Black Cotton Soils exhibit high swelling and shrinking when exposed to changes inrnmoisture content and are most troublesome soils leading to failure of civil engineeringrnstructures constructed on such soils. The purpose of this study is to investigate the effectrnof pozzolanic sand on improving the engineering properties of expansive soil collectedrnfrom Addis Ababa around Koyefeche condominium site.rnIn this research, pozzolanic sand found around Meki Town was used to treat problematicrnexpansive soil. The pozzolanic sand used for treating the expansive soil was pulverized tornpass certain sieve sizes. In this particular research, to consider the size effect of pozzolanicrnsand on treating the expansive soil, pozzolanic sand passing 150Î¼m and 75Î¼m sieve sizernwere used to treat the expansive soil. The pozzolanic sand crushed to a size less thanrn150Î¼m was blended to the soil up to 25% at 5% percentage increments whereas thernpozzolanic sand crushed to a size less than 75Î¼m was blended with soil to a maximum ofrn15% at 2.5% percentage increments by dry weight of the soil.rnSoil treated with the pozzolanic sand showed improvements to plasticity, strength andrnswelling. As soil is treated with pozzolanic sand the plasticity of the soil decreased, therndry density of the soil increased, the optimum moisture content decreased and the swellingrnpressure also decreased. Besides, the compressive strength of the soil increased as thernpercentage of pozzolanic sand is increasing. For the sizes of pozzolanic sand consideredrnthe pozzolanic sand crushed to a size less than 75Î¼m improved the engineering propertiesrnof expansive soil than the pozzolanic sand crushed to a size less than 150Î¼m when blendedrnwith the expansive soil.