Incorporation mechanism of titanium in Portland cement clinker and its effects on hydration properties

Titanium is a trace element that might be introduced by industrial wastes to the cement kilns. In this study, the effect of TiO2 on cement clinker phase formation, ion substitution as well as hydration properties were evaluated by free lime test, X-ray diffraction (XRD), scanning electron microscope-energy dispersive spectrometer (SEM-EDS), isothermal heat-conduction and nuclear magnetic resonance (NMR). Results indicated that titanium was preferentially incorporated in the interstitial phases, especially in C(4)AF, then C(3)A, C2S and C3S, which could be attributed to the lattice substitution of Ti4+ to Fe3+ in C(4)AF and consequently weakened its crystallinity. Meanwhile, the threshold limit for TiO2 incorporated into clinker was 1.5 wt%, beyond which, the crystal form of C(3)A transformed from cubic to orthorhombic. These changes of C(3)A and C(4)AF promoted the formation of C3S by decreasing the liquid phase viscosity during the sintering process. TiO2-doped promoted the hydraulic activity of clinker in initial period while postponed the hydration process of C3S in later period. However, after 28 days hydration, C-S-H gel polymerization degree did not decrease but improved as the TiO2 dosage was below 1.5 wt%. (C) 2017 Published by Elsevier Ltd.