Degradation of alkali activated slag mortars subjected to accelerated leaching

Alkali activated slag (AAS) is an amorphous aluminosilicate material consisting mainly of C-A-S-H gel with a considerably high pH of approximately 12-13. Alkali and earth-alkali ions (e.g., Na+, K+, and Ca2+), besides contributing to the alkalinity of the pore solution, serve as the charge-balancing ions for the Al3+ in the C-A-S-H gel [1]. The alkali ions are potentially leached out of the AAS matrix when the materials are exposed to acidic or even neutral environments, leading to the dissolution and consequently degradation of the C-A-S-H gel. Because of the absence of portlandite, a lower Ca/Si ratio of C-A-S-H gel, and a low porosity of the matrix, AAS is expected to be much more stable in acidic media than conventional cementitious materials [2]. Few studies have published on the alteration in AAS properties immerged in a 6M NH4NO3 solution (pH ~ 4.25). For example, Komljenovic et al. [3] reported the leaching of sodium and the decalcification of C-A-S-H gel, and strength reduction of AAS paste after up to 90 days of leaching. Varga et al. [4] investigated the effect of slag composition on the evolution of the gel structure of AAS paste after 21 days of immersion in a 6M NH4NO3 solution, and found that the C-A-S-H gel was more stable if slag precursor contained a lower MgO and higher Al2O3 content. As a consequence of the decalcification, the microstructure potentially change after leaching; however, its alteration has not been widely reported in literature. Furthermore, although the leaching process also depends significantly on the microstructure of AAS as it controls the diffusion of ions from the solution into the matrix, studies on how microstructure influencing the leaching process are still rare. Furthermore, the water-to-binder (w/b) ratio is known as one of the primary parameters affecting the microstructure of AASs, however, the effect of this parameter on the leaching resistance and microstructure after leaching of AAS remains a topic of active research. In order to address the gap in understanding the correlation between the microstructure of AAS and its leaching resistance in the concentrated NH4NO3 solution, this study aims to investigate the alteration of the C-A-S-H gel and the microstructure of AAS mortars after 28 days of leaching in a 6M NH4NO3 solution, with a focus on the effect of the w/b ratio. Different of techniques, including inductive coupled plasma-optical emission spectrometry (ICP-OES), 29Si magic-angle spinning nuclear magnetic resonance (MAS-NMR), and mercury intrusion porosimetry (MIP), have been used to provide information on and insight in the evolution of the AASs structure.