Immobilization of Cr(VI): Mechanism and performance of modified coal bottom slag in cement

To broaden the resource utilization of coal-fired furnace bottom slag (CBS) and provide a cost-effective solution for heavy metal pollution, this study modified CBS for enhanced application in heavy metal immobilization. The effects of four modifiers—calcium hydroxide (Ca(OH)₂), sodium lignosulfonate (SLS), citric acid (CA), and sodium silicate (SS)—on the immobilization of Cr(VI) in cement-based systems were systematically investigated via macro- and micro-scale analyses. The results demonstrated that while raw CBS and cement possess a certain inherent Cr(VI)immobilization capacity, all modified CBS groups exhibited significantly enhanced immobilization performance. Notably, the CA and SS-modified groups achieved the most substantial reduction in Cr(VI) leaching concentration, decreasing by 23.0% and 22.4%, respectively, at a higher Cr(VI) dosage of 0.2%, compared to the unmodified group. However, the addition of Cr(VI) impaired the mechanical strength, with reductions ranging from 9.2% to 29.1%. Microscopic analysis attributed this to the formation of larger flower-like and fibrous morphologies, which induced microcracking. Phase analysis indicated that Cr(VI) substitutes for Al³⁺ in ettringite (AFt) to form Bentorite (Ca₆Cr₂(SO₄)₃(OH)₁₂), contributing to the observed deterioration in 28-day mechanical performance. When the Cr(VI) content was lowered to the permissible limit of 0.015%, its adverse effect on 28-day compressive strength was significantly mitigated, with reductions confined to 3.95%–6.30%, while the immobilization rate still maintained an increase of 9.83%.