Effects of particle size, crystal phase and surface treatment of nano-TiO₂ on the rheological parameters of cement paste

In this study, two sizes of rutile nano-TiO₂ (NT) (20 nm and 50 nm), three crystal phases of NTs (anatase NT, rutile NT and polymorphic NT, a hybrid of 80% anatase NT and 20% rutile NT) and two surface-treated NTs (SiO₂-coated NT and SiO₂/Al₂O₃-coated NT) were employed to their effect on rheological behaviors of cement pastes. The water-to-cement ratio was fixed at 0.24, and the NTs were added to the paste at contents of 0, 0.1%, 0.3%, 0.5%, 0.7% and 1.4 vol% of cement, respectively. The rheological behaviors of cement pastes were characterized using two rheological parameters including yield stress and minimum viscosity. The H-B model was chosen to calculate the yield stress and the minimum viscosity was selected as the smallest apparent viscosity in the apparent viscosity-shear rate curve. Experimental results show that NTs have a significant effect on the rheological parameters of the cement pastes when the NT content exceeds 0.5 vol%. The yield stress and minimum viscosity of the cement pastes varies with NT particle size by changing the total surface area of the system. The rutile NT with smaller particle size produces larger yield stress and the minimum viscosity. Cement pastes with three different crystal phases of NTs show distinct rheological behaviors. The cement paste with the rutile NT has the largest yield stress and minimum viscosity, while cement paste with the polymorphic NT has the smallest yield stress and minimum viscosity. The yield stress and the minimum viscosity of the cement paste with SiO₂/Al₂O₃-coated NTs are larger than those of the cement paste with SiO₂-coated NTs owing to a smaller electrostatic repulsion.