Growth of gel microstructures between stressed silica grains and its effect on soil stiffening
Laboratory tests on microscale are reported in which two amorphous silica cubes were compressed in a liquid environment, namely in solutions with different silica ion concentrations for up to four weeks. Such an arrangement represents an idealized representation of two sand grains. The grain surfaces and asperities were examined in Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) for fractures, silica gel growth, and polymer strength. In 500ppm solution, silica gel structures a few hundred microns long appeared between stressed silica cubes. In 200ppm solution, silica deposits were found around damaged grain surfaces, while at 90ppm (below silica solubility in neutral pH), fibers a few microns in length were found growing in cube cracks. AFM pulling tests found polymers with strength in the order of 100nN and length between 50 and 100 nm. After aging, size of silica gel is in the order of 10-100 µm with intergranular strength in the order of 0.01-1 mN. We concluded that prolonged compression produced damage in grains, raising local Si ion concentration, and accelerating precipitation, polymerization and gelation of silica on grain surfaces enhancing soil strength at the microscale, hence most likely contributing to the aging phenomenon observed at the macroscale. Mica surfaces near stressed silica contacts were also found to enhance silica gel growth.
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