Microstructural and mechanical analysis of magnesium chloride stabilization in highly plastic swelling clayey soils

Abstract

In recent years, there has been an increasing interest in investigating the use of non-traditional additives for stabilizing problematic soils. As the demand for eco-friendly alternatives to cement rises, magnesium chloride, a widely used deicer and dust suppressor, has emerged as a potential choice. This study aims to provide a comprehensive understanding of the microstructural changes that occur and affect the macro behavior of treated bentonite (B) and yellow marl (YM). To achieve this, MgCl2 solution was added to the soils at 3, 6, 9, and 12 percent by dry weight of the soil, and samples were cured for 7, 14, and 28 days at 5°C, 25°C, and 35°C. The mechanical properties of the treated soils were then evaluated using the unconfined compression test, direct shear test, and pressure chamber test (SWCC), while microstructural analysis techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDAX), and Fourier transform infrared spectroscopy (FTIR) were employed to examine the mechanism of MgCl2 stabilization. The results indicate that adding MgCl2 and extending the curing period significantly increased both soils' unconfined compressive strength (UCS). However, the UCS value decreased for treated samples cured at temperatures higher than 25°C due to an incomplete cation exchange process and the reduction of apparent cohesion. A part of the gained strength from apparent cohesion and matric suction in the unsaturated samples was lost when the samples reached full saturation during the direct shear test. Changes in the particle size, pore size, and pore void distribution due to the MgCl2 stabilization affected the SWCCs of the treated soils. Microstructural analyses revealed the formation of magnesium hydration products, such as magnesium silicate hydrate (M-S-H) and magnesium aluminate hydrate (M-A-H), which contributed to the strength increase by increasing grain size, filling the pores, binding fine particles within coarse grains, and forming a flocculated structure through recrystallization of MgCl2 and the formation of cementitious gel. Additionally, for B, adding MgCl2 led to soil flocculation through ion exchange, while for YM, the same process occurred due to the greater surface tension of the saline solution encircling the particles.