Degradation of alkali-activated Fe-rich slag in magnesium sulfate solution

The potential of Fe-rich non-ferrous metallurgical slag (NFMS) in alkali-activated materials has been demonstrated, exhibiting promising mechanical strength and fire resistance. However, the durability of alkali-activated NFMS (AA-NFMS) in terms of sulfate salt resistance remains unexplored. Additionally, the variability in the chemical composition of NFMS presents challenges in directly assessing the durability of AA-NFMS. This study investigates the interplay between the Ca/Si and Mg/Ca ratios in Fe-rich NFMS and the resulting MgSO4 resistance of the AA-NFMS made thereof. To do so, mass change and compressive strength retention were monitored for six months on AA-NFMS immersed in 5 wt% MgSO4, whereas microstructure was investigated using scanning electron microscope (SEM) and electron probe microanalyzer (EPMA). The results show that the MgSO4 solution attacked selectively the reacted binder matrix of AA-NFMS rather than the unreacted slag. The corrosion primarily occurs at the sulfate exposure surface of AA-NFMS, forming a dense, thin layer of Mg(OH)(2), which acts as a barrier and impedes further attack. Dissolution of Ca is the most pronounced among the major elements (Ca, Fe, Al, and Si) in the NFMS. The mass of the specimens increased linearly with the initial Ca content. The medium Ca concentration (similar to 10%) retained the highest strength.