Performance of steel slag as sustainable aggregate in heavy-haul railways

In response to increasing environmental constraints, industrial by-products have been increasingly considered as alternative geomaterials to mitigate the extraction of virgin aggregates from new quarries. In this context, the behaviour of steel slag and limestone ballast was investigated, focusing on the interplay between stress state and particle breakage. A combined experimental and numerical approach was adopted, including cyclic triaxial tests performed under varying principal stress ratios to evaluate long-term permanent deformation, resilient behaviour, and particle breakage quantified by the ballast breakage index. The results show that both materials exhibit a transition from shakedown to plastic creep behaviour as the principal stress ratio increases, with a critical threshold identified at σ1/σ3 ≈ 7.0, beyond which particle breakage increases markedly. Steel slag consistently exhibited lower particle breakage than limestone under comparable stress conditions, despite showing similar resilient moduli. Finite-element simulations were conducted to assess track-scale behaviour under heavy-haul loading, confirming that both materials satisfy performance criteria for ballast layers. From a granular mechanics perspective, the findings highlight the strong coupling between stress state, particle breakage, and permanent deformation in ballast materials, while supporting the use of steel slag as a mechanically robust and environmentally sustainable alternative for railway ballast.