No abstract

Interparticle potential energy calculations were performed to investigate the mechanisms by which a new class of concrete admixtures, generally referenced as poly(carboxylic acid)‐type (PC) superplasticizers, which aid in dispersing cement particles, are formed. These calculations consisted of long‐range Van der Waals, electrostatic, and steric interactions. The repulsive potential that resulted from electrostatic interactions was negligible, which would allow cement particles to flocculate in the absence of steric contributions. A model was developed to describe the adsorption behavior of these superplasticizers, which consisted of grafted polyethylene oxide (PEO) chains on a PC backbone on cement surfaces. Using this adsorption model, the influence of the length of the PEO molecular chain and the density per unit area on the steric contribution was quantified. Steric hindrance effects were the dominant stabilizing mechanism in this system. As expected, enhanced stability was observed with increasing adlayer thickness (and/or density). The results of this study may be useful in designing the molecular structure of this new and important class of dispersion aids for cement‐based systems.

We discussed the influence of sulfate ion on the fluidizing mechanism of polycarboxylate-based superplasticizers (PC) in cement paste by considering adsorption characteristics of PC and steric hindrance effects. The fluidity, amount of adsorbed PC on cement particles and sulfate ion concentration in aqueous phase were measured using cement paste with some dosages of PC and alkali sulfate. The fluidity of cement paste decreased with increasing alkali sulfate. Clear correlation between the amount of PC adsorbed and the fluidity did not obtain. The fluidity decreased linearly with increasing sulfate ion concentration. We had showed that sulfate ion reduced the thickness of adsorbed PC and steric hindrance effects. The results of this study suggested that sulfate ion reduced the fluidity of cement paste mainly because of reducing steric hindrance effects with any dosage of PC.

No abstract