Fly ash influences on sulfate-heave in lime-stabilised soils

The paper describes a study carried out to investigate how fly ash influences sulfate-heave in lime-stabilised soils. Five different clays having a range of total potential sulfate contents were tested. Quicklime was added at 3·0, 4·5 and 6·0% (by mass of dry soil) and two fly ashes with different characteristics were used at levels of 6, 12, 18 and 24%. A ground granulated blastfurnace slag was also considered for comparative purposes. Swelling tests followed the BS EN 13286-49:2004 accelerated (40°C) volumetric method. Mellowing period, exposure temperature, mineralogical development, unconfined compressive strength and porosity of the test mixes were also considered in studying fly ash effects on the process. Swelling generally increased with overall sulfate content in the lime-stabilised mixes and reduced with increasing fly ash level and coarseness. The mechanisms involved appeared to depend on the characteristics of the fly ash used. In the case of coarse fly ash, this was attributed to a mainly physical phenomenon associated with increased pore space in the system to accommodate expansive sulfate products forming, which contrasted with enhanced fly ash reactivity and increased strength of the lime-stabilised structure for fine material.