Impact of Inorganic Salts on Rheology, Strength, and Microstructure of Excess-Sulfate Phosphogypsum Slag Cement
Excess-sulfate phosphogypsum slag cement (EPSC), offering the potential for large-scale phosphogypsum (PG) utilization, has drawn significant attention. However, its susceptibility to salt erosion in marine/saline environments remains unquantified, hindering engineering applications. This study, the...
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| author | Zhe Chen Zixin Xue Yong Xia Chunli Wu Junming Mai Weisen Liu Yuan Feng Jianhe Xie |
| author_facet | Zhe Chen Zixin Xue Yong Xia Chunli Wu Junming Mai Weisen Liu Yuan Feng Jianhe Xie |
| author_sort | Zhe Chen |
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| description | Excess-sulfate phosphogypsum slag cement (EPSC), offering the potential for large-scale phosphogypsum (PG) utilization, has drawn significant attention. However, its susceptibility to salt erosion in marine/saline environments remains unquantified, hindering engineering applications. This study, therefore, systematically investigates the effect of various salts (NaCl, MgCl<sub>2</sub>, KCl, and Na<sub>2</sub>SO<sub>4</sub>) at different concentrations (0.5–1.5%) on the hydration mechanism and performance of EPSC using rheometry, strength tests, and microstructural characterization (XRD/SEM-EDS). The findings reveal that EPSC exhibits low initial yield stress and plastic viscosity, both of which increase over time. The addition of Na<sup>+</sup>, Cl<sup>−</sup>, and SO<sub>4</sub><sup>2−</sup> ions promotes hydration and flocculent structure formation in the EPSC paste, thereby enhancing the yield stress and plastic viscosity. In contrast, Mg<sup>2+</sup> and K<sup>+</sup> ions inhibit the hydration reaction, although Mg<sup>2+</sup> temporarily increases the plastic viscosity by forming Mg(OH)<sub>2</sub> during the initial stage of the reaction. Both Na<sub>2</sub>SO<sub>4</sub> and NaCl improve mechanical properties when their concentrations are within the 0.5–1.0% range; however, excessive amounts (>1%) negatively impact these properties. Significantly, adding 0.5% NaCl significantly improves the mechanical properties of EPSC, achieving a 28-day compressive strength of 51.06 MPa—a 9.5% increase compared to the control group. XRD and SEM-EDX analyses reveal that NaCl enhances pore structure via Friedel’s salt formation, while Na<sub>2</sub>SO<sub>4</sub> promotes the early nucleation of ettringite. However, excessive ettringite formation in the later stages of the hydration reaction due to Na<sub>2</sub>SO<sub>4</sub> may negatively affect compressive strength due to the inherent abundance of SO<sub>4</sub><sup>2−</sup> in the EPSC system. Therefore, attention should be paid to the effect of excessive SO<sub>4</sub><sup>2−</sup> on the system. These results establish salt-type/dosage thresholds for EPSC design, enabling its rational use in coastal infrastructure where salt resistance is critical. |
| format | Article |
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| spelling | doaj-art-2b1b327e7cd045ca87e2f24f90517fc62025-08-20T03:16:46ZengMDPI AGBuildings2075-53092025-07-011513234810.3390/buildings15132348Impact of Inorganic Salts on Rheology, Strength, and Microstructure of Excess-Sulfate Phosphogypsum Slag CementZhe Chen0Zixin Xue1Yong Xia2Chunli Wu3Junming Mai4Weisen Liu5Yuan Feng6Jianhe Xie7Guangdong Building Materials Institute Co., Ltd., Guangzhou 511455, ChinaGuangdong Building Materials Institute Co., Ltd., Guangzhou 511455, ChinaGuangdong Building Materials Institute Co., Ltd., Guangzhou 511455, ChinaGuangdong Building Materials Institute Co., Ltd., Guangzhou 511455, ChinaGuangdong Building Materials Institute Co., Ltd., Guangzhou 511455, ChinaSchool of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou 510006, ChinaSchool