A connection between structure and reactivity in alkali-modified calcium aluminosilicate glasses for sustainable cements

Abstract

The primary reactive components in supplementary cementitious materials such as fly ashes and slags are calcium aluminosilicate (CAS) glasses. The structures of CAS glasses can be modified by alkali ions, which in turn affects their pozzolanic reactivity in cementitious environments. In this study, twenty-one CAS glasses, both alkali-free and alkali-modified with up to 20 mol% Na₂O and/or K₂O, have been synthesized and analyzed using Raman spectroscopy and X-ray powder diffraction (XRPD). The local heterogeneity of the synthetic glasses, caused by aluminum avoidance and the association of non-bridging oxygens primarily with silicon tetrahedra, significantly influences their dissolution behavior and pozzolanic reactivity in simulated cementitious environments. This is further supported by isothermal calorimetry and XRPD measurements on blended Portland cement pastes containing 35 wt% glass. Additionally, the glasses exhibit congruent dissolution behavior, unlike their crystalline counterparts. These findings may allow for tailoring of synthetic glasses for applications as supplementary cementitious materials.