Role of fly ash in the mitigation of swelling in lime stabilised sulfate-bearing soils

Lime stabilisation is widely used to improve both plastic properties and strength of cohesive soils with high clay contents, most commonly in highway construction. The process principally involves the reaction of lime with clay alumino-silicate phases, producing cement-like products. It is a rapid reaction and improvements occur within a few days. The process is now highly mechanised and large areas can be stabilised quickly and at low economic and environmental cost. However, sulfate phases encountered in some clays can also react with lime resulting in the formation of ettringite. This is an expansive reaction and can produce volume stability problems due to swelling of the stabilised soil. These damaging effects often become visible after completion of the pavement and there are significant costs to rectify the problem. Fly ash has a well established history of use in suppressing sulfate attack of concrete and grout. Although a different material the chemistry involved in lime-induced ettringite formation in clay is broadly similar and there is growing interest in determining whether fly ash can play the same role in this application. An extensive study, reported here, was, therefore, initiated to examine the role of fly ash in the lime-stabilisation process and its influences on ettringite-induced swelling. Issues including optimum quantities and timing of intermixing fly ash with clay / lime, as well as the effect of their characteristics, were examined.