Mechanisms of sulfate heave prevention in lime stabilized clays through pozzolanic additions

Stabilizing sulfate-bearing clay soils with lime can lead to heave problems through the formation of expansive minerals such as ettringite, and cause damage to supported structures. Adding ground granulated blastfurnace slag (ggbs) before compaction has been noted to contribute to strength development and to reduce swelling of the stabilized soil. Limited availability of ggbs in certain geographical areas means that alternative materials may be needed for this role. A potential candidate for this is fly ash, which has long-term reactivity and a track record in mitigating sulfate swelling in other types of construction, e.g. concrete and grout. A research project was therefore initiated to examine this. A series of clay soils with potential for sulfate heave were investigated using 3% lime and various percentages of fly ash and ggbs. Volumetric swelling tests were carried out on various mixes and it was found that, with increasing levels of fly ash, the swelling of soils gradually reduced. To explore the underlying mechanisms, porosity and mineralogical phase development testing were carried out. Mercury Intrusion Porosimetry results indicate that there is a relationship between the extra porosity introduced by fly ash or ggbs and the observed reduction in swelling, while levels of ettringite formation were generally similar for the range of combinations tested. This supports the hypothesis that provision of space for the formation of expansive products is a principal mechanism by which these materials minimise heave.