Flowability and structural evolution of CaO-SiO2-MgO-Al2O3 slag with varying MgO/CaO ratios:Experiments and MD simulations

•Viscosity first decreases and then increases with increasing MgO/CaO ratio.•The free running temperature decreases significantly with increasing MgO/CaO ratio.•Degree of polymerization of slag is the main determinant of viscosity change.•Connected forms of the slag microstructure are clearly observed by MD simulations. In this study, the effect of MgO/CaO mass ratio on the flowability and structure of CaO-28 wt.%SiO2−MgO-22 wt.%Al2O3 slag was investigated. It was found that the viscosity first decreased and then increased with increasing MgO/CaO ratio, while the free running temperature monotonically dropped. FTIR and Raman analyses indicated that the [AlO4] and [SiO4] tetrahedral structures underwent a change from complex to simple and back again as the MgO/CaO ratio increased. XPS analysis confirmed that the bridging oxygen first decreased and then increased with increasing MgO/CaO ratio, whereas the free oxygen and non-bridging oxygen presented the opposite trend. Molecular dynamics simulations demonstrated that the Al atoms mainly existed in the form of [AlO4] and [AlO5], and the [AlO4] unit first decreased and then increased upon increasing the MgO/CaO ratio. Both spectral analysis and simulations indicated that the degree of polymerization of the slag structure is a key factor in determining the viscosity variation.