I coated a stone surface with K-siilcate. Apparently, it does noe seal the surface since the water penetrae the stone easily and the absorption coefficient does not change too much. However, the BET measurements show a very small specific surface area of just 1.4 m2 per g. What's wrong?
Fang xu
In colloidal terms silicate you have a gel. These are hydrated silicates. The gel contains water. Water penetrates the stone (porous xerogel). However, there are few pores in the stone.
Yuri Mirgorod
Thank you very much for the answer. I should specify better: I diluted the silicate and used this to coat a porous stone. Even for a quite thick coating film made with relatively high silicate concentration, for which I could see and touch with a finger the presence of the silicate film, the water absorption was not affected too much. The surface is quite rough, and the contact angle is quite small (14°). Separately, I put the silicate solution in a petra dish and let it to cure (air curing) for about a week until the mass no longer changed. Finally I did BET N2 adsorption. The specific surface area was just 1.4 m2 per g. During heating to 150°C to get rid of the physisorbed water (about 15% weight loss), water bubbles literally burst out. Probably they were encapsulated. I attach two SEM images of the silicate films of different densities, coated on the stone surface. The silicate is a commercial product and I used it as received without adding anything except for water. The producer declared that they use Na (probaly NaOH) to stabilize the solution.
Fang xu
After heating the gel (hydrated silicate), you got a second xerogel on the surface of the first xerogel (stone).
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