The natural weathering of EAF (Electric Arc Furnace) and LF (Ladle Furnace) steel slag was evaluated through changes in the mineralogical and elemental composition. For that purpose, black steel slag and mixture of black and white steel slag were collected from two forest tracks, where they had been used as filler 19 and 35years ago respectively in a protected mountain area. Primary/original and secondary/degradation compounds were identified by spectroscopic techniques (Raman Spectroscopy, X-Ray Diffraction (XRD) and Scanning Electron Microscopy/Energy Dispersive X-Ray Spectroscopy (SEM/EDS)). Among secondary compounds, brucite (Mg(OH)2), portlandite (Ca(OH)2), thaumasite (Ca3Si(CO3)(SO4)(OH)6.12H2O) were identified. Secondary compounds indicated the reactivity of the slag with the surrounding environment (underground waters, atmosphere, lands and organisms), and volume change of the material. This effect could promote fractures in the road and thus, increase the possibility of leaching of hazardous elements (HE), present in the slags, to lands, rivers, etc. Besides, potentially toxic compounds such as hashemite (BaCrO4) and crocoite (PbCrO4) were identified as Cr(VI), which means a potential hazard to the surrounding environment and human life, since the sampling location is a mountain area with recreational activities. Cr(VI) can affect to the growth and development of plants, soil microbial communities, animals and cause allergy, asthma and respiratory tract cancer in humans. Moreover, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) allowed us to observe similar ranges of elemental concentrations in slag samples of the two forest tracks, with the exception of Ca, Mg, Sr, Pb, Ni and As concentration values. They were higher in the forest track with mixture of black and white slag than in the track with only black slag, and therefore, more likely to be leached and to be an environmental risk over time. By contrast, Na, V, Cr and W values were higher in the track with only black steel slag.