Compressive strength is a crucial indicator for assessing the stability of the cemented tailings backfill (CTB) matrix. The stringent requirements of traditional non-destructive testing (NDT) methods (e.g., ultrasonic pulse velocity and electrical resistivity) regarding the geometry, section flatness, and integrity of the core sample severely limit their use in the in-situ measurement of CTB strength. Therefore, this study proposes a novel NDT technique for assessing the in-situ strength of CTB made from ground granulated blast furnace slag (slag) by means of image analysis. Strength tests and image analysis are carried out in 258 specimens maintained at different solid concentrations (71-79 %), binder contents (6-14 %), activator ratios (from 28:2-20:10) and curing temperatures (5- 40degree celsius). In addition, microscopic investigations are carried out to clarify the underlying mechanisms contributing to the macroscopic features. Increasing solid content, desulfurized gypsum to calcium carbide slag (DG/CS) ratio, and curing temperature results in increases in UCS of 87.9-199.9 %, 171.3-421.2 %, and 27.5-173.8 %, respectively. The strength-enhancing effect of increasing the curing temperature diminishes with time, from 173.8 % at 3 days to 27.5 % at 28 days. Strength initially rises and then abruptly decreases as binder content rises. The grayscale value generally has an opposite trend to strength, but the former is less sensitive to changes in influencing factors. An obvious exponential relationship is found between strength and grayscale for all specimens (R2 = 0.92). Compared with traditional NDT methods, the application of image analysis is not restricted to the shape, size, and integrity of CTB and therefore provides a faster, easier, and less costly tool for the quality evaluation of field CTB matrix.
