Effects of titanium gypsum on high calcium fly ash system: Physical and microscopic properties, hydration, and Fe transformation

Abstract

Titanium gypsum (TG) is a solid waste generated in the sulfate method of TiO2 production. The stockpiling of TG causes harmful environmental issues, but TG could be reused for manufacturing green cementitious materials with fly ash (FA) and calcium carbide residue (CCR). However, the effects of TG on the high-calcium FA–CCR system and the transformation rule of Fe compound in TG remain unclear. Therefore, FA–CCR and FA–natural gypsum–CCR were used as the control group. The effects of TG on the physical properties, phase composition, and microstructure of the FA–CCR system were studied. Furthermore, the transformation role of Fe in TG was revealed by calculating the thermodynamic properties and performing X-ray photoelectron spectroscopy. Results demonstrated that the appropriate addition of TG was found to shorten the setting time and increase the compressive strength of the FA–CCR system, whereas the excessive addition will cause a negative effect. The small crystal size of TG has a greater contribution to the physical properties compared with the negative effect of iron impurities. The addition of TG can optimize the pore structure of the FA–CCR composites. TG reduced the hydration rate of C-(A)-S-H gels at an early age but promoted ettringite generation and polymerization of C-(A)-S-H at the late stage. Fe(OH)3 in TG plays an insert-filling role at an early age, but this compound could transform into Fe(OH)63− and form ettringite in the late hydration stage, thereby promoting the growth of ettringite crystals. The relative conversion rate of Fe(OH)3 to Fe(OH)63− at 60 days is 11.8%.