Temperature and Particle-size Effects on the Formation of Silica Gels from Silica Sols

Abstract

Silica nanoparticles (silica sols) based gels have increasingly been used as alternative grouting material for sealing the small fractures in the tunnel walls. Gelling of silica nanoparticles at room temperature has been investigated thoroughly but gelling at different temperatures scarcely investigated. At the same time temperature is one of major factor which can affect the long-term stability of grouted silica. In this work we have investigated the gelling of three different types of silica sols (Levasil CS40-213, Levasil CS40-222, and Levasil CS30-236) having different particle sizes, in 0.28 M NaCl at 10, 20 and 30 °C. Aggregation process, starting from the addition of salt to the gelling point, was monitored by measuring the time dependent particle size distribution. Electrospray scanning mobility particle sizer (ES-SMPS) was used to measure the aggregating. These measurements were complemented by rheological measurements in order to get a relationship between changes in aggregate structure and in the viscosity of silica suspension. Data from the temperature dependent gel time measurements were used to calculate the activation energy. At room temperature, silica sols with smallest average particle size showed the shortest gel times whereas the sols with the largest particle size showed the longest gel time. However, at increasing temperature shorter gel times were seen for all the sols. Temperature dependent rheological measurements showed similar trends in viscosity changes as seen for gel times i.e., increased temperature leads to quicker increase in the viscosity and a sharp increase in viscosity near the gelling point. Our calculations of fractal dimensions showed that in the gel network there are still many free particles which continuously incorporated into the gel network. Apparent activation energies calculated for CS40-213, CS40-222, CS30-236 were 13.40, 23.36 and 41.45 kJ/mol, respectively. These values are lower than values reported for silica in the literature. Moreover, temperature dependent zeta potential measurements show that zeta potential get less negative as temperature increase. The above mentioned measurements are at odd what has been reported in literature but we have provided plausible explanation of these results.