Model Systems for Investigation of the Stability and Rheological Properties of Cement-Based Materials

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Title: Model Systems for Investigation of the Stability and Rheological Properties of Cement-Based Materials

Author(s): N. Mikanovic, C. Jolicoeur, K. Khayat, and M. Page

Publication: Special Publication

Volume: 235

Issue:

Appears on pages(s): 323-356

Keywords: admixtures; bleeding; calcium carbonate; cement; coagulation; flocculation; hydration; paste; rheology; sedimentation; segregation

Date: 3/22/2006

Abstract:
The growing use of fluid concrete increases the need for understanding the conditions under which these materials can undergo bleeding and segregation. However, the interfacial and colloidal phenomena, which control water and solids migration in cementitious systems, are inherently complicated by the hydration of the cement components. Hence, to unravel the specific role of chemical admixtures on the stability of cement-based systems, the mode of action of these admixtures should also be investigated in dense colloidal slurries of ‘un-reactive’ minerals. Several highly insoluble minerals, having specific surface areas comparable to that of a Portland cement, were thus evaluated for this purpose. The state of flocculation of these materials in dilute and concentrated slurries was examined through sedimentation and rheological measurements under various conditions, and the results compared to observations on similar slurries containing cements. The comparison showed that calcium carbonate (CaCO3) exhibits surface and colloidal properties very similar to “un-hydrating” cement particles. In fact, CaCO3 pastes can be made to accurately reproduce most of the kinetic properties of a cement paste, including bleeding, sedimentation and all dynamic viscosity parameters. It is therefore proposed that CaCO3 pastes can be used to adequately model ‘physical-type’ effects occurring in cementitious systems at very early stage of hydration, i.e., in the first hour.