Influence of nano-TiO2 on physical and hydration characteristics of fly ash-cement systems

This paper presents the effect of nano-TiO2 (NT) on physical and hydration characteristics of cement-based materials containing different fly ash (FA) contents ranging from 10% to 30% (by weight) as partial replacement of cement. The fluidity of mortars and the setting time of pastes were tested with different NT dosage. The compressive strength of mortars is measured at 3, 7, 28 and 90 days. Results show that NT addition could accelerate initial and final setting and decrease the fluidity, while FA had the opposite effects. Introducing NT would lead to a considerable increase in compressive strength at early ages while it had an adverse effect on the later age strength. However, the positive functions of FA in late strength could offset the negative influence of NT and the optimum contents of NT and FA are 3% and 20%, respectively. Then the influence mechanisms of NT on properties of FA-cement materials are investigated by hydration heat, X-ray diffraction (XRD), differential scanning calorimetry-thermogravimetry (DSC-TG), scanning electron microscope (SEM) and nuclear magnetic resonance (NMR). The results of analyses indicate that incorporating NT promoted the early hydration of cement and accelerated the formation and precipitation of hydration products. Furthermore, in the strength development process, NT could raise the pozzolanic reaction degree of FA and lead to the formation of a C-S-H type with longer chain and higher Al:Si ratio. Also, the interfacial transition zone (ITZ) of FA mortars were improved by NT.