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.

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This paper studies the effectiveness of alkaline activation of low-calcium fly ash on the improvement of residual granitic soils to be used on rammed-earth construction. Different liquid:solid ratios, alkali concentrations and Na2O : ash ratios were tested. Effect of calcium hidroxide, sodium chloride and concrete superplasticiser is also reported. Compressive strength up to 7 days at 60ºC was determined. Results show that, in terms of mechanical strength, there is an optimum value for the activator:solids ratio and the alkali concentration, and that a decrease in the Na2O:ash ratio results in strength increase. No improvement was observed with the sodium chloride or the superplasticiser, while the calcium produced only a short term increase in strength. SEM/EDS analysis were used to analyse microstructural development, showing that strength is fairly related to the Si:Al and Na:Si ratios. Research highlights  Low calcium fly ash activated with a sodium-based solution.  No significant improvement with sodium chloride or superplasticiser.  There is an optimum liquid : solid ratio and alkali concentration for strength development.  An alkali : ash ratio reduction results in a significant strength increase.  Microstructure is significantly affected and can be related to strength variation.

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