2011 Volume 65 Issue 1 Pages 552-559
Polycarboxylate-based superplasticizer（PC）is used to be dispersed the suspended colloidal particles, thereby improving the fluidity of such suspensions. The driving force for dispersing of PC might be contributed by the steric repulsion arising from overlap of polymer layers of PC adsorbed on surface of colloidal particles. According to enormous past studies, the repulsion force might be represented as a function of the thickness of the layer adsorbed on the colloidal particle surface. Therefore, it is of great importance to estimate the conformation of adsorbed PC and its thickness. However, for concentrated suspension, it is too difficult to directly measure the conformation and thickness of PC adsorbed on the surface of cement particles.
In this study, we estimated the thickness of adsorbed polymer layer of PC using the two different models：the rheological model for prediction of relative viscosity of concentrated suspension proposed by Chougnet et al. and the steric repulsion model to predict the relation between the thickness and steric repulsion, which is proposed by Alexander and de Gennes. Using the former model, one can estimate an effective volume fraction of aggregate of suspended particles. Since the effective volume fraction of aggregate can be expressed as a function of the interparticle force Fa and the shear stress τ, the interparticle force can be calculated. The interparticle force is a sum of electrostatic repulsion, van der Waals attraction and steric repulsion. As the electrostatic repulsion and van der Waals attraction can be calculated according to DLVO theory, the steric repulsion can be determined. Using Alexander and de Gennes’s model, the thickness of polymer dispersant adsorbed on the suspended particles can be predicted. We used fine CaCO3 powder as the suspended particle, and two different PCs that consist of different graft polymer with different length. The results of rheological approach studies indicated that the viscosity of suspensions added two PCs almost the same and thus the thicknesses of adsorbed PCs were estimated to be the same despite length of graft polymer with different length. This implies that the conformation of adsorbed PC does not agree with the previous conformation model that the main chain is adsorbed on the surface and polyethylene oxide chain is stretched into the solution. In this manner, the thickness is proportion to the length of grafted polymer. Thus the main chain of adsorbed PC partially becomes detached on the surface of particles.
