separation of cement paste is related to the value of its point of yielding - with the increase of the latter the resistance of the system to water separation increases. The point of yielding depends on the content and average size of cement particles, the nature of particle size distribution, their shape, structure and character of the surface, the physical properties of the liquid and solid phases and many other factors. As a consequence, the yield strength and water separation are virtually unpredictable and difficult to control properties of cement paste.

Author: A.S. Brykov

Section: Science and production

Keywords: bleeding, yield stress, cement paste