Electrosteric Repulsion Induced by Superplasticizers between Cement Particles -An Overlooked Mechanism?

摘要

Dispersion mechanisms of superplasticizers have received much attention over the past years. Recent developments have brought very efficient superplasticizers where the dominant stabilizing mechanism is thought to be via steric repulsion. These new superplasticizers contain an adsorbing backbone onto which non adsorbing side chains are grafted with the objective of getting them to stretch out into the solution from the cement particle surface and induce the steric repulsin upon approach of other particles. Another feature of these polymers is that they induce only very small zeta potentials. Calculations of interaction energies indicate that these polymers act predominantly through steric repulsion. However, the same calculations could lead to the conclusion that all polymers can only act through steric repulsion. The calculation of the steric and electrostatic contributions are greatly dependent on the polymer adsorption conformation and the distribution of charge at the particle surface associated with these adsorbed polyelectrolytes. Many of the assumptions made in calculating interparticle forces are not necessarily good approximations for polyelectrolytes. This paper discusses the limits of the approximations currently used in such calculations and presents a more accurate model for the calculation of these forces. The main result, applicable for a wide range of superplasticizers, is that both electrostatic and steric repulsions should be taken into account, provided the electrostatic charge can be assumed to lie at the outer-bound of the adsorbed layer of superplasticizers. Such information is of primary importance for understanding and solving cement and superplasticizer incompatibilities, as well as for developing novel products.