Effect of Functional Group of the Inorganic Additives on Index and Microstructural Properties of Expansive Soil

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Expansive soils exhibit volume-change behaviour on exposure to moisture changes due to the presence of the montmorillonite mineral. The swell–shrink behaviour of these soils could lead to instability of the structures found on them, specifically the light-weight structures and pavements. Chemical stabilisation of expansive soils has always proven to be more successful in mitigation of swelling, compared to other methods of removal, replacement, and structural alterations. The present study focuses on the behaviour of a representative expansive soil treated with a conventional inorganic additive (lime) and less explored additives like calcium chloride and magnesium hydroxide, with a view to understand the effect of functional group of the inorganic additives in curtailing the swelling behaviour of the soil. The phenomenon of stabilisation is compared in terms of index and engineering properties based on the quantity of additives and curing period adopted. The macrostructural properties are evaluated through the changes in the plasticity characteristics, stress–strain behaviour, swell and permeability properties. The microstructural aspects of the treated soils are determined in terms of specific surface area, pore size measurements and minerology. Microlevel and macrostructural parameters are correlated to understand the soil-additive interaction and to quantify the extent of stabilisation and permanence of the additive induced changes. The lime and Mg(OH)2-treated samples effectively alleviate plasticity characteristics of the soil. However, strength of the lime-amended soil gets significantly improved, contributing to nearly 180% increase in strength compared to 100% and 45% increase for CaCl2- and Mg(OH)2-amended soils. Further, the additives also completely nullify the swelling phenomenon. The microstructure properties, SSA and pore size of the treated soils are concomitant with the macrolevel properties, such as permeability, swell and strength characteristics of treated soils, which further confirm the particle orientation of the soil on additive amendment. These analyses give a clear understanding of the orientation of the soil fabric, which dominates the overall behaviour of the soil-additive composites.