of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou 510006, ChinaSchool of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou 510006, ChinaExcess-sulfate phosphogypsum slag cement (EPSC), offering the potential for large-scale phosphogypsum (PG) utilization, has drawn significant attention. However, its susceptibility to salt erosion in marine/saline environments remains unquantified, hindering engineering applications. This study, therefore, systematically investigates the effect of various salts (NaCl, MgCl<sub>2</sub>, KCl, and Na<sub>2</sub>SO<sub>4</sub>) at different concentrations (0.5–1.5%) on the hydration mechanism and performance of EPSC using rheometry, strength tests, and microstructural characterization (XRD/SEM-EDS). The findings reveal that EPSC exhibits low initial yield stress and plastic viscosity, both of which increase over time. The addition of Na<sup>+</sup>, Cl<sup>−</sup>, and SO<sub>4</sub><sup>2−</sup> ions promotes hydration and flocculent structure formation in the EPSC paste, thereby enhancing the yield stress and plastic viscosity. In contrast, Mg<sup>2+</sup> and K<sup>+</sup> ions inhibit the hydration reaction, although Mg<sup>2+</sup> temporarily increases the plastic viscosity by forming Mg(OH)<sub>2</sub> during the initial stage of the reaction. Both Na<sub>2</sub>SO<sub>4</sub> and NaCl improve mechanical properties when their concentrations are within the 0.5–1.0% range; however, excessive amounts (>1%) negatively impact these properties. Significantly, adding 0.5% NaCl significantly improves the mechanical properties of EPSC, achieving a 28-day compressive strength of 51.06 MPa—a 9.5% increase compared to the control group. XRD and SEM-EDX analyses reveal that NaCl enhances pore structure via Friedel’s salt formation, while Na<sub>2</sub>SO<sub>4</sub> promotes the early nucleation of ettringite. However, excessive ettringite formation in the later stages of the hydration reaction due to Na<sub>2</sub>SO<sub>4</sub> may negatively affect compressive strength due to the inherent abundance of SO<sub>4</sub><sup>2−</sup> in the EPSC system. Therefore, attention should be paid to the effect of excessive SO<sub>4</sub><sup>2−</sup> on the system. These results establish salt-type/dosage thresholds for EPSC design, enabling its rational use in coastal infrastructure where salt resistance is critical.https://www.mdpi.com/2075-5309/15/13/2348excess-sulfate phosphogypsum slag cement (EPSC)inorganic saltsrheological behaviormechanical propertiesmicrostructure |
| spellingShingle | Zhe Chen Zixin Xue Yong Xia Chunli Wu Junming Mai Weisen Liu Yuan Feng Jianhe Xie Impact of Inorganic Salts on Rheology, Strength, and Microstructure of Excess-Sulfate Phosphogypsum Slag Cement Buildings excess-sulfate phosphogypsum slag cement (EPSC) inorganic salts rheological behavior mechanical properties microstructure |
| title | Impact of Inorganic Salts on Rheology, Strength, and Microstructure of Excess-Sulfate Phosphogypsum Slag Cement |
| title_full | Impact of Inorganic Salts on Rheology, Strength, and Microstructure of Excess-Sulfate Phosphogypsum Slag Cement |
| title_fullStr | Impact of Inorganic Salts on Rheology, Strength, and Microstructure of Excess-Sulfate Phosphogypsum Slag Cement |
| title_full_unstemmed | Impact of Inorganic Salts on Rheology, Strength, and Microstructure of Excess-Sulfate Phosphogypsum Slag Cement |
| title_short | Impact of Inorganic Salts on Rheology, Strength, and Microstructure of Excess-Sulfate Phosphogypsum Slag Cement |
| title_sort | impact of inorganic salts on rheology strength and microstructure of excess sulfate phosphogypsum slag cement |
| topic | excess-sulfate phosphogypsum slag cement (EPSC) inorganic salts rheological behavior mechanical properties microstructure |
| url | https://www.mdpi.com/2075-5309/15/13/2348 |
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