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

摘要

Despite the widespread occurrence of sulfur in both natural and man-made materials, the S-33 nucleus has only rarely been utilised in solid-state NMR spectroscopy on account of its very low natural abundance (0.76%), low NMR frequency (v(0) = 30.7 MHz at B-0 = 9.4 T), and significant nuclear quadrupole moment (spin / = 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 / > 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 S-33 STMAS NMR experiments at B-0 = 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 S-33 STMAS NMR study of the cement-forming mineral ettringite (Ca6Al2(SO4)(3)(OH)(12).26H(2)O) at B-0 = 9.4 T and 20.0 T, resolving a discrepancy in the literature between two previous conventional S-33 MAS NMR studies and obtaining an alternative set of S-33 NMR parameters that is simultaneously consistent with the MAS and STMAS data at both field strengths.