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

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 appear to be directly correlated with the ability of the ions (i.e., Na⁺, Mg²⁺, H⁺) accompanying SO₄^2- to alter the pH of the pore solution in the samples. By correlating the changes that occurred to the phase composition and the structure of the paste with the pH data from the equilibrated solutions, this study has provided important mechanistic insight on the fundamental sulfate-induced degradation reactions occurring in hydroxide-activated slag.