Magnesium sulfate resistance of alkali/slag activated silico-manganese fume-based composites

The performance of room-cured alkali-activated mortar, developed using silico-manganese fume (SiMnF) as primary source material (SM), was investigated by immersing the specimens in 5% MgSO₄ solution up to 40 weeks. The effect of quantity of ground blast furnace slag (GBFS) and the concentration of alkaline activator was investigated by varying the ratio of GBFS/(GBFS + SiMnF) (0 and 0.3) and molarity of NaOH_aq (4 and 10 M). The stability of the developed binder under MgSO₄ environment was evaluated by assessing the changes in the physical, chemical, mechanical and microstructural properties. The GBFS-admixed high-alkaline binder system developed a maximum strength of 49.4 MPa while the ultimate residual strength and mass loss on exposure to sulfate environment were 60.1% and −9.59%, respectively. As expected, a proper synergy of GBFS and alkalis enhanced the microstructural densification and dissolution of SMs. This resulted in moderate stability of the binder by imparting amorphousity to the deterioration products and retaining a polymerized Si-O-T chain. The GBFS-admixed mild-alkaline system exhibited a moderate strength of 39.6 MPa while the ultimate residual strength and mass loss were 36.5% and −13.24%, respectively. The progressive deterioration, due to sulfate exposure, was attributed to the existence of unreacted Ca²⁺ grains which aided easy decalcification and dealuminization, increased crystallinity of gypsum and concomitant formation of brucite and calcite. The GBFS-free system developed maximum strength of 20.6 MPa while the ultimate residual strength and mass loss were 60.8% and −11.28%, respectively. The formation of quartz and absence of sulfate phases resulted in a relatively less damage due to the dearth of CaO.