Effect of Alkali-Activated Fly Ash on Soft Soil Stabilization

Introduction: Soft soils often require stabilization before they can safely support infrastructure. Although cement is widely used for stabilization, its production is associated with high carbon emissions. This study explores the use of alkali-activated Class F fly ash as a sustainable alternative binder for improving the engineering performance of soft soil collected from Chattogram, Bangladesh.

Methods: Soft soil specimens were prepared by incorporating 10-25% of Class F fly ash binder with sodium-based alkaline activators of 20% to 80% of binder weight while maintaining a constant water-to-binder ratio of 0.45. The stabilized samples were cured for 7 days,  28 days and 56 days to examine the influence of fly ash content, activator dosage, and curing time. Unconfined Compressive Strength (UCS) tests were conducted, and the modulus of elasticity was obtained from the corresponding stress–strain curves to evaluate the stiffness of the treated soil.

Results: The experimental results showed a clear improvement in both strength and stiffness following geopolymer stabilization. Specimens containing higher fly ash contents generally developed greater UCS and modulus of elasticity, particularly after longer curing periods. Soil mix with highest fly ash content (i.e. 25% of soil weight) gained the maximum increase in UCS of 30 times than the raw soil. The alkaline activator dosage also influenced the performance of the mixtures, although the improvement was most pronounced at the optimum dose of activator of 50% of binder weight. The maximum increase in modulus of elasticity was also observed at the same composition and curing period. These observations indicate that the mechanical behavior of the stabilized soil depends on the combined effects of binder composition and curing conditions.

Conclusions: The findings demonstrate that alkali-activated Class F fly ash can effectively enhance the engineering properties of soft soil while reducing dependence on conventional cementitious binders. This environmentally sustainable stabilization approach has strong potential for deep mixing and other ground improvement applications in regions where soft soils are prevalent.