In the present study, in order to make geopolymer concrete for use in concrete dams, 6 mixing designs including 288 concrete samples of control concrete containing Portland cement and slag geopolymer concrete containing 0 to 8% nanosilica and 1 to 2% polyolefin fibers were prepared. 7, 28 and 90 days processing were subjected to mechanical and durability tests. In order to study the microstructure in this study, XRF and SEM tests were used. The results show that the compressive strength, tensile strength, water impermeability, capillary water absorption for geopolymer concrete containing 8% nanosilica and 2% polyolefin fibers (as an optimal design) compared to control concrete, by 3%, respectively, 9 %, 56% and 65 times at 90 days of processing. In this regard, the maximum values ​​of compressive strength and tensile strength obtained were 82.96 and 5.51 MPa, respectively, and the minimum amount of water penetration in concrete was 13 mm and the minimum rate amount of water capillary absorption was 0.049 Cm/s^1/2 for geopolymer concrete. SEM images showed that the geopolymerization process was enhanced by producing more hydrated gels and improving the microstructure of geopolymer concrete by increasing the presence of silica nanoparticles.