Influence of Free Lime Precipitated in a Grain Boundary of Wüstite on Volume Fraction of Free Lime in Steelmaking Slag Determined via Cathodoluminescence Imaging

In developed countries, nearly half of steelmaking slag is reused in road construction. Determining free lime content in steelmaking slag is important because the hydration of free lime can result in the road fracturing. Recently, we have developed a simple method to rapidly determine the area fraction of free lime via cathodoluminescence (CL) imaging of steelmaking slag based on orange illuminated areas corresponding to free lime. However, the areas of free lime that precipitated along grain boundaries of wüstite were overestimated in CL images because the thickness of the precipitated free lime was not evaluated properly. In this study, we investigate the grain-boundary microstructures of wüstite and propose a method to correct the overestimated areas of the free lime. Scanning transmission electron microscope observation revealed that the average thickness of free lime precipitated in the grain boundaries was 250 nm, which was approximately 20 times smaller than that evaluated via the CL image. Assuming that the thickness of the free lime precipitated in the grain boundaries was 250 nm as a whole, the area fractions decreased by 60%-80%, compared with those without the thickness correction. Considering the CL sampling depth of a few microns, the proposed method can determine the volume fraction of free lime in steelmaking slag more precisely, hereby aiding in the prevention of road expansion in the reuse of steelmaking slag.