THE CHEMISTRY OF AIR-COOLED BLAST FURNACE SLAG (ACBFS) RELATED TO ENVIRONMENTAL EXPOSURE ASSOCIATED WITH REUSE

This dissertation contains three chapters encompassing research on air-cooled blast furnace slag chemistry, environmental impacts related to testing, and additional options for beneficial use as a carbon capture utilization and storage (CCUS) technology. Air-cooled blast furnace slag (ACBFS) is often used as road construction aggregate in Indiana and elsewhere. However, several incidences of adverse environmental effects have been reported from sites where slag has been used, including pollutant discharges to waterways, even though these slags had been previously approved for use by passing State of Indiana leaching procedures. The motivation of the first study was to test several hypotheses posed by industry about ACBFS leaching and better understand water quality impacts. Ten slags were tested from two suppliers according to leaching procedures adopted by the Indiana Department of Transportation. Additional material and leachate chemical analyses were conducted. In this study, no relationship was found between slag age and leachate composition. The results of this work indicate that the parameters in existing ACBFS acceptance tests did not help predict or prevent adverse environmental impacts of ACBFS use. Rather, fundamental studies of material chemistry and field investigations are recommended where ACBFS has been proposed to be used. In Chapter 4 this is explored in greater detail. The highly alkaline nature of ACBFS is directly attributable to the high quantity of calcium and magnesium oxides commonly organized structurally as akermanite (Ca2MgSi2O7) and other related alkaline minerals found in most blast furnace slags (BFS). The objective of the second study was to investigate the dissolution of finer ACBFS samples with the highest akermanite wt %, evaluating dissolution behavior of finely ground ACBFS (< 250 μm) at low solution pH (~ 4.0). Specifically, the akermanite-equivalent alkalinities of five ACBFS samples were determined using pH-stat titrations, in which powdered suspensions of ACBFS were maintained at a constant low pH through addition of nitric acid. Results showed that, in the absence of atmospheric carbonates at pH 4, significant but not complete dissolution did occur, indicating that at typical pH ranges encountered in the field unbound ACBFS will likely still produce highly alkaline leachate when it encounters water. The final titrated alkalinity was consistent with akermanite as the dominant mineral component. In Chapter 5 (i.e., the third study), a potential method of reducing or preventing leaching of alkalinity through direct mineral carbonation was evaluated. The influences on the following parameters on mineral carbonates were studied: (1) Extraction of calcium ions, and (2) carbonation efficiency. In addition, ACBFS stability was investigated experimentally as a function of reaction temperature, type of base, and post-carbonation pH. Results show that while there is an increase in the percentage of calcium carbonate (CaCO3) in ACBFS samples when temperature is increased, there were minimal changes in carbonated slag pH when subjected to leachate. While the method may have utility in stabilizing slag, for the amount of slag that can be carbonated, the amount of CO2 emissions that can be treated is not significant. From a stabilization perspective as it pertains to leaching impacts, there was not a significant reduction in leachate pH from carbonated ACBFS under the conditions assessed. However, given the results, the process outlined in this study, given more refinement (elevated pressure, increased solid-to-liquid ratio, retention times, etc.), may result in significantly reduced environmental impacts associated with ACBFS leachate.