Journal
JOURNAL OF WUHAN UNIVERSITY OF TECHNOLOGY-MATERIALS SCIENCE EDITION
Volume 34, Issue 3, Pages 638-647Publisher
WUHAN UNIV TECHNOLOGY
DOI: 10.1007/s11595-019-2098-7
Keywords
sulfate attack; calcium aluminosilicate hydrate; thermodynamic stability; microstructure
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Funding
- Anhui Key Laboratory of Advanced Building Materials, Anhui Jianzhu University [JZCL201602KF]
- Major State Basic Research Development Program of China (973 Program) [2015CB655101]
- State Key Laboratory of High Performance Civil Engineering Materials [2015CEM005]
- Natural Science Foundation of Hebei Province [E2016209283]
- Hubei Key Laboratory of Roadway Bridge and Structure Engineering (Wuhan University of Technology) [DQZDJJ201504]
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The thermodynamic stability of sulfate ions on synthesized calcium aluminosilicate hydrate (C-A-S-H) microstructure with different Ca/Si ratios and Al/Si ratios was investigated by XRD, SEM-EDS, Si-29 and Al-27 nuclear magnetic resonance (NMR) and thermodynamic modeling. The results indicate that sulfate attack leads to both decalcification and dealumination for C-A-S-H gels, and the amount of corrosion products (gypsum and ettringite) decreased gradually with decreasing Ca/Si ratios of C-A-S-H. Sulfate ions can also promote the polymerization degree of C-A-S-H gels, improving its resistance to sulfate attack. Moreover, the 4-coordination aluminum (Al[4]) in C-A-S-H, 5-coordination aluminum (Al[5]), 6-ccordination aluminum (Al[6]) in TAH (third aluminum hydrate) and Al[6] in monosulfate or C-A-H (calcium aluminate hydrate) can be transformed into Al[6] in ettringite by sulfate attack. Furthermore, through thermodynamic calculation, the decrease of Ca/Si ratios and increase of Al/Si ratios can improve the thermodynamic stability of C-A-S-H gels under sulfate attack, which agrees well with the experiment results.
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