Strength enhancement of calcium carbide residue-stabilized clay with perlite powder and water treatment sludge

Clay soils, particularly kaolinite, often exhibit low strength and high deformation, necessitating stabilization for geotechnical applications. This study investigates the potential of calcium carbide residue (CCR) and its partial substitution with perlite powder (PP) and water treatment sludge (WTS) to enhance the strength and microstructure of kaolinite clay. To capture both mechanical response and structural evolution, unconfined compressive strength (UCS) and ultrasonic pulse velocity (UPV) tests were conducted at 7, 28, and 56 days of curing. The microstructural changes were also investigated using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) analyses to reveal cementitious gel formation. The findings show that the addition of 4% CCR increased UCS by about 10 times to 1395 kPa, while partial replacement with 25% PP and 10% WTS further enhanced UCS by nearly 14 times, reaching 1783 and 1744 kPa, respectively. Similarly, UPV increased from approximately 280 m/s in untreated clay to over 1500 m/s in stabilized blends, indicating a denser and more cohesive microstructure. The results of secant modulus (E 50 ) and energy absorption capacity (E u ) followed the same trend as UCS. Partial replacement of CCR with 25% PP led to the highest long-term improvements, producing E 50 of 1021 MPa and E u of 535 kJ/m 3 , while substitution with 10% WTS also yielded substantial gains, reaching an E 50 of 1011 MPa and an E u of 520 kJ/m 3 . These enhancements corresponded to increases of up to 18 times and 9 times over untreated soil, confirming greater stiffness but a shift toward more brittle behavior. Microstructural analysis revealed the formation of tightly bonded cementitious gels and superior particle packing, which directly account for the pronounced improvements in the strength and stiffness of the stabilized samples. Overall, the results demonstrate that CCR, particularly when partially substituted with PP or WTS, presents a strong potential as a sustainable alternative to conventional cement-based stabilizers, offering significant improvement in soil performance along with environmental benefits.