Sulfuric acid resistance of alkali/slag activated silico-manganese fume-based mortars

The resistance of alkaline-activated silico-manganese fume (SMF) and blast furnace slag (BSF)-based mortar, prepared by varying BFS and NaOH_aq composition, during 20 weeks of exposure to 5% sulfuric acid (H₂SO_4aq) of initial pH = 0.24 and pure water (controlled) was studied. The deterioration was monitored by visual, alkalinity, mass, compressive strength and microstructural tests. The set of specimens synthesized included: (a) BFS-free high alkaline, (b) SMF-BFS blended high alkaline, and (c) SMF-BFS blended mild alkaline system. Their residual strengths after 20 weeks of acid exposure were 20.1, 18.6, and 16%, respectively. BFS-free specimens exhibited high resistance to acid attack due to the dearth of Ca in SMF. BFS-admixed mild alkali system resulted formation of more gypsum and dealumination attributed to incomplete dissolution of Ca grains leading to severe spalling. BFS-admixed high alkaline system underwent moderate resistance due to dense microstructure formed by additional C-S-H, K-A-S-H, and C-Mn-H gel which enhanced retention of the polymerized framework.