by Hyun-Joon Kong
Co-Chairs: Professor Stacy G. Bike and Professor Victor C. Li
This dissertation investigates a dispersion/stabilization technique to improve the fluidity of heteroflocculating concentrated suspensions and applies the technique to develop self-compacting Engineered Cementitious Composites (ECC), defined as a cementitious material which compacts without any external consolidation in the fresh state while exhibiting strain-hardening performance in the hardened state.
To meet the criteria of micromechanical design to achieve the ductile performance and processing design to attain high fluidity, this work has focused on preparing cement suspensions with low viscosity and high cohesiveness at a particle loading determined by the micromechanical design. Therefore, the goal of this work is to quantify how to adjust the interactions between the cement particles, which tend to strongly flocculate due to electrostatic and van der Waals attractive forces. For this purpose, a strong polyelectrolyte, melamine formaldehyde sulfonate (MFS), to disperse the oppositely-charged particles present in the cement dispersion, is combined with a non-ionic polymer of a high molecular weight, hyroxypropylmethylcellulose (HPMC). The combination of these two polymers to prevent re-flocculation leads “complementary electrosteric dispersion/stabilization”. Through extensive studies on polymeric adsorption and its effects on the particle surface properties and rheological properties of the concentrated suspensions, the appropriate polymer concentrations and mixing procedure to attain the desired electrosteric layers on the surfaces have been determined. As a result, suspensions with the desired fluidity for processing are obtained.
To quantify the roles of the two polymers in imparting stability, a heteroflocculating model suspension was developed, which facilitates the control of the interactions typical of cement suspensions, but without irreversible hydration. This model suspension is composed of alumina and silica particles, which bear surface potentials of opposite sign at intermediate pHs, as well as a have comparable magnitude of the Hamaker constant as compared to cement particles. As a result, the model system displays not only van der Waals attraction but also electrostatic attraction between dissimilar particles and electrostatic repulsion between similar particles. Rheological studies of the model system stabilized by MFS and HPMC show behavior identical to that of the cement suspensions, allowing the model system to be used to interpret the role of the stabilizers in altering the system microstructure and fluidity.
Finally, the self-compacting performance of fresh ECC mix made with the electrosterically stabilized fresh matrix mix and the ductile strain-hardening performance of the hardened ECC were demonstrated. This illustrates the beneficial effects of electrosteric stabilization on the processing of the composites that containing reinforcing fibers.
