Preparation of superfine and semi-hydrated flue gas desulfurization gypsum powder by a superheated steam powdered jet mill and its application to produce cement pastes

Abstract

The low reactivity, high moisture content, and gap-graded particle size distribution of flue gas desulfurization (FGD) gypsum clearly restrict its large-scale application, particularly for cement production. In this paper, we utilize a superheated steam powdered jet mill with different classifier frequencies (20, 35, and 45 Hz) to modify FGD gypsum. Then, the resulting 5% superfine flue gas desulfurization (SFGD) gypsum powder was applied with cement clinker to produce cement pastes. A series of characteristics (water content, particle size distribution, crystalline phase and morphology) of FGD gypsum before and after jet mill grinding were analysed. The setting time, porosity, mechanical strength and hydration heat, were determined, followed by XRD, SEM and TG/DSC analyses, to explore the performances of cement pastes with FGD gypsum and SFGD gypsum addition. Jet mill grinding with superheated steam significantly reduced the water content and particle size (D50 decreases from 33.810 to 2.667 µm) and produced semihydrated gypsum powders. Superfine and semihydrated FGD gypsum dissolution adequately provided SO42- ions, leading to much more hydration heat release and ettringite generation in the clinker-SFGD gypsum system. Moreover, the addition of superfine FGD gypsum powders helped to achieve even more hydration products, a denser inner structure and better mechanical performance of the cement pastes. Therefore, jet mill grinding with superheated steam could be a novel and effective approach to modify FGD gypsum, which possesses great potential for use in the cement industry.