Effect of gypsum on the properties of composite binder containing high-volume slag and iron tailing powder

Abstract

• Composite binder with high-volume slag and ITP shows low early compressive strength. • The addition of gypsum compensates for the poor early properties of composite binder. • The addition of gypsum negatively affects later-age properties of composite binder. • Adding gypsum increases Ca/Si ratio and S/Si ratio but doesn’t affect Al/Si ratio. • High-volume slag and ITP can be used to produce high performance green concrete. The effect of gypsum on the properties of composite binder containing high-volume slag and iron tailing powder was studied by measuring compressive strength, hydration heat, bound water content, hydration products, pore structure and morphology. The addition of gypsum increases early-age compressive strength and bound water content, but the opposite trend is observed at later ages. A higher quantity of gypsum leads to a higher third exothermic peak and a higher total hydration heat. Although incorporating gypsum promotes early hydration of cement and slag, it limits their further hydration at later ages. Adding gypsum forms a large amount of ettringite and densifies early-age pore structure, but it coarsens later-age pore structure. The addition of gypsum increases Ca/Si and S/Si ratios of C-S-H, but it doesn’t affect Al/Si ratio. The addition of gypsum compensates for the poor early-age properties of composite binder containing high-volume slag and iron tailing powder, and the later-age properties are acceptable.