Solidify/stabilise a heavy metal-contaminated soil using a novel steel slag-based binder

Basic oxygen furnace slag (BOFS) is an alternative to cement due to its similar mineral compositions. However, the low reaction activity of BOFS limits its application in the solidification/stabilisation of contaminated soils. This study presented a systematic investigation on the performance of monopotassium phosphate (MKP)-activated BOFS to solidify/stabilise a mixed lead (Pb)-, zinc (Zn)- and cadmium (Cd)-contaminated site soil. The activation mechanisms of BOFS and immobilisation mechanisms of heavy metals were explored by measurement of pH, specific surface area and X-ray diffraction analysis on binder pastes. Furthermore, the solidified/stabilised soils were subjected to a toxicity characteristic leaching procedure, unconfined compression tests, a sequential extraction procedure and acid neutralisation capacity tests. The results showed that the specific surface area of the MKP-activated BOFS was much higher than that of the un-activated BOFS. The formation of hydroxide precipitates, ion exchange of hydroxyapatite and absorptivity and encapsulation of calcium silicate hydrates were the primary immobilisation mechanisms of heavy metals. The addition of the MKP-activated BOFS significantly decreased leachability and improved soil strength, compared to that of the non-activated BOFS. With MKP activation, the BOFS could transform more amounts of heavy metals from soluble fraction to stable fraction and improve the acid neutralisation capacity of the solidified/stabilised soil.