The possibility of using granulated blast-furnace slag (GBFS), furnace bottom ash (FBA), and their combination as fine aggregates in concrete was studied by performing experiments. These materials were used without applying any preprocesses such as sieving and grinding. The compressive, flexural, and split tensile strengths of concretes with natural sand replaced with GBFS, FBA, and GBFS plus FBA at 10, 20, 30, 40, and 50% were examined at a fixed water-cement ratio (w/c). The percentages represent the replacement percentage of fine aggregate by GBFS, FBA, or their combination and were evaluated depending on weight basis. Also, microstructure and water absorption capacity of concrete were researched. Test results showed that concrete strength decreases with increasing replacement ratio with respect to reference concrete. In addition, FBA decreases the strength of concrete more than GBFS. In particular the strength of concrete was detrimentally affected when the replacement ratio was beyond 40%. The microstructure studies showed that different pore structures were formed in the concrete depending on the replacement material, that is, GBFS or. FBA. It is concluded that the main reason for the strength reduction in new concrete is the formation of a porous concrete structure. Moreover an increase trend in water absorption capacity was observed for both replacement materials.