The growth in the building construction activity sector is increasing leaps and bounds with boast to infrastructure. The growing requirement for River Sand (RS) is having an adverse impact on the environment, necessitating the urgent need for an alternate construction resource. Finding a different substitute or replacement for natural river sand is therefore urgently needed. When silica and alumina-rich materials react with alkaline liquid, a geopolymeric material is created. In this study, fly ash geopolymer sand (FAGS ), was manufactured using fly ash and an alkaline activator solution as an alternative to river sand, and was used in M 25 grade concrete. FAGS was prepared a combination of sodium hydroxide and sodium silicate and was cured at 100 ͦ C for one hour. In comparison to RS, FAGS has a similar specific gravity (2.53), pH (11.48), water absorption (3.5%), and bulk density (1.762 kg/m3). By substituting FAGS for RS in concrete, compressive strength for 7 days, 14 days and 28 days was evaluated for varying percentages of FAGS . The replacement was done at 25%, 50%, 75% and 100% of natural sand with FAGS . The compressive strength at 7 days and 14 days with 25% and 50% replacement was 16.6 MPa and 22.6 MPa for both replacements respectively, and the 28-day strength with both these partial replacements was found to be 28.7 MPa. When a higher percentage of replacement was done beyond 50%, there was a huge reduction in compressive strength. It was found that the replacement with 75% and 100% use of FAGS, compressive strength was 7.3 MPa and 6.4 MPa at 7 days, 13.5 MPa and 12.8 MPa at 14 days and 14. MPa at 28 days respectively. The mechanical strength of the FAGS was found to be virtually identical to that of the concrete made with the river sand, indicating that FAGS might be utilized as a partial alternative material in place of river sand in concrete. In order to take the role of RS as a permanent material in building technology, more research work has to be carried out on the durability features of the concrete containing FAGS.