The combined use of electric arc furnace slag fines (EAFSF) and ladle furnace slag fines (LFSF) for soil stabilisation was not evaluated in previous works. Previous studies reported limited information on the microscale behaviour of steel slag materials used in soil stabilisation. This work evaluated synergistic effects provided by the combination of EAFSF and LFSF on the stabilisation of clayey and sandy soils. It provided a detailed microscale analysis of raw materials and slag-soil mixes with scanning electron microscopy, X-ray diffraction, X-ray fluorescence, Raman spectroscopy, thermogravimetric analysis, and derivative thermogravimetry. Their compaction behaviour, unconfined compressive strength (UCS), CBR index, swelling, and resilient modulus were also determined. Filler effects, cementitious, pozzolanic, and ion-exchange reactions were discussed. A positive synergistic behaviour was observed when combining 10% LFSF and 5% EAFSF (by soil weight), which provided a structural arrangement that provided increases in UCS and CBR up to 651% and 1500%, respectively
