The effect of a radiation-initiated polyester on the physicomechanical properties of a sand/clay-polyester composite has been investigated. The unsaturated polyester resins containing various styrene contents were used at different sand/clay composites from 10 to 70 wt % with respect to the weight of sand and clay together; polymerization was conducted using gamma-irradiation at 50 kGy. Also, different particle sizes of clay (namely 1.25-0.80, 0.80-0.50, and <0.50) were used. The compressive strength, apparent porosity, and water absorption-in addition to infrared spectroscopy of the composite samples-were studied. Results indicate that the compressive strength decreases with an increase in sand/clay, as well as the styrene content in the unsaturated polyester resin, whereas the apparent porosity and water absorption of the composite samples increase. Infrared spectra showed the appearance of new bands, which indicated a chemical reaction between polyester and clay constituents.