Study on Ions Leachability and Microstructure of FGD Gypsum–Based Material Mixed with Cement and Slag by Isothermal Calorimetry and Low-Field Nuclear Magnetic Resonance
Publication: Journal of Materials in Civil Engineering
Volume 34, Issue 12
Abstract
Flue gas desulphurization (FGD) gypsum is a kind of industrial solid waste containing impurities (i.e., heavy metal ions and ), which may lead to threats to the ecological environment and public health. To safely use the FGD gypsum, this study investigated the mechanical strength, releasing property of and leachability of heavy metal ions of a series of FGD gypsum (40%) based mortar with various cement to slag ratios. The hydration process, pore structure, and morphology of the sample were then characterized and analyzed by isothermal calorimetry, low-field nuclear magnetic resonance (NMR), and scanning electron microscope with energy dispersive X-Ray analysis (SEM-EDx). It was found that the highest mechanical strength could be obtained when the C/S ratio was 1:5, and the decrease of C/S ratios from 4:2 to 1:5 leads to less releasing from the mortar. The heavy metal leachability of the mortars is lower than that of the limitation of solid waste stipulated by the specification of GB5085.3-2007, and less of the heavy metal are leached from the sample as the C/S ratios decrease. The heat flow for the paste sample with 40% of FGD gypsum was delayed as the cement to slag ratio decreased from 5:1 to 2:4. In addition, the released amount of ions linearly decreases with the pore size of gel pores and capillary pores but increases with the pore size of macropores, and the ettringite in the paste exhibits the transformation from the needle to the clubbed shape when smaller cement to slag ratio was used. This work helps to understand how to reduce the heavy metals leachability of FGD gypsum from cementitious material and provides baseline for safely using the FGD gypsum for the preparation of building materials.
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Data Availability Statement
No data, models, or code were generated or used during the study.
Acknowledgments
The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Nos. 52108240 and 51678441) and the Science and Technology Commission of Shanghai Municipality (Nos. 19DZ1202702 and 19DZ1201404). Gratitude is also extended to the support from Shanghai Postdoctoral Excellence Program (2019–2020) and Visiting Scientist project in Darcy Center of TU/e (2019–2020). We also thank Niumag Electric Corporation (Shanghai, China) for its assistance in performing these measurements.
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Published In
Journal of Materials in Civil Engineering
Volume 34 • Issue 12 • December 2022
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© 2022 American Society of Civil Engineers.
History
Received: Nov 16, 2021
Accepted: Apr 8, 2022
Published online: Oct 12, 2022
Published in print: Dec 1, 2022
Discussion open until: Mar 12, 2023
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