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.