Shear Behavior and Microstructure of Controlled Low-Strength Materials Prepared from Yellow River Alluvial Soils

To comparatively evaluate the shear behavior of controlled low-strength materials (CLSM) prepared from different local soil sources, three representative soils from the Yellow River alluvial plain, namely, silt, silty clay, and sand, were used to prepare CLSM with a cement–slag–fly ash–gypsum blended cementitious binder. Triaxial shear tests and scanning electron microscopy (SEM) observations were conducted to compare the failure modes, stress–strain responses, strength characteristics, and hardened microstructures of the three CLSM types under different binder contents and confining pressures. The specimens generally exhibited inclined shear planes, conjugate shear planes, vertical cracks, and plastic bulging. Their stress–strain responses could generally be divided into four stages: linear elastic deformation, plastic yielding, strain softening, and residual stabilization. Within the tested binder-content ranges, the peak strength generally followed the order of sand-based CLSM > silt-based CLSM > silty clay-based CLSM. On average, the residual strength retained approximately 75% of the peak strength. The failure stress states of the tested CLSM could be reasonably represented by the Mohr–Coulomb criterion within the investigated confining-pressure range, and preliminary empirical relationships were established within the tested ranges to estimate peak strength, residual strength, and shear strength parameters. SEM observations suggested that C–S–H-like gel and needle-like products appeared to fill pores and form cemented connections between soil particles, providing a possible qualitative interpretation of the macroscopic strength differences among the three CLSM types. These findings provide a basis for shear strength evaluation and the mix design of CLSM prepared from Yellow River alluvial soils.