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The interaction of ions (calcium, alumina, silica, and sulfate) in the presence of water is very complex to indentify, and hence, to predict the behaviour of soil. An attempt has been made to understand the mineralogical and micro-structural changes due to soil-lime-gypsum reactions. Further, the associated mechanisms behind changes in the properties of lime treated gypseous soil with fly ash are presented. A series of experimental works are performed to find out the compaction characteristics, strength and swell behaviour of lime treated soil with gypsum and fly ash. The samples collected after completion of experimental works are used for micro-analysis (X-ray diffraction and scanning electron microscope). The results show that use of fly ash enhances the strength of lime treated gypseous soil by 1.92 folds. Also, the rapid swell, which is observed in gypseous soil with lime treatment, is reduced by 33% after using fly ash. The micro-analysis confirms that formations of cementitious compounds such as calcium silicate hydrate (CSH), calcium aluminum oxide hydrate (CAOH), calcium aluminum silicate hydroxide hydrate (CASHH) and ettringite needles lead to change in the geotechnical properties of soil. Further, it has been found that availability of water influences significantly on the formation of ettringite minerals and, thereby subsequent changes in properties of soil.