EFFECTS OF DIFFERENT VISCOSITY MODIFYING ADMIXTURES ON RHEOLOGY OF CEMENT PASTE MATRIX OF SELF- CONSOLIDATING

摘要

Better characterization of the rheological behavior of cement pastes is a first step to study the rheological properties of self-consolidating concrete (SCC). Cement paste has to be sufficiently fluid to ensure the fluidity of the SCC itself and sufficiently viscous to support the coarse aggregates. Viscosity modifying admixtures (VMAs) along with adequate superplasticizer content has proved to be very effective in these two properties (high deformability, a good resistance to segregation and consistency) of SCC. However, little is known about the interactions between superplasticizers, VMAs and pastes. The major aim of this paper is to study the role and the influence of VMAs. Pastes were made with cement containing limestone filler and fly ash following the formulation of paste of self-consolidating concrete. Superplasticizers and different type and dosage of VMAs were incorporated in the study. The influence of different type and dosage of VMAs; inorganic VMAs (e.g. microsilica) and organic VMAs (based on polysaccharide, starch and cellulose derivatives) combined with superplasticizer on the flow properties and the rheology of cement paste was studied and assessed. The effect of water/cement ratio (W/C) on the early hydration of cement pastes and on the development of mortar strength was also in the focus of this study. The paste measurements were conducted both on empirical tests, namely mini-slump and rheometer. At constant water-to-binder ratio, the increase of viscosity modifying admixtures (VMA) cause decrease of flow and increase of yield stress and plastic viscosity. It was found that polysaccharide based VMA is the most effective one. Yield stress of cement paste changes with time due to the hydration reaction. Cement hydration was monitored by isothermal calorimetry and thermogravimetry (TGA). The organic VMA show almost no influence on early cement hydration and the development of compressive strength. However, the inorganic VMA cause an acceleration of cement hydration and a higher compressive strength at the age of 1 day.