The Strength, Permeability, and Microstructure of Cement–Bentonite Cut-Off Walls Enhanced by Polypropylene Fiber

Cement–bentonite cut-off walls are widely used in geoenvironmental engineering such as landfill liners and contaminated site remediation, due to their low permeability and structural stability. However, excessive cement use reduces the swelling capacity of bentonite and increases environmental burdens. This study proposes incorporating polypropylene fibers (PPFs) into cement–bentonite cut-off walls to improve their performance under lower cement dosages. A total of 16 formulations were tested with different cement and fiber contents. Unconfined compressive strength (UCS), direct shear, and falling head permeability tests were conducted over 7, 14, and 28 days, respectively. Microstructural changes were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that compared to the conventional high-cement mixture without fibers, a formulation with moderate cement content and 2% PPF achieved higher compressive strength, comparable shear strength, and significantly lower permeability. Microstructural analysis confirmed that fiber addition enhanced cement hydration and preserved bentonite, forming a compact microstructure with reduced porosity. Furthermore, cost and carbon emission analyses revealed that the above optimized formulation reduced both material cost and embodied carbon by approximately 12.5% and 22.3%. These findings provide a sustainable and cost-effective approach to improve the mechanical and hydraulic performance of cement–bentonite cut-off walls.