Abstract
Monosulfoaluminate (Ca4Al2(SO4)(OH)12∙6H2O) plays an important role in anion binding in Portland cement by exchanging its original interlayer ions (SO42− and OH−) with chloride ions. In this study, scanning transmission X-ray microscope (STXM), X-ray absorption near edge structure (XANES) spectroscopy, and X-ray diffraction (XRD) were used to investigate the phase change of monosulfoaluminate due to its interaction with chloride ions. Pure monosulfoaluminate was synthesized and its powder samples were suspended in 0, 0.1, 1, 3, and 5 M NaCl solutions for seven days. At low chloride concentrations, a partial dissolution of monosulfoaluminate formed ettringite, while, with increasing chloride content, the dissolution process was suppressed. As the NaCl concentration increased, the dominant mechanism of the phase change became ion exchange, resulting in direct phase transformation from monosulfoaluminate to Kuzel’s salt or Friedel’s salt. The phase assemblages of the NaCl-reacted samples were explored using thermodynamic calculations and least-square linear combination (LC) fitting of measured XANES spectra. A comprehensive description of the phase change and its dominant mechanism are discussed.
Highlights
The NaCl aqueous solution is a frequent environment for cement-based materials
Al2 O3 -Fe2 O3 -mono (AFm) phases have a layered structure that mainly consists of octahedral Ca(OH)6 sheets; one-third of Ca atoms are replaced with Al, and Fe(III) is present as a relatively minor substituent
Calcium cations are present in a seven-fold coordination with a distorted octahedral Ca(OH)6 polyhedra, and additional water molecules exist in the interlayer space
Summary
The NaCl aqueous solution is a frequent environment for cement-based materials. For example, marine and offshore concrete structures are generally exposed to seawater and corrosion of reinforcing steel bars by chloride attack is of significant concern in the deterioration of reinforced concrete (RC) structures [1,2]. If the phase transition discontinuously occurs at a specific chemical environment, it is unlikely that the continuous adsorption isotherms (e.g., Freundlich or Langmuir isotherms) can properly describe the adsorption process of chloride ions by monosulfoaluminate. For this reason, a further study is necessary to understand the detailed process of the phase change due to the chloride binding for monosulfoaluminate in saline environments. Thermodynamic calculations were conducted to theoretically deduce a possible phase change of monosulfoaluminate under a saline environment using a previously reported database for cementitious substances and compared with the quantified results obtained from the XRD and XANES analysis
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