Property Changes and Mechanisms in Lime-Stabilised Kaolinite in the Presence of Metal Sulphates

    Student thesis: Doctoral Thesis


    In recent times, environmental issues concerning mineral extraction and disposal of waste material have arisen world-wide. Thus the advantages of employing soil stabilisation as opposed to granular material in highway construction and foundations (i.e. cost effectiveness, material savings, and waste disposal management) have become more apparent. In soil lime-stabilisation, however, there has been increased concern over the enormous heaving that can occur when sulphate bearing soils are encountered and, to a lesser degree, the relatively slow strength development which is produced.

    Thus, the major objective of this work has been first to establish the fundamental material property changes which occur when clay soils are stabilised with lime in the presence of the most commonly encountered sulphates in the soil (i.e. sulphates of calcium (gypsum), magnesium, sodium and potassium), and then in addition, establish whether inclusion of ground granulated blastfurnace slag (GGBS), which is used with Portland Cement (PC) and PC-blends to combat sulphate attack in concrete, would impart similar sulphate resistance when included with lime in the stabilisation of clay soils.

    In order to achieve these objectives, carefully controlled laboratory investigations have been carried out on compacted cylinders of lime-stabilised industrial kaolinite of high purity (84% kaolinite) containing various levels of the four sulphates. The cylinders were all moist cured at 30┬░C and 100% relative humidity for periods of up to 20 weeks and their strength gain monitored. In addition, expansion of the cylinders has been monitored both during moist curing and during soaking. For one of the sulphates (gypsum), cylinders have been produced in which the lime was systematically replaced with GGBS and the effect of this replacement on both strength and swelling has been established. in order also to establish the effectiveness of the findings to real soils, a natural sulphate bearing clay soil, Kimmeridge Clay, was included in the investigations.

    The investigations have confirmed the detrimental swelling and beneficial strength effects of gypsum addition, and also established the adverse effects, in terms of strength development and swelling, of the presence of the sulphates of magnesium, sodium and potassium. More importantly, enhancement of strength development and dimensional stability has been achieved by incorporating GGBS in clay-lime mixes, especially in the presence of gypsum. For example, the 7-day unconfined compressive strength (UCS) ofkaolinite-6wt.% lime-gypsum mixes was increased by as much as 1.5-3 times (50-200% increase) depending on the gypsum content, by the substitution of approximately 80% of the lime with slag. Meanwhile, the linear expansion during soaking of the most expansive 7 day moist cured cylinders (27%) was reduced to less than 5% by a similar lime substitution. Similar, but slightly lower strength and swelling improvements were achieved with Kimmeridge Clay -6wt.% lime mixes (i.e. 1.5-2.5 times (50-100% increase┬╗ by the substitution of between 40% and 80% of the lime with slag, confirming the successful application of the findings to lime-stabilisation of natural sulphate bearing clay soils. In both cases, the improvement is even more significant at 28 days and at higher total stabilizer (lime + slag) contents. The findings have produced considerable industrial interest and there are on-going pilot trials currently taking place.

    Recommendations for further investigations include the establishment of long term trends, so as to eliminate any possibility of potential long-term adverse effects, particularly when the apparently beneficial GGBS is present.
    Date of AwardSept 1997
    Original languageEnglish

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