ABSTRACT: The engineering properties of a clay soil and its mixtures (5, 10 and 15% by weight) with a plaster mortar additive (PMA) were studied through a series of soil mechanical tests to investigate possibilities to improve its undesired/problematic plasticity, strength, compaction and consolidation characteristics. The tests included the Atterberg limits, shear box, compaction, consolidation, and unconfined compressive strength tests. The results demonstrated that using PMA can significantly enhance the soil properties and serve as a soil stabilizer. Adding PMA led to a decrease in the plasticity values of the soils. Moreover, it was observed that the highest maximum dry unit weight and the lowest optimum moisture content were achieved when 15% PMA was added. The soil strength properties reached maximum values when the mixtures contained 15% PMA. Additionally, an optimal coefficient of volume compressibility (mv) was obtained when the PMA ratio in the mixtures was 10%. It was concluded that the plaster mortar additive used in this study could significantly improve the geotechnical parameters of the soil.