Synopsis: The authors aim to investigate the effect of using colloidal nano-silica on the properties of concrete especially for the case of fly ash concrete. The study includes a laboratory study on six concrete mixtures in which three have 30% of the cement content replaced with fly ash, while the others were normal cement concrete mixtures. Two ratios of colloidal nano-silica were added to concrete with and without fly ash to examine its effect. Testing was conducted to assess the reactivity and the durability of the studied mixtures, including adiabatic temperature test, compressive strength test, splitting tensile strength test, and rapid chloride penetration test. Results show the addition of nano-silica can enhance the reactivity and early age strength of fly ash concrete mixtures to match normal concrete mixtures. Furthermore, the use of nano-silica improved the mechanical properties and reduced the permeability of concrete.