Preparation of blended geopolymer from red mud and coal gangue with mechanical co-grinding preactivation

In preparing geopolymer based on red mud (RM), preactivation processes that are generally energy-intensive are employed, including two indispensable steps, namely, calcination and fine grinding. A novel co-grinding preactivation method without calcination was proposed in this study. RM was pretreated by mix grinding with blended coal gangue (CG) (weight ratio of 8:2) for 20 min. The powdered mixture was characterized by alumina silicate dissolution efficiency test, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy. The corresponding geopolymer was also synthesized and characterized by XRD and scanning electron microscopy to assess the mechanical properties and polymerization reaction. Results showed that under co-grinding effect, the main alumina silicates of the RM/CG mixture were transformed into low-coordinated or low-polymerized phases. The phases were then dissolved in alkaline solution and repolymerized. Alkali activation with high compressive strengths completely formed dense amorphous geopolymer matrices. This work demonstrated that CG, as a milled conditioner, could efficiently expedite the preactivation of RM by grinding without calcinations. Moreover, an RM/CG-based geopolymer was successfully synthesized from the preactivated mixture.