A high-resolution natural abundance 33S MAS NMR study of the cementitious mineral ettringite

Despite the widespread occurrence of sulfur in both natural and man-made materials, the 33S nucleus has only rarely been utilised in solid-state NMR spectroscopy on account of its very low natural abundance (0.76%), low NMR frequency (ν0 = 30.7 MHz at B0 = 9.4 T), and significant nuclear quadrupole moment (spin I = 3/2, Q = −69.4 mb). Satellite-transition magic angle spinning (STMAS) is an NMR method for obtaining high-resolution spectra of half-integer quadrupolar nuclei (spin I > 1/2) in solids and is notable for its intrinsic sensitivity advantage over the similar multiple-quantum (MQMAS) method, especially for nuclei with low NMR frequencies. In this work we demonstrate the feasibility of natural abundance 33S STMAS NMR experiments at B0 = 9.4 T and 20.0 T using a model sulfate sample (Na2SO4 + K2SO4 in a 1 : 1 molar ratio). Furthermore, we undertake a natural abundance 33S STMAS NMR study of the cement-forming mineral ettringite (Ca6Al2(SO4)3(OH)12·26H2O) at B0 = 9.4 T and 20.0 T, resolving a discrepancy in the literature between two previous conventional 33S MAS NMR studies and obtaining an alternative set of 33S NMR parameters that is simultaneously consistent with the MAS and STMAS data at both field strengths.