The compressive strength is the primary parameter used for the design, control, and performance assessment of cementitious materials. However, this value is strongly influenced by specimen geometry, which has led to the introduction of shape coefficients to convert compressive strength results between different specimen types, particularly between cubes and cylinders. While this topic has been extensively investigated in concrete, very limited research has addressed the shape coefficient in soil-cement or cement-treated base materials, despite their widespread use in pavement construction. Aiming to bridge this gap, this study systematically analyzes the unconfined compressive strength (UCS) of soil-cement specimens with different geometries. Two soil-cement mixtures with distinct physical and chemical characteristics were tested at various curing ages (7, 28, and 90 days) using cylindrical specimens (150 mm diameter × 180 mm height) and cubic specimens (150 mm edge). The results show that the UCS in cylindrical specimens (UCScyl) was consistently higher than that of cubic specimens (UCScub), although the difference decreased with increasing compressive strength. By combining all datasets, a single conversion factor of 1.04 was derived, resulting in an equation, UCScyl = 1.04·UCScub, with an excellent determination coefficient (R2 = 0.99), enabling reliable conversion between cubic and cylindrical UCS results for soil-cement.
