Mechanistic study on the effect of sulfate ions on polycarboxylate superplasticisers in cement

Sulfate ions are known to potentially impede the performance of polycarboxylate superplasticisers (PCEs) in cement. In this study, the effect of up to 2% potassium sulfate on two types of PCE composed of different molar ratios of methacrylic acid (MA) and methoxy-polyethylene glycol methacrylate ester (MPEG-MA) was studied, and the mechanism behind the effect of sulfate was investigated. Mini slump tests revealed that the dispersing ability of both PCEs was negatively impacted by sulfate, with the less anionic PCE being more affected than the more anionic one. This result coincided with adsorption and zeta potential measurements. These indicate that in the presence of high sulfate concentrations, the less anionic PCE adsorbs in lower amounts. A mechanistic study revealed that, contrary to previous findings by other authors, the PCE molecules do not significantly shrink and coil in the presence of elevated sulfate concentrations. Instead, it was observed that the delayed addition of potassium sulfate leads to desorption of already adsorbed PCE. Accordingly, competitive adsorption between PCE molecules and sulfate ions takes place. PCEs exhibiting higher grafting density are more negatively affected in their dispersing ability than PCEs showing higher anionic character. Thus, the latter are less sensitive to variations in cement composition.