Abstract
The durability of alkali-activated silico-manganese fume (SMF)-based mortar specimens was investigated by immersing them in 5% Na2SO4 solution and water (controlled environment) for up to 40 weeks. The effect of blast furnace slag (BFS) content [R = BFS/(BFS + SMF) = 0 and 0.3] and alkali concentration (4 and 10 M NaOHaq) was studied. Visual appearance, variation in alkalinity, mass loss, compressive strength, and microstructural changes were recorded. The maximum strength of 49.4 MPa was noted in BFS-blended high-alkaline binder system while the ultimate residual strength and mass loss were 88.8% and - 2.4%, respectively. The high sulfate-resistance of this binder is attributed to the dense matrix resulting from the formation of C & x2500;S & x2500;H, K & x2500;A & x2500;S & x2500;H, and C & x2500;Mn & x2500;H. BFS-blended mild-alkaline system exhibited incomplete precursor dissolution thereby gaining a moderate strength of 39.6 MPa with the residual strength and mass loss of 74.1%and - 4.4%, respectively. The decomposition of the specimens on exposure to Na2SO4 may be ascribed to high pH differential from the mobility of Na+, SO42-, and OH- in and out of the mortar leading to formation of additional gypsum and calcite. The BFS-free system displayed excellent stability under sulfate environment owing to the dearth of Ca in SMF. This was notably aided by increased precipitation of quartz together with the formation of degradation products and despite that still attained low strength of about 20 MPa. It is postulated that Ca/Si, Ca/Mn, Si/Mn, Ca/K, and Si/K ratios are the controlling factors influencing the sulfate-resistance of the developed binders.