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Characterization of Monochromate and Hemichromate AFm Phases and Chromate-Containing Ettringite by1H,27Al, and53Cr MAS NMR Spectroscopy

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The calcium aluminate hydrate AFm and AFt phases formed upon hydration of Portland cement have an important role in the stabilization and solidification of hazardous chromate ions in hardened cement. AFm monochromate (Ca4[Al(OH)6]2(CrO4)12H2O), AFm hemichromate (Ca4[Al(OH)6]2(CrO4)0.5(OH)12H2O) and the chromate-containing AFt phase, Ca6[Al(OH)6]2- (CrO4)324H2O, were synthesized and investigated by1H,27Al, and53Cr MAS NMR spectroscopy.27Al quadrupolar coupling parameters (CQ, ηQ) and isotropic chemical shifts (δiso) were determined for the three phases, including two distinct Al sites in chromate-AFt, as observed by27Al MAS and MQMAS NMR. Two dominant peaks are apparent in the1H MAS NMR spectra of each of the phases. For the AFm phases, these resonances are assigned to framework hydroxyl groups (1.7–2.0 ppm) and water molecules/hydroxyls (5.0–5.5 ppm) in the interlayer. For chromate-AFt, the peaks are ascribed to framework hydroxyl groups in the [Ca6Al2(OH)12]6+ columns (~1.4 ppm) and water molecules (~4.8 ppm) associated with the Ca ions.53Cr MAS NMR spectra acquired at 22.3 T for the samples show a narrow resonance for both chromate AFm phases, whereas indications of three distinct Cr resonances are apparent for the chromate AFt. The absence of any second-order quadrupolar effects in the53Cr NMR spectra strongly suggests that the chromate ions are highly mobile in the anionic sites of the AFm and AFt structures. The NMR data reported in this work are in agreement with the reported crystal structures for the chromate AFm and AFt phases and may be useful for identification and characterization of chromate fixation in cementitious systems, complementing information gained from conventional powder X-ray diffraction studies.

StatusUdgivet - mar. 2022

Bibliografisk note

Funding Information:
Funding: The authors acknowledge access to the 950 MHz NMR spectrometer at the Danish Center for Ultrahigh Field NMR Spectroscopy at iNANO, Aarhus University, funded by a Ministry of Higher Education and Science grant (AU-2010-612-181). The Carlsberg Foundation is acknowledged for an equipment grant (CF19-0498).

Publisher Copyright:
© 2022 by the authors. Licensee MDPI, Basel, Switzerland.

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