Physical, mechanical, chemical, and environmental characterization of stockpiled BOF slag as railway ballast material

Abstract

Crushed stone from natural sources is commonly used as ballast in railways. The material is a good ballast, but its extraction is destructive. Therefore, this study characterizes basic oxygen furnace (BOF) slag as an alternative material for railway ballast in Kazakhstan with respect to physical, mechanical, chemical and environmental properties. The characterized BOF slags are of two types (top and bottom) with 10 years minimum age. Grain size distribution, specific gravity, absorption capacity, flakiness, lightweight particles, and 3D image analysis for forms and angularity were used for physical characterization. These BOF slags were found to have high specific gravity and a predominantly spherical shape, with larger particles displaying lower sphericity and greater angularity than smaller ones. The mechanical characterization of the BOF slag were carried out using various methods, including LA abrasion, Micro-Deval, aggregate crushing value, aggregate impact value, volumetric expansion, and fouling. BOF slag aggregates demonstrated exceptional strength with good resistance to fragmentation, crushing, surface abrasion, and sudden loading. The expansion rate is considerably lower than the standard limit, and the fouling rate is better than limestone and similar to granite. X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were all used for chemical characterization. Both BOF slags contain CaO and Fe2O3 with a high peak of SiO2 and portlandite, which suggests that most of the free lime has undergone hydration due to the natural aging of the BOF slag. Regarding the environmental characterization, the tests employed were organic impurity, freeze and thaw, and sodium-based soundness. No organic impurity was observed in the BOF slag, and the BOF slag demonstrated good resistance to weathering and degradation. The characteristics of the investigated BOF slag obtained from laboratory testing were analyzed to confirm its performance for implementation in railway ballast in compliance with the specific requirements, primarily the GOST standard, which is commonly employed in Russia and former Soviet republics, including Kazakhstan. As a result, both BOF slags demonstrated total compliance with the specifications, and the possibility of using stockpiled BOF slag for railway ballast material in Kazakhstan was established.