Structural evolution of sodium silicate solutions dried to amorphous solids

Sodium silicate solutions with a molar SiO2:Na2O ratio of 3.3 were dried at temperatures up to 100 °C to solid, amorphous and transparent materials with water contents between 45 and 12 wt%. A time–temperature-transition diagram of the specific drying process was established. The dried materials were investigated by optical microscopy, X-ray diffraction, electron microscopy, and thermal analysis. Materials with higher water contents had structural units with sizes of 50–100 nm, whereas materials with lower water contents did not show a structure in this size range. At least two thermal effects were detected and are discussed with respect to composition, glass transition and structure of the materials. Hot stage microscopy was applied to correlate these effects to a liquid–solid transition. A model of the structural evolution of the materials during drying is discussed.