The effect of construction moisture content on the mechanical, shear and environmental characteristics of clay stabilized with cement and CS: A micro and macro study

High-plasticity (CH) clayey soils are usually unsuitable for civil engineering applications due to their low density, high porosity, and poor shear behavior. This study investigates the effectiveness of partially replacing cement with copper sludge (CS), an industrial by-product, to improve the engineering performance of CH soils. The parameters evaluated in this study include the CS content (0–30% of cement weight), curing time (7, 28, and 90 days), moisture content (OMC±2%) and confining pressure (400–800 kPa). For this purpose, unconfined compressive strength (UCS), indirect tensile strength (ITS), unconsolidated-undrained triaxial (UU), ultrasonic pulse velocity (UPV), California bearing ratio (CBR), scanning electron microscope (SEM), and toxicity characteristic leaching procedure (TCLP) tests were performed. The results showed that replacing 15% of cement with CS was the optimum content. At this replacement level, the UCS, ITS, and UU strength (at 600 kPa confining pressure) increased significantly from 310 kPa, 42 kPa, and 450 kPa in untreated soil to 7157 kPa, 795 kPa, and 7190 kPa, respectively. These enhancements correspond to increases of 8%, 10%, and nearly 16 times in UCS, ITS, and UU strength. Also, the specimens prepared on the OMC+2% performed better than those prepared at or below OMC. Statistical analysis showed a strong exponential relationship between UPV and the mechanical parameters with coefficients of determination (R²) > 0.80. In addition, SEM images confirmed the denser structure and reduced porosity in CS-treated samples. TCLP test results also showed that the heavy metal leaching rate was within the permissible environmental limits and that CS utilization was environmentally safe. This study comprehensively investigated for the first time the simultaneous effects of moisture content, CS replacement ratio, and curing time on the engineering, microstructural, and environmental performance of stabilized soil. The findings demonstrated the potential of using UPV as a rapid and non-destructive tool in predicting the performance of stabilized soil.