Nano-materials have been found to enhance the properties and performance of different materials. Nano-TiO2 (NTs), for instance, have been widely used to improve the mechanical properties and durability as well as smart/multifunctional properties of cementitious composites. Nano-TiO2 exists in different crystal phases: rutile NT, anatase NT, and polymorphic NT. Despite the extensive research on NT-based cementitious composites, a review of the literature reveals that most research focuses mainly on mechanical and photocatalytic properties with limited attention on the rheological properties. Moreover, the few studies on rheological properties only cover three aspects, namely, the multiple properties, effects of rheological behavior between NT and fillers, and the influence of external conditions on the rheological properties, and not the influence of the internal physicochemical characteristics of NT.
Rheological properties are of great importance because they strongly determine the homogeneity and workability of cementitious composites. Additionally, they strongly relate to cement hydration and mechanical properties as well durability of the cementitious composites after hardening. To this note, researchers at the Dalian University of Technology: Dr. Hongyan Li, Professor Jian Ouyang, and Professor Baoguo Han in collaboration with Professor Siqi Ding from The Hong Kong Polytechnic University and Professor Liqing Zhang from the East China Jiaotong University, studied the influence of internal physicochemical characteristics of NT, i.e., particle size, crystal phase and surface treatment on the rheological behavior of cement paste. Their work is currently published in the journal, Construction and Building Materials.
In their approach, NTs with different particle sizes (20nm and 50nm), crystal phases (rutile, anatase, and polymorphic), and surface treatment (SiO2 coated and SiO2/Al2O3 coated) were utilized. The water-cement ratio was taken to be 0.24. Their effects on the rheological parameters of cement pastes were explored based on two rheological parameters, that is, yield stress and minimum viscosity, which were calculated using the H-B model and selected as the minimum viscosity from the viscosity-shear rate curve, respectively.
The authors observed that the NTs exhibited significant effects on rheological parameters of the cement, especially at content exceeding 0.5 vol%, while the yield stress and minimum viscosity of the cement paste varied with the NT particle sizes. For instance, rutile NT with smaller particle sizes produced minimum viscosity and maximum yield stress. Furthermore, cement pastes with different crystal phases exhibited different rheological properties. For example, rutile NT exhibited minimum viscosity and maximum yield stress while that with polymorphic NT exhibited the smallest yield stress and minimum viscosity. The effects of surface treatment of NTs on the rheological properties depended mainly on the interactions between the electrostatic repulsion and pozzolanic surface reaction. Compared to SiO2-coated NTs, the yield stress and minimum viscosity of SiO2/Al2O3-coated NT were remarkably larger.
In summary, the study presented a systematic study of the effects of the internal physicochemical characteristics of NT, i.e., particle size, crystal phase, and surface treatment on the rheological parameters of cement paste. Results showed that at higher content exceeding 0.5 vol%, NTs have significant effects on the rheological parameters of cement paste. In a Statement to Advances in Engineering, the authors expressed their confidence their findings would advance the synthesis of (ultra)high-performance and smart/multifunctional cementitious composites for different applications (e.g., self-compacting, 3D printing, self-sensing, self-cleaning, photocatalysis, and killing bacteria and viruses).
Li, H., Ding, S., Zhang, L., Ouyang, J., & Han, B. (2020). Effects of particle size, crystal phase and surface treatment of nano-TiO2 on the rheological parameters of cement paste. Construction and Building Materials, 239, 117897.