Collapsible soils such as loess, which are naturally unsaturated, are a type of problematic soil that has high resistance in dry conditions but when wet and saturated suddenly collapses, creating hazards for structures built on them. Therefore, the stabilization of such soils with cement can be effective. Because cement production is costly and one of the most important causes of air pollution in the world, replacing a portion of cement with an environmentally friendly additive such as natural zeolite (NZ) is of great importance. The present study aimed to quantify the influence of cement and zeolite as well as recycled polyester fiber (PET) on the unconfined compressive strength (UCS) of loess soil using a series of unconfined compression tests. The results indicated that the UCS of samples stabilized with 4% and 8% cement were substantially enhanced by increasing zeolite replacement to an optimum value of 10% and 30%, respectively. The highest resistance was achieved in mixtures with a water content (omega) equal to 1.2 times optimum water content (omega opt). Additionally, soil stabilization with a combination of cement, zeolite, and recycled polyester fiber significantly increased UCS. The addition of PET to a zeolite-cement-loess mixture caused an increase in failure strain. To overcome the brittle behavior of cemented loess, the combination of PET and zeolite in a cement-loess mixture is very effective.
