Composition-structure relationships for calcium aluminosilicate glasses

Calcium aluminosilicate (CAS) glasses serve as ideal model systems for understanding the composition-structure relationships underpinning the performance of supplementary cementitious materials (SCMs) due to their simpler chemistry and reduced phase heterogeneity. Here, we investigate the structure of a broad compositional range of CAS glasses, including unprecedented high-CaO compositions, using X-ray diffraction (XRD), solid-state nuclear magnetic resonance (NMR), and Fourier transform infrared (FTIR) spectroscopy. With increasing CaO content, the XRD hump maxima shifts towards higher diffraction angles and causes downfield shifts in the ²⁹Si and ²⁷Al NMR isotropic shift maxima, indicating reduced interatomic distances and decreased electron density around Si and Al nuclei, respectively. These trends, consistent with a depolymerized structure that is more compact and contains a higher number of non-bridging oxygens, also correlate strongly with changes in Si–O-Si and Al–O–Al bond angles predicted by atomistic simulations. FTIR spectra reveals shifts in T-O-T' bond vibrations to lower wavenumbers with increasing CaO, signifying a transition to less polymerized Qⁿ (Si, Al) species. Collectively, our results demonstrate the role of CaO in promoting network depolymerization, a crucial factor for SCM reactivity, and provide valuable insights into the structural evolution of CAS glasses as a function of composition.