Characterization of Monochromate and Hemichromate AFm Phases and Chromate-Containing Ettringite by 1H, 27Al, and 53Cr MAS NMR Spectroscopy

The calcium aluminate hydrate AFm and AFt phases formed upon hydration of Portland cement have an important role in the stabilization solidification hazardous chromate ions hardened cement. monochromate (Ca4[Al(OH)6]2(CrO4)·12H2O), hemichromate (Ca4[Al(OH)6]2(CrO4)0.5(OH)·12H2O) chromate-containing phase, Ca6[Al(OH)6]2-(CrO4)3·24H2O, were synthesized investigated by 1H, 27Al, 53Cr MAS NMR spectroscopy. 27Al quadrupolar coupling parameters (CQ, ηQ) isotropic chemical shifts (δiso) determined for three phases, including two distinct Al sites chromate-AFt, as observed MQMAS NMR. Two dominant peaks are apparent 1H spectra each phases. For these resonances assigned to framework hydroxyl groups (1.7–2.0 ppm) water molecules/hydroxyls (5.0–5.5 interlayer. ascribed [Ca6Al2(OH)12]6+ columns (~1.4 molecules (~4.8 associated with Ca ions. acquired at 22.3 T samples show a narrow resonance both whereas indications Cr AFt. absence any second-order effects strongly suggests that highly mobile anionic structures. data reported this work agreement crystal structures may be useful identification characterization fixation cementitious systems, complementing information gained from conventional powder X-ray diffraction studies.