Abstract:To reveal damage evolution and brittle-ductile synergistic behavior of the gypsum-slag-cement (GSC) ternary cementitious system under uniaxial compression, this study conducted uniaxial compression tests at different curing ages by adjusting the ratios of gypsum, cement, and slag. Combined with digital image correlation (DIC) and scanning electron microscopy (SEM) techniques, a systematic analysis was performed from three perspectives: macroscopic mechanical response, crack propagation characteristics, and microstructural evolution. Results indicated that with extension of curing age, compressive strength, elastic modulus, as well as crack initiation and damage stresses of GSC system significantly increased, while post-peak failure gradually transitioned from typical brittleness to quasi-ductility. Increasing the proportion of cement content helped enhance the material''s strength, stiffness, and overall bonding capacity, but led to increased brittleness. Gypsum content exhibited a significant nonlinear influence on material properties. A proportion of 10% effectively promoted internal stress redistribution and synergistic multi-crack propagation, thereby improving toughness and cracking resistance, whereas excessive gypsum tended to cause a decline in structural continuity and deterioration of mechanical performance. Comprehensive comparison revealed that the formula G10S70C20 demonstrated the highest compressive strength and superior deformation coordination ability at curing age of 28 d, with cracks exhibiting a typical symmetrical "X" pattern, indicating optimal comprehensive performance. The findings suggested that synergistic regulation of gypsum and cement was the key to achieving both strength enhancement and an optimized brittle-ductile balance in the GSC system.