Laboratory Investigation on Physical-Mechanical Characteristics and Microstructure of a Clayey Gypsiferous Soil in the Presence of Chemical Accelerator

Gypsiferous soils are considered as problematic soils that cause damage to engineering infrastructures, particularly hydraulic structures. The exposition of gypsiferous soils to the steady flow leads to the dissolution of gypsum particles, significantly changing the soil engineering characteristics. In this paper, by employing physical, chemical, and mechanical experiments, the effect of raw gypsum with various percentages (0%, 3%, 6%, 9%, 12% and 15%) on the geotechnical parameters of a low plasticiy clay was investigated through leaching. Electrical conductivity test and ionic concentration analysis were used to assess the trend of gypsum dissolution. The results showed that leaching and increased gypsum content significantly degraded soil engineering characteristics, such as strength and compressibility. Acidic accelerator (diluted acetic acid) speeded up the chemical reaction rate and, at the same time, affected the geotechnical properties of the soil more than water. Mineralogical and microstructural studies (x-ray diffraction, scanning electron microscopy and energy-dispersive x-ray spectroscopy) were carried out to evaluate the interaction between gypsum and soil particles and verify laboratory test results. The x-ray diffraction patterns indicated the formation of new peaks such as calcium aluminate hydrated (CAH) and calcium silicate hydrated (CSH) and the altered structure of pure soil following the addition of raw gypsum. Scanning electron microscope images confirmed the presence of cementitious compounds and changes in the texture of gypsiferous soil in comparison with pure soil. Energy dispersive x-ray interpretation revealed changes in the chemical composition of soil blended with gypsum. The laboratory test results were corroborated by microstructural analyses.