Progressive damage of solid-waste coal-based geopolymer mortar based on acoustic emission: New indicators for precursor characterization of instability

This study aims to investigate the damage mode and instability characteristics of coal-based solid-waste geopolymer (CSG) under loading and thoroughly understand the CSG's crack evolution rules and failure behaviors. The stress-time curve of CSG under loading and the AE's energy, amplitude, and frequency characteristics are analyzed through a uniaxial compression experiment and acoustic emission (AE) real-time monitoring. The crack types are classified based on RA (rise time/amplitude) and AF (average frequency), and the crack propagation at different stress stages is explored. The results showed that the CSG's strength decreases as the cement-to-sand ratio decreases, but the post-peak stress curve exhibits an apparent 'plateau' phenomenon. In addition, the first gathering zone of AE amplitude values moves in a low-stress direction, and the instability stage delays the damage and energy accumulation. The proportion of tensile cracks gradually increases with the load at different stress stages. The continuous damage of CSG with a high cement-to-sand ratio under post-peak stress is closely related to the increase in shear cracks. Based on the fractal dimension of RA calculated by the fractal theory, a new instability sign index r -value is proposed, and the proposed r -value is compared to the traditional b -value. It was found that the significant decline of the b -value usually occurs at the peak stress, whereas the sudden drop point of the r -value appears 20-45 s before the b -value. This instability sign index provides a more secure method for evaluating the instability failure of CSG. • Crack initiation and propagation in CSG are primarily tensile, with shear cracks secondary; the transition between crack types becomes more pronounced as stress increases. • A new index, the r -value, is proposed based on the fractal theory by calculating the fractal dimension of RA. • The sudden drop point of r-value appears before b-value, which can advance the damage precursor by 20-45 s.