Sustainable stabilization/solidification of electroplating sludge using a low-carbon ternary cementitious binder

Electroplating sludge (ES), due to its high dosages of heavy metals (HMs), is classified as hazardous waste, presenting risks to both health and environment. Stabilization/solidification (S/S) technology is extensively applied in ES treatment. This study designs a low-carbon ternary cementitious binder, limestone calcined clay cement (LC3), for S/S treatment of ES and investigates the immobilization mechanism of HMs in the crystalline products and calcium alumino-silicate hydrate (C-A-S-H) within LC3. Results show that CrO42- and Cd2+ are primarily stabilized in aluminate crystalline products by replacing SO4 2- and Ca2+ in the double-layered plate structure, respectively. The remaining CrO42- is captured by the (---SiOOH)2AlCa+ groups in C-A-S-H generated from substitution of aluminum tetrahedra to bridging silicon tetrahedra. For positively charged Cd2+, it is mainly stabilized through ion exchange with Ca2+ on the C-A-S-H chains. The pozzolanic reaction of calcined clay increases the Al/Ca and Si/Ca atomic ratios in C-A-S-H, enhancing the immobilization efficiency of C-A-S-H to CrO4 2- and Cd2+. The synergistic effect of calcined clay and limestone, along with the dissolution of aluminosilicates in calcined clay, changes the structure and composition of hydration products, facilitating the chemical binding of CrO42- and Cd2+ by aluminate products. Moreover, the extra hydration products generated by the pozzolanic reaction and the formation of numerous carboaluminate phases strengthen the physical encapsulation performance to HMs. Regarding actual S/S performance, multi-scale evaluation results show that the designed LC3 binder achieves sustainable S/S of ES with high strength, low carbon emission, low cost, and low energy consumption.