Water Glass as a Sealer for Plaster and Concrete
The utility of sodium silicate, liquid glass or water glass (as it is better known) is not just limited to making sand molded castings or metal molds. Apart from working as an excellent binder for sand molds or giving an instant antique patina to ceramic products, sodium silicate also works as an effective sealer - it is very commonly used to seal concrete surfaces.
The problem with concrete is that it is naturally quite porous. There are a large number of pores on the surface of concrete which make it susceptible to air and water. Even oil, acids and other liquids can easily permeate concrete structures and weaken them over time.
How to Waterproof
Liquid sodium silicate can be applied on the concrete as a sealer. It will penetrate the concrete and chemically react with the portlandite to turn it into calcium silicate hydrate. This not only makes the concrete surface water repellant, but also much stronger, denser and wearable. The same reaction also permanently binds the silicate to the concrete. It will not wear out unless and until the concrete itself wears away.
And these same benefits of water glass can be transferred to plaster gypsums like plaster of Paris too. Plaster gypsum also suffers from the same drawbacks of porosity and weakness. Applying a thin coat of aqueous sodium silicate on unpainted plaster surfaces like frescos or stuccos can make them effectively repellant to water and other liquids. The porosity is significantly reduced and the plaster also becomes stronger and long-lasting. The impermeable layer further helps the plaster to resist damage and staining.
Coming to the art studio, sodium silicate can be used to coat plaster gypsum molds and casts. The only criteria is that it should be applied after the plaster (or even concrete) has cured initially - say after 7 days or so. Let it dry naturally. The sodium silicate will penetrate the surface, seal the pores and form a natural and transparent finish on the plaster.
On the other hand, some people even add sodium silicate in solid form to the plaster mix itself to make it tough and water-resistant. In both forms, sodium silicate enables us to achieve strength and impermeability without using any synthetic materials at all.
Water glass is odorless, non-toxic and non-inflammable. It effectively resists moisture and high temperatures too. It proves to be a very economical sealer that stays strong and rigid for long. Another benefit is that sodium silicate is harmless to the environment too.
However, you should be careful when handling sodium silicate as it can irritate or burn the skin and eyes on contact. Protect yourself with glasses, gloves and apron or overalls when using sodium silicate. Keep the area well-ventilated to avoid unnecessary inhalation.
Store sodium silicate in a cool and dry place away from heat, acids or food materials. It's better not to let it freeze either.