Preparation of coal gangue based composite alkali‐activated cementitious materials and its application in solidification/stabilization of chromite ore processing residue

Abstract Chromite ore processing residue (COPR) generated during chromium salt production contains hexavalent chromium (Cr(VI)), posing risks to ecosystems and human health. Meanwhile, hundreds of millions of tons of coal gangue (CG) accumulate annually in China, causing significant ecological impact. Thus, CG and blast furnace slag (BFS) were selected to solidify/stabilize COPR in this study. CG and BFS were prepared into a binary system alkali‐activated composite cementitious material (AACM) by using a composite alkali activator. The single‐factor and orthogonal experimental results showed that the compressive strength of the AACM (28 days) can reach 46.7 MPa with the water glass modulus of 1.4, 10% alkali content, 0.32 solid–liquid ratio, and initial 24 h curing temperature of 50°C. The orthogonal experiment of three factors and three levels demonstrated that the importance ranking was as follows: curing temperature (C) > water glass modulus (A) > solid–liquid ratio (B). Furthermore, the optimal formulation of AACM enabled effective solidification/stabilization of COPR. The specimen with a 30% COPR content exhibited a compressive strength of 20.7 MPa, demonstrating that its mechanical properties remained favorable. Moreover, it was found that the leaching concentrations of Cr(VI) and total Cr in solidified bodies (≤40%COPR addition) were both below safe limits in two different kinds of leaching tests, that is, sulfuric acid and nitric acid method and toxicity characteristic leaching procedure, indicating the solidified bodies exhibited the great stability. X‐ray diffraction, Fourier Transform Infrared Spectroscopy, and scanning electron microscope analyses confirmed the ability of the CG‐based composite AACM to effectively immobilize COPR.