Resolution of the Two Aluminum Sites in Ettringite by ²⁷Al MAS and MQMAS NMR at Very High Magnetic Field (22.3 T)

Abstract

Ettringite (Ca6[Al(OH)6]2(SO4)3·26H2O) is the first hydration product formed during Portland cement hydration. 27Al MAS NMR has been used in a wide number of studies to detect and quantify ettringite in hydrated cement blends by the observation of a single, narrow resonance at 13–14 ppm. This work reports the first observation of resonances from two distinct Al sites in octahedral coordination for ettringite, employing 27Al MAS and MQMAS NMR at an ultrahigh magnetic field (22.3 T). Thereby, the 27Al NMR spectra are in agreement with the most accepted trigonal model for the ettringite structure. 27Al quadrupole coupling parameters and isotropic chemical shifts for the two Al sites are determined from simulations and least-squares optimization of slow-speed 27Al MAS NMR spectra of the satellite transitions. These data reveal that the local environments for the two octahedral Al sites are very similar, in accord with the most recent XRD refinements of the ettringite structure. Finally, the significant improvement in spectral resolution by the application of an ultrahigh magnetic field is illustrated by the detection of the two Al sites from ettringite in a hydrated cement mimicking the composition of a calcium sulfoaluminate cement.