<p>This study explores the potential of secondary treated wastewater (STW) from three wastewater treatment plants as a viable and sustainable alternative to potable tap water in the production of concrete. In addition to utilizing STW, the concrete mixtures were enhanced with supplementary materials: 10% fly ash, a by-product of coal combustion, and varying dosages (1% to 3%) of sodium nitrite, known for its corrosion-inhibiting properties. The dual aim was to improve the environmental sustainability of concrete while maintaining or enhancing its structural integrity. To evaluate the impact of these modifications, the study conducted a series of standardized performance tests, including assessments of workability using the slump cone method, as well as mechanical property tests for compressive strength, split tensile strength, and flexural strength. The results indicated a 25% reduction in workability for concrete mixed with STW compared to traditional tap water, likely due to variations in the chemical composition of the wastewater. Despite this reduction, the decrease in mechanical strength was relatively minor—compressive strength dropped by only 2.91%, split tensile strength by 4.95%, and flexural strength by 1.75%. These decreases are primarily attributed to the inclusion of fly ash and sodium nitrite rather than the water source itself. To further analyze performance, machine learning models were applied to predict compressive strength. Among them, the Random Forest model demonstrated the highest accuracy, achieving an R² value of 0.87 and a mean squared error (MSE) of 0.86. The findings suggest that STW, in combination with fly ash and sodium nitrite, offers a promising alternative for sustainable concrete production without significantly compromising performance.</p>