Solidification mechanism of heavy metal of red mud–based engineered cementitious composites

Red mud (RM) exhibits significant potential for the preparation of cementitious materials, but the presence of heavy metals in RM hinders its large-scale utilisation. This study systematically investigates the speciation and leaching behaviour of heavy metals in RM, while elucidating the immobilisation mechanisms mediated by hydration products and stabilising agents. Analytical results revealed the following descending order of heavy metal concentrations in RM: chromium (Cr, 754 mg/kg) > lead (Pb, 78.2 mg/kg) > nickel (Ni, 41.8 mg/kg) > copper (Cu, 33.2 mg/kg) > arsenic (As, 15.0 mg/kg) > cadmium (Cd, 0.77 mg/kg). Upon reaching 120 days of curing, stabilisation efficiencies plateaued at 99.91% for As, 97.62% for Cu, 97.02% for Ni, 96.83% for Cd, 95.91% for Cr, and 91.11% for Pb. Concurrent leaching concentrations were reduced to 0.001 mg/l (As), 0.059 mg/l (Cu), 0.093 mg/l (Ni), 0.0015 mg/l (Cd), 2.30 mg/l (Cr), and 0.51 mg/l (Pb). Comparative analysis of stabilisation agents demonstrated the following efficacy order: 5A zeolite < fly ash < attapulgite. This treatment enhanced immobilisation rates to 99.74% for Pb (8.86% increase) and 99.40% for Cr (4.28% increase). The findings of this study offer valuable insights into the sustainable and scalable application of RM.