Upcycling waste cement paste powder into a composite cement-free heavy metal stabilization binder

Lacking economic and practical solutions for the increasing disposal of toxic heavy metals (HM) became a globally concerning issue. In this study, recycled cement paste powder (RCP) derived from industrial waste was utilized to develop a composite cement-free binder (CFB) consisting of RCP, carbonated RCP (CRCP) and thermal activated RCP (TRCP) for Zn²⁺ and Pb²⁺ stabilization. A series of experiments and analysis were conducted by using testing apparatus including X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). The compressive strength and toxicity characteristic leaching procedure (TCLP) of the hardened specimens were also tested. The results showed that i) The stabilization capacity of RCP, CRCP, TRCP and CFB-50% (CRCP: TRCP in 1: 1) significant improved comparing with previous data and achieved a maximum value of 1661.48 mg/g and 2840.10 mg/g for Zn²⁺ and Pb²⁺ respectively; ii) Both carbonation and thermal activation enhanced the Zn²⁺ and Pb²⁺ stabilization; iii) the stabilization mechanism of Zn²⁺ and Pb²⁺ mainly included precipitation, ion-exchange and surface encapsulation; iv) the composite binder can be used as long-term engineering backfill with required strength after rehydration (>2.1 MPa), excellent CO₂ sequestrating ability and significant HM-stabilization capacity with leaching stability.