Mixtures of soil, fly ash and an alkaline solution made from sodium hydroxide and sodium silicate were artificially moulded using different sodium hydroxide concentrations and water contents. The chemical reactions involved in the alkaline activation of fly ash produce a geopolymeric gel that links the soil particles with cementitious bonds. Two different soils were tested, and the effect of the activator and water content on workability, stiffness and strength was carefully evaluated by means of unconfined compression strength tests, indirect tensile strength tests and seismic wave measurements. The results indicate that the addition of water has a negative effect on stiffness, in both soils, and in strength, for the coarse soil.