Joachim Pakusch

Adsorption of polyelectrolytes and its influence on the rheology, zeta potential, and microstructure of various cement and hydrate phases

In this study the influence of polycarboxylate-based polyelectrolytes on the particle interaction among tricalcium silicate (C(3)S, main clinker phase), calcium silicate hydrates (CSH), and calcium aluminate sulfate hydrates (ettringite) (main hydration phases) has been examined. These phases are the constituents of major concern during early hydration of cement suspensions. The results of zeta potential measurements on single mineral phase experiments show that the phases C(3)S and CSH are positively charged in synthetic pore solution (liquid phase of hydrating cement suspension), whereas the ettringite is negatively charged. Due to these opposite charges, ettringite crystals should coagulate with CSH phases and/or deposit on surfaces of the much larger C(3)S clinker particles. This behavior was proven by cryo-microscopic analysis of high-pressure frozen cement suspensions, which illustrates the consequences of colloidal mechanisms on the microstructure of early cement suspensions. Furthermore, it is shown that the polyelectrolytes have a much higher adsorption affinity to ettringite surfaces (hydrate phase) compared to silicate surfaces. However, the results from rheology experiments reveal that the presence of polyelectrolytes has a strong impact on the suspension properties of all investigated mineral phases by decreasing yield stress and plastic viscosity. From the results it can be concluded that the ettringite is the dominant mineral phase in terms of the state of dispersion which includes particle-particle and particle-polyelectrolyte interaction in the bulk cement system.