Nanoscale Chemical Degradation Mechanisms of Sulfate Attack in Alkali-activated Slag

Abstract

Chemically induced material degradation is a major durability issue facing many technologically important materials systems, including conventional and new sustainable cementitious materials. In this study, the nanoscale chemical degradation mechanisms have been elucidated for an amorphous sodium hydroxide-activated slag paste (one type of sustainable cement) exposed to different types of sulfate-bearing solutions (i.e., Na2SO4, MgSO4, and H2SO4), by combining synchrotron-based X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and X-ray pair distribution function (PDF) analysis. The XRD, FTIR, and PDF results show that the chemistry and structure of the paste is essentially immune to Na2SO4 attack, whereas exposure to 5–10 wt % MgSO4 and H2SO4 cause complete disintegration of the main binder gel (i.e., sodium-containing calcium–(alumino)–silicate–hydrate), along with formation of magnesium–silicate–hydrate or silica-rich gels and extensive precipitation of gypsum. These differences a...