Experimental investigation on mechanical behaviour, hydraulic conductivity, and microstructure of cement-stabilized soil mixed with biochar

To expand the application of biochar in civil engineering, this study investigates the feasibility of partially replacing cement with biochar in cement-stabilized soil. Unlike previous studies that primarily focus on mechanical performance, this work systematically examines the coupled behaviour of mechanical properties, hydraulic conductivity, and microstructure. The mechanical behaviour, hydraulic conductivity, and microstructure properties were examined through unconfined compressive strength (UCS), splitting tensile strength (STS), and scanning electron microscopy (SEM) tests. The macroscopic test results demonstrate that the performance of cement-stabilized soil, considering both its mechanical strength and hydraulic conductivity, first improves and then declines with increasing biochar content. In particular, a 1% biochar replacement of cement was found to be the optimal ratio, yielding the highest UCS and STS values alongside the lowest hydraulic conductivity. A predictive UCS model incorporating curing age and cement content was established, and an empirical correlation between UCS and hydraulic conductivity was also developed. SEM observations reveal that biochar promotes hydration, reduces pore size, and increases matrix compactness without chemically altering hydration products. These findings clarify the strengthening mechanism of biochar in cement-stabilized soil and provide a quantitative reference for engineering applications.