Evaluation of Mechanical Properties and Micro-Mechanism of Alkali-Activated Material with CG Under Ultrasonic Treatment from a Fractal Perspective

To mitigate the adverse effects of particle agglomeration in alkali-activated coal gangue-based cementitious (AAM–CG) materials, ultrasonic treatment and fractal theory, combined with microscopic analysis techniques were employed to investigate the physical activity of coal gangue (CG) and the microscopic mechanisms of AAM–CG materials. The results indicate that ultrasonic treatment effectively enhances the mechanical properties of AAM–CG materials. With increasing ultrasonic duration, the compressive strength initially rises and then declines, whereas it shows a continuous upward trend with increasing ultrasonic power. The optimal dispersion of CG particles in AAM–CG materials was achieved under ultrasonic treatment at 840 W for 4 min, resulting in a peak compressive strength of 106 MPa. This represents a 28.8% enhancement compared to non-sonicated controls. Ultrasonic treatment effectively disperses agglomerated particles, fully activates CG reactivity, promotes the formation of cementitious phases, improves pore-filling effects, and optimizes the internal pore structure of the material. Compared to untreated samples, the fractal dimension of the pore structure increased after ultrasonic treatment, harmful pores decreased, and porosity was reduced by 32%. This study expands the application of ultrasonic technology in the preparation of alkali-activated geopolymers and provides an efficient activation method for the resource utilization of CG.