This research investigated the unsaturated behavior of gypseous soils with two different gypsum contents. To reduce their collapse potential, these soils were treated with cement. The optimum cement content was determined through collapse potential tests on soil samples with varying cement contents. Saturated and unsaturated triaxial tests were conducted on both pure and cement-treated gypseous soils to evaluate and compare their shear strength behavior under varying suctions. As water retention capacity plays an important role in soil unsaturated behavior, the Soil Water Retention Curves were obtained for all specimens using a suction-control oedometer in both wetting and drying paths. Furthermore, to assess the effects of wetting and drying cycles, unsaturated triaxial tests after hydraulic hysteresis were performed. The results highlighted the critical role of matric suction in enhancing the shear strength and stiffness of both pure and cement-treated gypseous soils. However, wetting and drying during hydraulic hysteresis, weakened the gypseous soil matrix, reducing stiffness and strength—more severely in high-gypsum soil. Cement treatment effectively mitigated the adverse effects of gypsum dissolution and enhanced the mechanical behavior of gypseous soils under unsaturated conditions.

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