Quantitative insight into aluminum structures in CaO–Al2O3–SiO2 system via Raman and 27Al MAS-NMR spectroscopies

Abstract In order to aid the research and development of Al2O3-based materials, a comprehensive study of the structural properties of CaO–Al2O3–SiO2 systems was carried out via Raman and MAS-NMR spectroscopies. The spectral results showed that Raman scattering coefficients of Q2(Al), Q3(Al), Q4(Al), and [AlO6] units calibrated by 27Al MAS-NMR were 1, 0.554, 0.374, and 1.468, respectively. Using these Raman scattering coefficients to quantitatively analyze the aluminum structures, the results showed that CaO could promote the transformation from six-coordinated aluminum to four- and five-coordinated state with simultaneous depolymerization of four-coordinated aluminum. And CaO could exert a stronger depolymerization on [AlO4]-tetrahedrons than [SiO4]-tetrahedrons. Moreover, non-bridging oxygen number per coordinated atom (NBO/T) corresponding to transition of aluminum speciation were acquired. The values of NBO/T of the transformations from coexistences of [AlO6] and [AlO4] to coexistences of [AlO6], [AlO5], and [AlO4] to coexistences of [AlO5] and [AlO4] were 0.94 and 1.38, respectively.