The present study focused on fabricating cement-free mortar based on ground granulated blast furnace slag incorporating low-calcium fly ash as precursors in an alkaline solution of sodium hydroxide and sodium silicate. The main purposes of the study were not only to find the optimum mixture proportions to confirm the fly ash role in contributing to the longterm mechanical properties and durability but also to expand the use of fly ash in cement-free binders based on ground granulated blast furnace slag. The ground granulated blast furnace slag was replaced with fly ash in the cement-free mortar at various levels of 30, 40, 50, 60, and 70% by mass. The cement-based sample as a control sample was prepared for comparison purposes. Results showed that the cement-free mortar samples with fly ash had the more superior longterm mechanical properties (i.e. flexural and compressive strengths and volumetric compaction via ultrasonic pulse velocity test) and durability (i.e. total charge passed via rapid chloride penetration test and sulfate resistance via length change of samples exposed to sulfate solution), lower water absorption and porosity, and denser microstructure than the control sample up to 120 days. The 40% replacement of ground granulated blast furnace slag by fly ash as an auxiliary source of alumina and silica was the optimum ratio in enhancing the long-term mechanical properties and durability of the cement-free mortar samples, which is in agreement with reducing the porosity and water absorption. The results of the environmental analysis indicate that using cement-free mortar samples has much lower environmental impacts. The cement-free mortar sample using 60% ground granulated blast furnace slag and 40% fly ash is found as an optimal mixture in terms of technical and environmental features.