The reuse of steel slag, a large-scale solid waste from steel production, has good social and environmental benefits. The application of a steel slag asphalt mixture is mainly hindered by its volume expansion in water. The expansion of steel slag can be inhibited by oxalic acid. The expansion rate and adhesion of steel slag were investigated, and the immersion stability of steel slag and its asphalt mixture was evaluated by water erosion. By means of XRD, XRF, TG, SEM, etc., the influence mechanism of oxalic acid and water erosion on the properties of steel slag and its asphalt mixture was discussed. The results show that oxalic acid can not only inhibit the expansion of steel slag but also improve its crush resistance, with a reduction in the expansion rate of steel slag by 53%. Oxalic acid is able to leach alkaline metal elements, reducing its adhesion with asphalt. After 10 days of water erosion, the rutting stability and bending crack resistance of the treated steel slag mixture decreased by 37% and 43.2%, respectively. Calcium oxalate is generated on the surface of treated steel slag, which improves the surface compactness, effectively inhibits the expansion of steel slag caused by water erosion, and improves the performance of steel slag and its asphalt mixture. Water erosion can accelerate the hydration and shedding of calcium-containing substances on the surface of steel slag, reduce the adhesion of steel slag, and lead to degradation in the performance of steel slag and its asphalt mixture. Oxalic acid is able to effectively inhibit the expansion of steel slag, and the treated steel slag can be used as recycled aggregate in asphalt mixture, effectively solving the problems of road aggregate deficiency and environmental pollution caused by steel slag.
The effective resource utilization of steel slag from bulk solid waste can achieve good social, environmental and economic benefits. In order to restrain the volume expansion of steel slag and apply it as a building material, in this work, oxalic acid was used for the treatment of steel slag, and the mechanism of oxalic acid restraining the expansion of steel slag was explored. This study recovered the main metal ions in steel slag while restraining its volume expansion. The volume stability of the eroded steel slag and the phase composition, microstructure, roughness and pore size distribution of the steel slag after oxalic acid erosion were investigated. The hydration process of the steel slag was characterized using the pH value, the ion distribution of the leachate was measured, and the leachate was recovered via ultraviolet catalysis and evaporation crystallization. The results showed that a large number of calcium and iron minerals in the steel slag reacted with adipic acid to form calcium oxalate, which precipitated on the surface and opened pores in the steel slag during the process of adipic acid erosion, which improved the compactness of the steel slag’s surface and improved the volume stability of the steel slag. After erosion, the surface roughness, specific surface area and porosity of the steel slag decreased, and the average pore diameter (μm) decreased from 1.717 to 0.208. In addition, the pH value was stable at approximately 7.3 over long-term interaction with water. The leachate was mainly composed of iron, calcium and silicon ions, accounting for 35.43%, 17.17% and 17.05%, respectively, which were recovered by ethanol and ammonia to obtain calcium oxalate, ferrous oxalate, ammonium oxalate and a metal hydroxide. The hydration activity of the steel slag treated with oxalic acid decreased and the volume expansibility of the steel slag was effectively restrained. The expansion rate of the steel slag decreased from 3.59% to 1.69% and the volume stability of the steel slag improved. The leachate recovery efficiency was high and was close to 100%. The utilization of steel slag as a resource was realized and the environmental problems caused by the steel slag were effectively solved.
This paper investigates the use of steel slag in the place of basalt coarse aggregate in Stone Mastic Asphalt-13 (SMA-13) gradings in the early forming of an experimental pavement and evaluates the test performance of the mixes, combined with 3D scanning techniques to analyse the initial textural structure of the pavement. Laboratory tests were carried out to design the gradation of the two asphalt mixtures and to assess the strength, chipping and cracking resistance of the asphalt mixtures using water immersion Marshall tests, freeze–thaw splitting tests, rutting tests and for comparison with laboratory tests, while surface texture collection and analysis of the height parameters (i.e., Sp, Sv, Sz, Sq, Ssk) and morphological parameters (i.e., Spc) of the pavement were performed to assess the skid resistance of the two asphalt mixtures. Firstly, the results show that a substitution of steel slag for basalt in pavements is a good alternative for efficient resource utilization. Secondly, when steel slag was used in place of basalt coarse aggregate, the water immersion Marshall residual stability improved by approximately 28.8% and the dynamic stability by approximately 15.8%; the friction values decayed at a significantly lower rate, and the MTD did not change significantly. Thirdly, in the early stages of pavement formation, Sp, Sv, Sz, Sq and Spc showed a good linear relationship with BPN values, and these texture parameters can be used as parameters to describe steel slag asphalt pavements. Finally, this study also found that the standard deviation of peak height was higher for steel slag–asphalt mixes than for basalt–asphalt mixes, with little difference in texture depth, while the former formed more peak tips than the latter.
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