This study investigates the enhancement of fly ash-cement mortar performance through tetraethyl orthosilicate (TEOS) modification, focusing on both strength development and durability parameters. A comprehensive experimental program, including compressive strength tests, water absorption measurements, sorptivity assessments, and ultrasonic pulse velocity (UPV) evaluations, was conducted. The results demonstrated that partial replacement of cement with 30% fly ash not only improved 28-day compressive strength to 46.81 MPa but also significantly refined the microstructure, as evidenced by reduced water absorption and higher UPV values. The incorporation of TEOS at 0.5%, 1%, and 1.5% further enhanced permeability without compromising strength, with the 1% TEOS mixture (S10) achieving optimal performance. This mixture exhibited the lowest water absorption (2.77%) and reduced sorptivity, attributed to the sol-gel formation of silica networks that densify the pore structure and impede water ingress. UPV results confirmed the structural integrity of TEOS-modified mortars, with velocities comparable to the highest-performing fly ash blend. FESEM analysis shows that incorporating fly ash and TEOS refines the cement mortar microstructure. This results in enhanced matrix densification and improved morphology. These findings underscore the efficacy of TEOS as a chemical modifier for producing durable, high-performance fly ash-cement composites, suitable for sustainable construction applications.