The effects of carbonation and hydration on the mineralogy and microstructure of basic oxygen furnace slag products

Basic oxygen furnace slag (BOFS) is one of many alkaline industrial residues. However, BOFS has a low utilization value, and it also may have environmental issues in the practical application. This paper performs a long-term study on the evolution of pH and compressive strength during the combined-curing through carbonation and subsequent hydration of BOFS. Results showed that the combined-curing is a promising and environmentally beneficial way to dispose and utilize BOFS, because it 1) permanently stores carbon dioxide, 2) makes full use of the activity of the minerals in BOFS, and 3) enhances the properties of building materials by improving the compressive strength, reducing the pH, and reducing the true density, without causing concerns on the properties of expansion and fracture. The pore structure was improved by the formed poorly crystalline calcium carbonate (CaCO3), crystalline CaCO3, and monocarboaluminate during the combined-curing process. The pH of the liquid phase initially decreased with the carbonation degree. After the CaCO3-coating was separated from the surface of basic phases, the late-age hydration of un-carbonated basic phases led to a rebound of pH. Rietveld refinement and the evolution of pH showed that the hydration rate decreased as the carbonation degree increased due to the absence of calcium hydroxide. Thus, the physico-chemical mechanism involved in the strength gain, and the alteration of the pH was explained through the composition and structure analysis. The above findings could be meaningful for the steel slag recycling, CO2 utilization and sequestration.