Identifying the role of fly ash properties for minimizing sulfate-heave in lime-stabilized soils

► Three clays of varying sulfate level and seven fly ashes covering a range of properties tested in lime-stabilized mixes. ► Coarser compacted soil structure obtained with higher fly ash levels and coarse, high loss-on-ignition (LOI), wet-stored material. ► Sulfate-induced heave (swelling) reduced with higher fly ash levels and coarse, high LoI, wet-stored material. ► High sulfate levels in fly ash reduce effectiveness of material in minimizing heave. ► Suggested methodology given for fly ash selection to minimize sulfate-induced heave. The paper describes a study carried out to examine the effects of fly ash in limiting damaging sulfate-heave (swelling) in lime-stabilized soils. This considered three clays with various sulfate levels; seven fly ashes covering a range of physical and chemical properties, and stored under different conditions (dry, stockpiled and ponded); and a typical quicklime. The initial part of the study examined the effects of fly ash (applied at levels of 6 to 24% by mass of dry soil) on compaction (in terms of density/moisture relationships) of the lime-stabilized soils (using 3.0% lime) to establish mix compositions and provide an insight to their resulting structure. This indicated that maximum dry density (MDD) and optimum moisture content (OMC) were influenced by the characteristics of fly ash (reducing and increasing respectively with increasing fly ash coarseness and loss-on-ignition (LoI; including that under wet storage)) and, depending on these, by the level of application. Tests for sulfate-heave were made following the BS EN 13286-49 volumetric swelling method. Fly ash gave systematic reductions in sulfate-heave with increasing application level up to 24% for the lime-stabilized soils tested. It was found that coarse, high LoI fly ashes, stored under wet conditions, were most effective in limiting the process, suggesting that the coarser structure obtained during compaction with these materials, i.e. lower MDD, is an important factor influencing this. The presence of high sulfate levels in fly ash reduced the effectiveness of the material in this role. The practical implications are considered and a nomogram relating fly ash fineness and application level, MDD and sulfate-swelling is developed, which demonstrates a possible methodology for material selection with regard to minimizing damage.