Leaching of heavy metals from solidified waste using Portland cement and zeolite as a binder

This study investigated the properties of solidified waste using ordinary Portland cement (OPC) containing synthesized zeolite (SZ) and natural zeolite (NZ) as a binder. Natural and synthesized zeolites were used to partially replace the OPC at rates of 0%, 20%, and 40% by weight of the binder. Plating sludge was used as contaminated waste to replace the binder at rates of 40%, 50% and 60% by weight. A water to binder (w/b) ratio of 0.40 was used for all of the mixtures. The setting time and compressive strength of the solidified waste were investigated, while the leachability of the heavy metals was determined by TCLP. Additionally, XRD, XRF, and SEM were performed to investigate the fracture surface, while the pore size distribution was analyzed with MIP.The results indicated that the setting time of the binders marginally increased as the amount of SZ and NZ increased in the mix. The compressive strengths of the pastes containing 20 and 40 wt.% of NZ were higher than those containing SZ. The compressive strengths at 28 days of the SZ solidified waste mixes were 1.2–31.1 MPa and those of NZ solidified waste mixes were 26.0–62.4 MPa as compared to 72.9 MPa of the control mix at the same age. The quality of the solidified waste containing zeolites was better than that with OPC alone in terms of the effectiveness in reducing the leachability. The concentrations of heavy metals in the leachates were within the limits specified by the US EPA. SEM and MIP revealed that the replacement of Portland cement by zeolites increased the total porosity but decreased the average pore size and resulted in the better containment of heavy ions from the solidified waste.

► Effectiveness in reducing the leachability of solidified waste using zeolite as a binder was higher than 80.0%.
► The concentration of Cr in the leachate was lower than the limit specified by the standards of the US EPA.
► Using zeolite as a binder exhibited higher total porosity and capillary porosity than containing OPC alone.
► Using NZ as a binder effectively reduced leaching more than using SZ.