Early hydration, mechanical strength and drying shrinkage of low-carbon alkali-activated Ti-extracted residues-fly ash cement and mortars

Ti-extracted residues (TRs) are the calcium-silicate-rich glassy wastes from titanium dioxide pigment industry. Utilization of TRs is rare because of its high amount of alkaline earth metal chlorides, e.g. CaCl2 and MgCl2 which are derived from the chlorination process for titanium extraction. In this work, the TRs were first used as a potential precursor to produce low-carbon alkali-activated binder and fly ash was blended with TRs to regulate the mechanical strength and early volume stability of this composite. The hydration behavior of alkali-activated TRs-fly ash (AATF) was studied through the isothermal calorimetry, and the pore solution ([Ca], [Si], [Al], [Na] and [OH–]) was investigated by the steel-die method and ICP-OES analysis. Mechanical strength and drying shrinkage behavior of hardened AATF mortars were also measured along with mercury intrusion porosimetry (MIP) and non-evaporable water analysis. The hydration assemblages of hardened AATF were evaluated via X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that an appropriate fraction of FA improved the hydration process. Moreover, the flexural and compressive strength reached to 6.8 and 63.2 MPa at 28 days when 20 wt% TRs was replaced by FA. The blends with higher replacement of FA exhibited a lower drying shrinkage at early days, but a comparable total shrinkage value of AATF mortars occurred at later ages due to the higher reaction degree and mesopore volume. The main hydrated products of hardened AATF pastes were composed of hydrotalcite and C-(N-)A-S-H gel from XRD analysis. Based on the experimental results, this work proposed a comprehensive recycling of TRs and FA and simultaneously developed a potential high-strength hydraulic binder in civil engineering.