In high-plasticity clays, low unconfined compressive strength and high swelling pressure cause problems in engineering structures. In this study; pumice, marble dust, and lime were mixed with high-plasticity clay to investigate their effects on the engineering properties of clay. Atterberg’s limits, standard compaction, unconfined compression test, and swelling experiments were performed by adding pumice, marble dust, and lime to clay in pre-defined ratios. After determining the optimum dosages for each additive, samples prepared at maximum dry density and optimum water contents were cured for 7, 28, and 90 days. Unconfined compression tests, triaxial compression tests, and CBR tests were performed on those samples. SEM, X-ray, and chemical analysis were also performed. It was observed that the additives used improved the engineering properties of high-plasticity clay.