This study aims to determine the effect of variations in composition and size of red sand grains on the quality of concrete. Concrete is made in the form of cubes 15 x 15 x 15 cm with the standard SNI K-175 with a composition of 1: 2: 3 cement mixture with FAS 0.5. In this study, variations in the composition of red sand (fine aggregate) were made by 0%, 5%, 10%, 15%, 20% of the weight of the fine aggregate used and the variation of red sand grain size of 80 mesh, 100 mesh, 120 mesh. After the

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... oncrete is 24 hours old, the mold is opened and given the code number as desired and treated in a water bath. After going through the immersion period 28 days later the pressure strength was tested and the XRD test. From the results of the study obtained the mechanical properties of the maximum concrete pressure strength test at a composition of 5% along with a decrease in the size of the red sand grain size (80 mesh, 100 mesh, 120 mesh) with pressure forces of 31.9 MPa, 35.4 MPa, and 35.5 MPa, respectively. This goes beyond the force of pressure set by the Indonesian National Standard Agency. XRD testing on concrete after smoothing, from the results of the XRD test obtained elements such as SiO2, Ca (OH) 2, CaO and Ta2O5 show that the graph shows a high silicon intensity value. Addition of red sand fine aggregate has an impact on improving the quality of concrete (the strength of pressure from SNI k-175 becomes K-400).