Utilization of low-quality desulfurized ash from semi-dry flue gas desulfurization by mixing with hemihydrate gypsum

This work aimed to investigate the feasibility of utilizing the low-quality desulfurization ash from semi-dry flue gas desulfurization by mixing with α-hemihydrate gypsum, and then the environmental issues resulted from low-quality desulfurized ash from semi-dry flue gas desulfurization can be resolved, and both economic and ecological benefits are brought to the related industries. Thus various contents of low-quality desulfurization ash were mixed into the α-hemihydrate gypsum and these pastes with desulfurization ash were prepared, and then the water requirements for the normal consistency, Zeta electrical potentials, setting times and strengths of the pastes with desulfurization ash were evaluated, also hydration products and microstructures of these mixtures were identified by Fourier transform infrared (FT-IR), X-ray diffraction (XRD) and Scanning electron microscope (SEM) respectively in this work. The results indicate that desulfurized ash increases the water requirement for the normal consistency, and it decreases Zeta electrical potential and strengths, and it prolongs the setting times. Higher water requirement for the normal consistency, lower absolute value of Zeta electrical potential, longer setting times and lower strengths will be found as more desulfurized ash is mixed into the paste. But the strengths of the pastes with 24% desulfurized ash still meet the requirement for the building. Lower absolute values of Zeta electrical potentials imply the reaction appearance of calcium oxide to calcium hydroxide. FT-IR and XRD results indicate that dihydrate gypsum is the main hydration product, and calcium sulfite, calcium carbonate and calcium hydroxide are also appeared in the hardened pastes with desulfurized ash and they do not participate in the reaction and there is not any new phase in the hardened pastes with desulfurized ash. SEM observation shows that desulfurized ash alters the dihydrate gypsum morphology and increases the pores in the hardened pastes. The crystal becomes thicker and more pores can be found in the hardened pastes as more desulfurized ash is mixed. These results will provide a reference for the desulfurized ash application into the hemihydrate gypsum and contribute to the sustainable management of low-quality desulfurized ash and find an approach for its application with a large scale.