Mixtures of silty sand, fly ash and an alkaline solution were tested in unconfined compression at 90 days of curing for evaluation of strength and, stiffness using local strain instrumentation. Measurements of seismic wave velocities were performed during the curing period, in order to evaluate the material geomechanical behavior evolution with time. These results were also compared with a specimen of soil and fly ash without alkali activator to analyze the effect of the activator. A significant increase in strength and stiffness due to the activator was observed which highlights the potential of this technique as a cement replacement bonding agent. The use of waste products like fly ash instead of cement makes this technique especially respectable for the environment, prospectively competitive economically and friendly for the sustainability of our resources.