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Sodium silicate is the common name for compounds with the formula Na2(SiO2)nO. A well-known member of this series is sodium metasilicate, Na2SiO3. Also known as waterglass or liquid glass, these materials are available in aqueous solution and in solid form. The pure compositions are colourless or white, but commercial samples are often greenish or blue owing to the presence of iron-containing impurities.
They are used in cements, passive fire protection, textile and lumber processing, refractories, and automobiles. Sodium carbonate and silicon dioxide react when molten to form sodium silicate and carbon dioxide:
In 1990, 4M tons of alkali metal silicates were produced. The main applications were in detergents, paper, water treatment, and construction materials.
The largest application of sodium silicate solutions is a cement for producing cardboard. When used as a paper cement, the tendency is for the sodium silicate joint eventually to crack within a few years, at which point it no longer holds the paper surfaces cemented together.
Sodium silicate is frequently used in drilling fluids to stabilize borehole wells and to avoid the collapse of bore walls. It is particularly useful when drill holes pass through argillaceous formations containing swelling clay minerals such as smectite or montmorillonite.
Concrete treated with a sodium silicate solution helps to significantly reduce porosity in most masonry products such as concrete, stucco, and plasters. A chemical reaction occurs with the excess Ca(OH)2 (portlandite) present in the concrete that permanently binds the silicates with the surface, making them far more durable and water repellent. This treatment generally is applied only after the initial cure has taken place (7 days or so depending on conditions). These coatings are known as silicate mineral paint.
It is used in detergent auxiliaries such as complex sodium disilicate and modified sodium disilicate. The detergent granules gain their ruggedness from a coating of silicates.
Water glass is used as coagulant/deflocculant agent in wastewater treatment plants. Waterglass binds to colloidal molecules, creating larger aggregates that sink to the bottom of the water column. The microscopic negatively charged particles suspended in water interact with sodium silicate. Their electrical double layer collapses due to the increase of ionic strength caused by the addition of sodium silicate (doubly negatively charged anion accompanied by two sodium cations) and they subsequently aggregate. This process is called coagulation/deflocculation.
Water glass is a useful binder of solids, such as vermiculite and perlite. When blended with the aforementioned lightweight aggregates, water glass can be used to make hard, high-temperature insulation boards used for refractories, passive fire protection and high temperature insulations, such as moulded pipe insulation applications. When mixed with finely divided mineral powders, such as vermiculite dust (which is common scrap from the exfoliation process), one can produce high temperature adhesives. The intumescence disappears in the presence of finely divided mineral dust, whereby the waterglass becomes a mere matrix. Waterglass is inexpensive and abundantly available, which makes its use popular in many refractory applications.