Effect of Waterproofed Nano-silica on Some Engineering Properties of Gypseous Soil

Gypsum soils are prevalent worldwide, and their primary engineering challenge lies in the loss of mechanical properties upon exposure to water due to the dissolution of the gypsum bond. Many previous studies have explored various additives to improve this soil type; among these, nanomaterials have emerged as promising candidates for enhancing the geotechnical properties of soil. This study compares the effects of adding different proportions of regular (hydrophilic) nano-silica and waterproof (hydrophobic) nano-silica to gypseous soil with varying gypsum contents (13%, 30%, and 45%). Laboratory tests were conducted on both semi-dry and submerged disturbed samples, including direct shear, collapsibility, and water absorption tests. For the semi-dry case, and in comparison to untreated samples, the cohesion of hydrophilic and hydrophobic nano-silica-treated soil showed improvement, reaching up to 112% and 30%, respectively. The angle of internal friction has lower sensitivity to the above additives with increases of 11% and 10%, respectively. However, for the soaked state, the soil cohesion decreased with the increase in the hydrophilic nano-silica ratio, whereas it improved by up to 70% for the waterproofed one. Interestingly, the angle of internal friction reached 53% and 54%, respectively. Regarding the collapsibility coefficient and water absorption, the nano-silica-treated soil exhibited contrasting behavior. Hydrophilic nano-silica increased the collapsibility coefficient and water absorption with increasing addition, while waterproof nano-silica showed significant improvements, resulting in a reduction of 5% to 59% in the collapsibility coefficient and a decrease in water absorption. These results suggest using hydrophobic (waterproofed) nano-silica instead of hydrophilic one to improve the engineering characteristics of gypseous soil.