Effect of the Side Chain Density and Length of Polycarboxylate Ether Superplasticizers on the Thixotropic Structural Build-Up of Cement Paste

Polycarboxylate ether superplasticizers (PCE) are of great importance to control interparticle interactions and hence the rheological properties of cement-based suspensions because their molecular structure can be modified according to the requirements. Nevertheless, the effect of individual parameters of the molecular structure on thixotropy is not yet fully understood. For a deeper understanding, specifically polymerized PCE with different side chain densities and side chain lengths were investigated. The effect of the molecular structure was studied by producing cement pastes with constant yield stress and determining the thixotropic structural build-up with a parallel plates rheometer. The results indicate that thixotropy increases with decreasing side chain densities and decreasing side chain lengths. This might be explained by the fact that PCE molecules with low side chain densities adsorb faster and to a larger extent on cement particles surfaces. Consequently, less PCE molecules are left in the pore solution to enable permanent adsorption on newly formed and growing hydration products, leading to a decrease of steric repulsion and hence to an increase in thixotropy. Furthermore, thixotropy increases in case of lower side chain lengths. This effect might be related to a decreased PCE layer thickness on the cement particle surface, increasing the probability to be overgrown by hydration products.