Comparative Analysis of the Use of Pasir Pantai Cermin and Deli River Sand as Fine Aggregates on the Compressive Strength of Normal Concrete

River sand remains the primary fine aggregate used in concrete, while beach sand—though abundant along the coast of North Sumatra—is rarely utilized due to concerns regarding salt content and impurities. This study compares the compressive strength of K-250 grade normal concrete (with a target $f'_c$ of 21.7 MPa) using either Pantai Cermin sand (washed three times) or Deli River sand as the sole fine aggregate (100% replacement). Mix proportions were determined in accordance with SNI 7394:2008, utilizing a water-cement ratio of 0.56, the same type of composite Portland cement, and a consistent slump of 12 cm for both variations. Three 15 × 30 cm cylinders for each variation were tested at 7 and 14 days, with results subsequently converted to 28-day equivalent values ​​using the age-conversion factors from PBI 1971. The average 28-day equivalent compressive strength was 18.23 MPa (84.0% of the target $f'_c$) for the river sand and 11.17 MPa (51.5%) for the beach sand, with a difference of 6.83 MPa at 7 days and 7.30 MPa at 14 days. Neither variation reached the target design strength. As the 28-day values ​​are estimates, it is recommended that direct 28-day testing, comprehensive sand characterization, and chloride content analysis be conducted before considering beach sand for use in structural concrete.