Compatibility between Polycarboxylate and Viscosity-Modifying Admixtures in Cement Pastes

摘要

Superplasticizers and viscosity-modifying admixtures (VMA) are often combined in the production of self-compacting concrete. The former provide high flow-ability and the latter are being increasingly used to enhance its stability. A deep knowledge of the mechanisms of action of both admixtures is needed for optimizing concrete design and for reducing possible technological problems on site. The aim of the present study is to provide a scientific understanding of the compatibility between a PCE and two different VMA admixtures, starch and welan gum-based. For this, the interaction of both types of admixtures with cement was first studied by determining adsorption isotherms. Rheological tests were also performed and showed that the studied VMAs do not modify notably the plastic viscosity, but they control the stability of cement pastes by increasing the yield stress (up to 100%). Moreover, it was found that the increase of yield stress depends on the mode of VMA addition, reaching higher values when the VMA is added after the PCE.