Reactive MgO enhanced carbonation resistance in Alkali-Activated Materials by forming M-S-H-type gels: Linking microstructure, strength, and phase stability

This study investigates the influence of reactive MgO on the carbonation behaviour of fly ash–blast furnace slag (FA–BFS)-based alkali-activated materials (AAMs). Five distinct mixtures were prepared, maintaining a constant 30% BFS content while progressively replacing FA with 5%, 10%, 15%, and 20% reactive MgO. The results demonstrate that increasing reactive MgO content significantly enhances the carbonation resistance of FA–BFS-based AAMs. Notably, the mixture containing 15% MgO exhibited no observable carbonation after 13 weeks of exposure. O2 diffusion, shrinkage strain, and N2 adsorption analyses further confirmed that mixtures with high MgO content were significantly less affected by carbonation, while samples with no or low MgO content showed substantial carbonation-induced damage. Microstructural analysis revealed that higher MgO content facilitated the retention of unreacted MgO (periclase), which plays a crucial role in buffering the pore solution. Upon carbonation, periclase transforms into brucite, hydrotalcite, and magnesium silicate hydrate (M-S-H)-type phases, thus stabilizing the matrix against carbonation-induced degradation. While hydrotalcite has been widely recognized as the primary carbonation sink in MgO-rich AAMs, our findings suggest a distinct early-stage carbonation mechanism in high MgO containing mixtures, and formation of M-S-H-type gel phases in AAMs post carbonation. X-ray diffraction (XRD) and thermodynamic modelling confirmed the formation of additional hydration products in high-MgO mixtures, including brucite, hydrotalcite, calcium sodium aluminosilicate hydrate (C-(N)-A-S-H), and M-S-H-type gels. These findings suggest the emergence of a hybrid binder system facilitated by MgO incorporation, that significantly improves the carbonation resistance of AAMs. This conclusion is supported by a combination of experimental techniques, including N2 adsorption–desorption isotherms, XRD, FTIR, and thermodynamic modeling using PHREEQC. Furthermore, thermogravimetric analysis (TGA) showed early decomposition of carbonate minerals around 300–400°C, particularly in mixtures with lower MgO content. This behavior is attributed to solid-state reactions between the silica gel formed from C-(N)-A-S-H decomposition and carbonate phases, highlighting the enhanced thermal and carbonation stability imparted by higher MgO incorporation.