Effect of fly ash on the unconfined compressive strength of black cotton soil

The strength of clayey soil can be altered by the addition of fly ash. Many factors of fly ash, such as reactivity, free lime content and silty nature, influence the strength of soil. This study examines the effect of varying fly ash content (at a constant percentage content of lime) and the effect of varying lime content (at a constant percentage content of fly ash) on the strength of black cotton soil. The study, conducted at constant percentage content of lime on the unconfined compressive strength of black cotton soil, with varying percentages of Neyveli fly ash and Vijayawada fly ash with and without curing, has shown that the effect of fly ash is mainly through pozzolanic reactivity and the silty character of fly ash. For Neyveli fly ash, having good reactivity, the pozzolanic reaction effect overrides the effect arising from its silty nature. For Vijayawada fly ash, having poor reactivity, the effect is similar to that of adding silt. It is noted that only reactive fly ash can increase the unconfined compressive strength of black cotton soil. From this study conducted with varying percentages of lime content it has been shown that at very low lime contents, sufficient strength can be achieved for a soil–reactive fly ash mixture; this cannot be attained for soils without fly ash.