Positive effects of sulfates on performance of cement paste incorporating low-grade, high-aluminum steel slag

Abstract

Steel slag with high aluminum (Al) content significantly delays the early hydration of cement when used as a supplementary cementitious material (SCM). To address this challenge, this study investigates the effects of gypsum (Gy) and sodium sulfate (SS) additions on the performance of cement pastes with 30 % high-Al ladle metallurgy furnace (LMF) slag. The mechanical properties, hydration kinetics, phase evolution, as well as the early-age hydration mechanism were explored. The results indicate that C₃S hydration is substantially inhibited under high-Al conditions, mainly attributed to the rapid dissolution and reaction of reactive aluminates (particularly mayenite) in the slag. Both Gy and SS additions promote the early hydration of the blended pastes, albeit through distinct mechanisms. The addition of Gy tends to suppress mayenite dissolution, thereby mitigating the retardation of C₃S hydration by postponing aluminum release; however, in sufficiently sulfated pastes (i.e., containing 15 % Gy), where renewed aluminate dissolution occurs after C₃S hydration, delayed hydration of C₃S compared to PC can still be observed. The addition of SS does not appear to affect mayenite dissolution due to the low calcium concentration; however, C₃S hydration is not inhibited, as the high alkalinity mitigates the inhibitory effects of aluminum on C₃S dissolution. Instead, SS promotes C₃S hydration, primarily by increasing the ionic strength of the solution. Consequently, the addition of sulfate (particularly SS) enhances early-age (within 3 days) compressive strength of the blended pastes, while excessive sulfate addition can compromise late-age strength.