Effect of Superhydrophobic Nano-SiO2 on the Hydraulic Conductivity of Expansive Soil and Analysis of Its Mechanism

The present work determined the influence of superhydrophobic nano-SiO2 on the hydraulic conductivity and pore size distribution of expansive soil, and analysed the mechanism of modification between superhydrophobic nano-SiO2 and expansive soil from a microscopic view. Superhydrophobic nano-SiO2 was added to expansive soil as a modifier. Our samples were of two types, i.e., unmodified (without nano-SiO2) and modified (with 0.2%, 0.4%, 0.6%, 0.8%, and 1.0% nano-SiO2 by weight of the parent soil). The hydraulic conductivity decreased with increasing nano-SiO2 content. Fourier transform mid-infrared test revealed that some silanols in soil and nano-SiO2 were dehydrated and condensed to form siloxanes. We inferred that nano-SiO2 can attach onto the surface of soil particles to form a hydrophobic membrane, which reduced the soil expansion and the change in pore size distribution. And microscopic tests showed that the pore volume and hydrophilicity of the soil samples decreased with increasing SiO2 content. According to the Young–Laplace equation, the minimum permeable pore radius was calculated in the hydraulic-conductivity test. With increasing nano-SiO2 content, the volume of permeable pore decreased. It had an excellent linear relationship with the hydraulic conductivity and permeable pore volume of samples containing different nano-SiO2 contents. Therefore, superhydrophobic nano-SiO2 could effectively reduce hydraulic conductivity by changing the pore size distribution of expansive soil.