The mechanism of basic oxygen furnace steel slag retarding early-age hydration of Portland cement and mitigating approach towards higher utilization rate

The incorporation of steel slag usually leads to retardation on the early-age hydration of cement, which limits its use as supplementary cementitious material in cement or concrete. Aiming to a better understanding of the retarding mechanism, the influences of two representative basic oxygen furnace steel slags on setting behaviour, hydration kinetics, aqueous and solid composition during early age were investigated. Different from the existing mechanisms, this study highlights the role of aluminate phase contained in steel slags in the retardation of hydration. The incorporation of steel slag with high aluminate phases content reduces C3S/aluminate phase ratio significantly. This reduction in C3S/aluminate phase ratio leads to undersulfation in the system, where aluminate phases react prior to C3S, resulting in flash setting, reduced C3S reaction, and thus the hindered early-age strength gain. This study also proposes an efficient way to alleviate the retarding effect of steel slag on early-age hydration by increasing the sulfate supply, which allows the system to react in a properly sulfated manner.