Synergistic Effects of MgO-Modified Activator and Aluminosilicate Materials on the Deterioration Performance of Alkali-Activated Slag Exposed to Chloride and Sulfate Solutions

Abstract

Alkali-activated slag (AAS) has emerged as a promising alternative to ordinary Portland cement (OPC) for use in aggressive environments, owing to its lower carbon emissions and superior resistance to corrosive media. Layered double hydroxides (LDHs), which include hydrotalcite and AFm-type phases, are identified as secondary hydration products of AAS. The unique layered structure of LDHs is posited to contribute significantly to the exceptional durability exhibited by AAS. The synergistic effects of MgO-modified activators, combined with metakaolin (MK) or fly ash (FA), on the phase evolution and chemical degradation processes of AAS exposed to aggressive solutions were studied through various experimental methods, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR),  thermogravimetric analysis (TGA), and SEM-EDS, along with thermodynamic simulation methods. The research results suggest that the reactive MgO modifier activator, in synergy with 20% or 40% MK, respectively, improved the compressive strength of AAS by approximately 30% and 70%. In NaCl solution, the incorporation of reactive MgO facilitates the precipitation of MgAl-LDHs, thereby enhancing the chloride binding capacity (CBC) of AAS (by around 30%). However, in sulfate solutions, the precipitation of MgAl-LDHs leads to an excess of calcium, which subsequently increases the precipitation of ettringite. Although the substitution of MK and FA enhances the sulfate resistance of AAS by reducing the CaO content in the reactive system, it does not promote the precipitation of MgAl-LDHs and Friedel’s salt, thereby adversely affecting the CBC of AAS.

Keywords: Alkali-activated slag, Reactive MgO, metakaolin, Fly ash, Deterioration performance