In this research, the effect of replacing cement with coal waste (at 0, 5, 10, 15, and 20% by weight of cement) on the mechanical and microstructural properties of sand-silt soil stabilized with 3% and 6% cement at ages of 14 and 90 days was investigated. To this end, standard proctor test, unconfined compressive strength, indirect tensile strength, ultrasonic pulse velocity, and scanning electron microscopy (SEM) tests were conducted. The results showed that for specimens stabilized with 3% cement, in all ages, adding coal waste caused a reduction in mechanical parameters. For specimens stabilized with 6% cement, substituting cement with 5% coal waste as the optimal amount led to a significant improvement in strength parameters and an increase in the wave transmission velocity of the sand-silt soil at 90 days of age. Analysis of SEM images confirmed that at this substitution percentage, the occurrence of active pozzolanic reactions and the consumption of calcium hydroxide led to the formation of secondary calcium silicate hydrate and calcium aluminosilicate hydrate gels, and by filling capillary pores, created a coherent matrix. Conversely, increasing the substitution amount to 20% due to the reduction of binding phases, led to an increase in microstructural porosity and a noticeable decrease in mechanical properties. The findings of this research show that the use of optimal amounts of coal waste, while enhancing soil stabilization quality, is an effective solution for reducing cement consumption and environmental hazards arising from mineral waste.