This research studied the use of alkaline activation of fly ash, as a silica and alumina amorphous source, to improve soft soils. A laboratory programme – including tests to study strength and deformability development, alternative curing methods, the effect of the different components of the grout, effect of raising initial temperature and comparison with a cement grout – was carried out. Laboratory tests revealed that the use of fly ash and alkaline activator resulted in a soil strength improvement up to 11·4, 16·7 and 43·4 MPa, at 28, 90 and 365 days curing, respectively. The most effective combinations obtained in the laboratory were chosen for the field application with jet grouting. The grout performed adequately to pass standard engineering specifications for soil mixing, achieving up to 26·4 MPa at 90 days curing. The main conclusion is the potential of alkaline activation for soil improvement, and therefore this research has created a basis for further studies.

Alkaline activation of fly ash (FA) was used to improve the mechanical performance of a silty sand, considering this new material as a replacement for soil-cement applications, namely, bases and subbases, for transportation infrastructures. For that purpose, specimens were molded from mixtures of soil, FA, and an alkaline activator made from sodium hydroxide and sodium silicate. Uniaxial compression tests showed that strength is highly increased by the addition of this new binder. The results described a high stiffness material, with an initial volume reduction followed by significant dilation. All specimens have clearly reached the respective yield surface during shearing, and peak-strength Mohr–Coulomb parameters were defined for each mixture. The evolution of the microstructure during curing, responsible for the mechanical behavior detected in the previous tests, was observed by scanning electron microscopy. These results were compared with soil-cement data obtained previously with the same soi...