Rheological Properties of Cement Pastes with Polycarboxylate Superplasticizers of Varied Backbone Stiffness

Polycarboxylate (PCE) superplasticizers greatly affect the rheological properties of cementitious materials. Yet, the systematical influence of the PCE structure parameter on the rheological properties of cement paste is not so clear. The authors investigated the effect of backbone methyl group content (backbone stiffness) of PCE samples on the yield stress, residual viscosity, and apparent viscosity of cement pastes at varied water-to-cement ratios ($\mathrm{w}:\mathrm{c}$ ratios). An obvious difference was observed at low $\mathrm{w}:\mathrm{c}$ ratio, which relied on the particle separation distance $H$. The solution viscosity affected the apparent viscosity through the contribution to plastic viscosity at low $\mathrm{w}:\mathrm{c}$ ratio. At high $\mathrm{w}:\mathrm{c}$ ratio, the difference between PCE samples was small at consistent fluidity of cement paste due to consistent packing density and $H$. At low $\mathrm{w}:\mathrm{c}$ ratio, large amount of PCE molecules remained unadsorbed. The value of $H$ approached its largest value and the apparent viscosity of cement pastes depended on the balance effect of $H$ and viscosity of the continuous phase. Due to the high backbone stiffness, PCE of high backbone methyl group content exhibited high adsorption affinity and high solution viscosity, yet a small size of a single molecule after adsorption. At low $\mathrm{w}:\mathrm{c}$ ratio, $H$ was small and the solution viscosity was high. The apparent viscosity of cement paste was therefore high.