PhD Dissertation Oral Defence, Nurmunira Muhammad: Development and investigation on strength and resilient modulus of a new bentonite-magnesium-alkalinization additive for road subgrade stabilization

Nurmunira Muhammad will defend the dissertation titled:

Development and investigation on strength and resilient modulus of a new bentonite-magnesium-alkalinization additive for road subgrade stabilization

The stabilization of problematic soils with chemical additives has become a popular practice globally. However, the mechanical and microstructural characterization of subgrade materials stabilized by alkalinization of raw silty sand, a typical soil in British Columbia (BC), Canada, has not yet been studied.

This study introduces the novel concept of using an alkaline activator, (a mixture of sodium hydroxide, SH (NaOH) and sodium silicate, SS (Na2SiO3)), along with magnesium chloride, L (MgCl2), to activate the silica and alumina components of silty sand. This chemical mixture named as a magnesium-alkalinization (MA) additive. Compaction and unconfined compressive strength (UCS) tests were used to assess the mechanical properties of the stabilized soil.

The UCS results revealed that a sample mixture containing an alkaline activator, S (SS/SH) ratio of 0.5, an alkaline activator-to-MgCl2 (L/S) ratio of 0.7, and 3% MgCl2 by dry weight of the soil was the optimum mix to improve the strength of the silty sand when cured for 28 days. The development of the MA additive was then used as a substitute component for developing the new chemical additive, which utilized a local bentonite product. The additive developed from the mixture of bentonite, MgCl2, and alkaline solution was introduced for stabilization of soil.

The mechanical and physicochemical results revealed that the addition of 40% bentonite was the optimum content for improving the strength of silty sand, which was then named as the bentonite-magnesium-alkalinization additive (BMA). The investigations on the mechanism of strength improvement were carried out on the optimum designed sample, based on microstructural analysis using X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), Energy Dispersive Spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR) The microstructure analysis confirmed the formation of the cementitious products.