To investigate the effect of dimensional changes on the mechanical properties of cemented tailings backfill (CTB), uniaxial compression tests are performed on square CTBs of four different sizes. Combining digital image correlation (DIC) and acoustic emission (AE) methods, the fracture process and AE behavior characteristics of backfills with different sizes are analyzed. The results show that as the backfill size increases, its uniaxial compressive strength decreases, and its strength stabilizes gradually when it measures 100 mm. Under uniaxial compression loading, surface cracks on the smaller specimens evolve rapidly and aggressively, with no significant shedding area, whereas the larger specimens show plastic failure. The cracks expand and penetrate gradually, forming a large shedding area. As the specimen size increases, the backwards trend of the peak value of the ringing count relative to the peak value of the stress becomes increasingly evident. Combining the change law between the r value and the average frequency centroid, the sudden drop point of r value and the lowest value of average frequency centroid can be regarded as the precursor of macroscopic damage.

In order to study the influence of the cement–tailing ratio and curing age on the mechanical properties of cemented backfill with unclassified tailings and improve the accuracy of the backfill monitoring method, the mechanical characteristics of the failure process of cemented backfill with unclassified tailings at different curing ages were studied using the acoustic emission and infrasound method. The results show that the peak strength and modulus of elasticity of backfill are positively correlated with the curing age. During the failure process of cemented backfill with unclassified tailings, the acoustic emission ringing count shows a “stabilizing–rising–falling–rising” trend, and the infrasonic ringing count will have a “multiple peak” phenomenon. The ib value of the acoustic emission signal generally increases with the extension of the curing age. The ib value of the infrasonic signal generally has a negative correlation with the increase of the curing age. When the ib value of the acoustic emission and infrasonic wave all start to decline in a jumping manner, this indicates that the backfill is about to be damaged. The dominant frequency ratio of acoustic emission signals (F_AE) is distributed between 0–4 during the failure process, and the total number of F_AE shows the phenomenon of “first decreasing and then increasing” with the curing age. The dominant frequency ratio of infrasonic signals (F_s) is distributed between 0–6. During the failure process of cemented backfill with unclassified tailings, the quantity ratio of class A and B of acoustic emission signals shows a gradually increasing trend, and the quantity ratio of class A and B of infrasonic signals shows a “decreasing–increasing–decreasing” trend, and the quantity ratio of class A and B of acoustic signals shows a sharp increase at the stage of entering the crack’s unstable growth stage.