Role of gypsum on microstructure and strength of soil

Mineralogical and microstructural changes in montmorillonitic soil blended with variable gypsum contents were carried out using X-ray diffraction analysis, scanning electron microscope and energy dispersive X-ray analysis to understand the alteration in strength behaviour. Fractured specimens after unconfined compressive strength testing, which were initially prepared by mixing different percentages of gypsum such as 1%, 2%, 4% and 6% after a curing period of 0, 7, 14 and 28 d were used for micro-level investigation. Alterations in pH value of gypsum–soil mixtures after different curing periods were monitored through pH tests. The X-ray diffraction analysis shows the formation of new peaks and breaking of parent mineral structure. Scanning electron microscope images also demonstrate the change in matrix and networking patterns of gypsum-blended soil in comparison with parent-soil matrix. Alteration in chemical composition of soil with gypsum is confirmed by energy dispersive X-ray analysis. It has been observed that the unconfined compressive strength of soil deteriorates on the addition of gypsum for both short-term and long-term periods. The variations in strength have been attributed to changes in mineralogical and microstructural changes with the formation of new minerals such as zeolite. Thus, the montmorillonitic soil cannot be stabilised with gypsum.