Growth of polymer microstructures between stressed silica grains: a chemo-mechanical coupling

Laboratory tests on the micro scale are reported in which two amorphous silica cubes were compressed in a liquid environment, in solutions with different silica ion concentrations, for up to 3 weeks. Such an arrangement represents an idealised representation of two sand grains. The grain surfaces and asperities were examined in the scanning electron microscope and atomic force microscope (AFM) for fractures, silica gel growth and polymer strength. In 500 ppm solution, silica gel structures a few hundred micrometres long appeared between stressed silica cubes. In 200 ppm solution, silica deposits were found around damaged grain surfaces, while at 90 ppm (below silica solubility in neutral pH), fibres a few micrometres in length were found growing in cube cracks. AFM pulling tests found polymers with strength of the order of 100 nN. It was concluded that prolonged compression produced damage in grains, raising local silicon ion concentration and accelerating precipitation, polymerisation and gelation of silica on grain surfaces, enhancing soil strength at the micro scale, and hence most likely contributing to the ageing phenomenon observed at the macro scale.