Superplasticizers adsorb at the solid−liquid interface between the particles and the aqueous phase. There, they impart a repulsive interparticle force, thus reducing or eliminating adhesion between particles in close proximity. 4 The attractive forces between the cement grains, silica fume, fly ash, and slag particles may originate from van der Waals5 or electrostatic forces (ion correlation, see the article by Pellenq and Van Damme in this issue), or from surface-charge inhomogeneities.
The term electrosteric stabilization is often used to describe how superplasticizers act as dispersants. Electrosteric stabilization is a combination of an electrostatic double-layer repulsion and a steric repulsion, where the relative importance of the respective contributions is closely related to the polymer segment density profile at the interface, the charge density of the polymer, and the ionic strength of the solution. To prevent particles from coming into close proximity with each other, this force must be sufficiently long-range. In cementitious systems, where the ionic strength is high ( 0.1 mol/L), electrostatics alone do not suffice. Thus, it is expected that extending the layer thickness of adsorbed super-plasticizers should improve their rheological performance in such systems.6, 7 This may be achieved with comb-type superplasticizers in which adsorption is driven by the ionic content of the backbone and the steric layer is enhanced by grafted non-adsorbing side chains that extend into the solution (Figure 2).
Figure 2. Schematic illustration of the force between two surfaces with adsorbed comb-like copolymers, as a function of separation distance. Steric repulsion prevents van der Waals interactions from developing strong attractive forces (negative values). Schematic illustrations (top) show surfaces (from right to left) before, at the onset, and during the overlap of their adsorbed layers, leading to steric hindrance that overcomes the van der Waals force. The dashed line in the graph is for a thinner adsorbed layer, which is shown before overlap-upon-layer-overlap in the lower illustration. The structure inset at lower right gives the generic composition of such comb-type copolymers.