Slags are a primary waste stream produced during the production of steel. Air-cooled Blast Furnace Slag (ACBFS) is a major type of steelmaking slag produced during the Blast Furnace smelting process. After the smelting process, the slag is slowly cooled under ambient conditions and stockpiled. As a by-product, slag is often sold and used as fill in construction projects, especially during construction and maintenance of highways. However, the environmental and human health impacts of slag have not been extensively studied. One of the primary concerns with ACBFS use during construction projects is the high pH (>9) and mineral alkalinity associated with its exposure to water (i.e., leachate). This is attributed to its fundamental minerology which largely has the same elemental composition of akermanite (Ca₂MgSi₂O₇). Akeramite, and other metal oxides (such as CaO) in slag, are readily soluble in their purest forms; however there is limited research on the dissolution of (these minerals from) ACBFS under ideal laboratory conditions, in order to estimate the long-term impacts of slag in the environment. Therefore, the objective of this study was to characterize the dissolution of ACBFS (obtained from various steel mill suppliers) under optimal dissolution conditions of low pH using fine and coarse particle sizes. The akermanite-equivalent alkalinity ACBFS was determined using a pH-stat, in which powdered suspensions of ACBFS were maintaining a constant low pH (= 4) through addition of nitric acid, recording the volume of acid addition as a function of time. In the absence of atmospheric carbonates, complete dissolution did not occur largely due to the formation of silica oxides (e.g., SiO₂); however quantitative release of calcium and magnesium occurred. The final titrated alkalinity was quite consistent with akermanite as the dominant mineral component (Ca₂MgSi₂O₇ + 4 H₂O = Ca²⁺ + Mg²⁺ + 2 SiO₂ + 4 OH^-). Understanding of ACBFS chemistry and kinetics can support the development of new slag testing procedures for environmental compliance, as well gain more understanding on how to optimize slag reuse methods, such as carbon capture using ACBFS.

Rasul Diop

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