Expansive soils cause serious problem in the civil engineering practice due to swell and shrinkage upon wetting and drying. Disposal of fly ash, which is an industrial waste in both cost-effective and environment-friendly way receives high attention in China. In this study, the potential use and the effectiveness of expansive soils stabilization using fly ash and fly ash-lime as admixtures are evaluated. The test results show that the plasticity index, activity, free swell, swell potential, swelling pressure, and axial shrinkage percent decreased with an increase in fly ash or fly ash-lime content. With the increase of the curing time for the treated soil, the swell potential and swelling pressure decreased. Soils immediately treated with fly ash show no significant change in the unconfined compressive strength. However, after 7 days curing of the fly ash treated soils, the unconfined compressive strength increased significantly. The relationship between the plasticity index and swell-shrinkage properties for pre-treated and post-treated soils is discussed.

Metakaolin, widely applied in concretes and cement-stabilized soft clays to improve their macro performance, was evaluated for use in compacted soils in highway and backfill engineering. The compactability and strength performance of cement-modified soils with metakaolin were evaluated in the terms of their applicability and effectiveness. The cement-based modified soil with metakaolin was more insensitive to water and more convenient in the field rolling compaction. The strengths, in addition to the unconfined compressive strength and splitting tension strength, were improved significantly. Up to a threshold ratio of metakaolin to cement ranging from 1/3 to 1/2 in this case, the strengths increased gradually, while they unexpectedly decreased thereafter. This phenomenon was different from the previously reported metakaolin applications in concretes and cement-stabilized soft clays, which was probably due to the water content and the ratio of metakaolin to hydrated calcium hydroxides of the cements. Microstructure analysis by X-ray diffraction, scanning electronic microscopy, thermo-gravimetric analysis and mercury intrusion porosimetry, demonstrated that the addition of metakaolin led to a higher quantity of hydration products and a denser micro-porosity distribution.

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