  Quaternary CaO-MgO-Al2O3-SiO2 glasses are important constituents of the
Earth's lower crust and mantle, and they also have important industrial
applications such as in metallurgical processes, concrete production and
emerging low-CO2 cement technologies. In particular, these applications rely
heavily on the composition-structure-reactivity relationships for CMAS glasses,
which are not yet well established. In this study, we developed a robust method
that combines force-field molecular dynamics (MD) simulations and density
functional theory (DFT) calculations with X-ray/neutron scattering experiments
to resolve the atomic structure of a CMAS glass. The final structural
representation generated using this method is not only thermodynamically
favorable (according to DFT calculations) but also agrees with experiments
(including X-ray/neutron scattering data as well as literature data). Detailed
analysis of the final structure (including partial pair distribution functions,
coordination number, oxygen environment) enabled existing discrepancies in the
literature to be reconciled and has revealed new structural information on the
CMAS glass, specifically, (i) the unambiguous assignment of medium-range atomic
ordering, (ii) the preferential role of Ca atoms as charge compensators and Mg
atoms as network modifiers, (iii) the proximity of Mg atoms to free oxygen
sites, and (iv) clustering of Mg atoms. Overall, this new structural
information will enhance our mechanistic understanding on CMAS glass
dissolution behavior, including dissolution-related mechanisms occurring during
the formation of low-CO2 cements.
