sodium silicate

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sodium silicate

sodium silicate, any one of several compounds containing sodium oxide, Na₂O, and silica, Si₂O, or a mixture of sodium silicates. Sodium orthosilicate is Na₄SiO₄ (or 2Na₂O·SiO₂); sodium metasilicate is Na₂SiO₃ (or Na₂O·SiO₂); sodium disilicate is Na₂Si₂O₅ (or Na₂O·2SiO₂); sodium tetrasilicate is Na₂Si₄O₉ (or Na₂O·4SiO₂). All these compounds are transparent, glassy or crystalline solids that have high melting points (above 800°C;) and are water soluble. They are produced chiefly by fusing sand and sodium carbonate in various proportions. The product is commonly known as water glass. The greatest single use of sodium silicates is as a raw material for making silica gel.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

Sodium silicate

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Sodium silicate, also known as water glass or liquid glass, available in aqueous solution and in solid form, is a compound used in cements, passive fire protection, refractories, textile and lumber processing, and art.

Properties

Sodium carbonate and silicon dioxide react when molten to form sodium silicate and carbon dioxide.

Sodium silicate is a white solid that is soluble in water, producing an alkaline solution. There are many kinds of this compound, including sodium orthosilicate, Na₄SiO₄; sodium metasilicate, Na₂SiO₃; sodium polysilicate, (Na₂SiO₃)_n; sodium pyrosilicate, Na₆Si₂O₇, and others. All are glassy, colourless and dissolve in water.

Sodium silicate is stable in neutral and alkaline solutions. In acidic solutions, the silicate ion reacts with hydrogen ions to form silicic acid, which when heated and roasted forms silica gel, a hard, glassy substance.

Uses

Metal repair

Sodium silicate is used, along with magnesium silicate, in muffler repair and fitting paste. When dissolved in water, both sodium silicate, and magnesium silicate form a thick paste that is easy to apply. When the exhaust system of an internal combustion engine heats up to its operating temperature, the heat drives out all of the excess water from the paste. The silicate compounds that are left over have glass-like properties, making a somewhat permanent, brittle repair.

Automotive repair

Sodium silicate can be used to seal leaks at the head gasket. A common use is when an alloy cylinder head motor is left sitting for extended periods or the coolant is not changed at proper intervals, electrolysis can "eat out" sections of the head causing the gasket to fail.

Rather than pull the head, a jar of "liquid glass" is poured into the radiator and allowed to circulate. The Waterglass is injected via the radiator water into the hotspot at the motor. This technique works because at 210–220 °F the sodium silicate loses water molecules to form a very powerful sealant that will not re-melt below 1500 °F. This approach is often used by disreputable used-car salespersons to disguise a leaking head gasket.

A sodium silicate repair of a leaking head gasket can hold for up to two years and even longer in some cases. The effect will be almost instant, and steam from the radiator water will stop coming out the exhaust within minutes of application. This repair only works with water to cylinder or water to Air applications and where the sodium silicate reaches the "conversion" temperature of 210–220 °F.

Cement uses

Sodium silicate has been widely used as a general purpose cement, but especially for applications involving cementing objects exposed to heat or fire. For example, sodium silicate has been provided in home first-aid kits and used in medical practice as a glue for holding human skin together at surface cuts. It has also been used as a general purpose paper cement.

One common example of its use as a paper cement was for producing paper cartridges for black powder revolvers produced by Colt's Manufacturing Company during the period from 1851 until 1873, especially during the American Civil War. Sodium silicate was used to seal combustible nitrated paper together to form a conical paper cartridge to hold the black powder, as well as to cement the lead ball or conical bullet into the open end of the paper cartridge. Such sodium silicate cemented paper cartridges were inserted into the cylinders of revolvers, thereby speeding the reloading of cap and ball black powder revolvers. This use largely ended with the introduction of Colt revolvers employing brass-cased cartridges starting in 1873.

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.

Food preservation

Sodium silicate was also used as an egg preservation agent in the early 20th Century with large success. When fresh eggs are immersed in it, bacteria which cause the eggs to spoil are kept out and water is kept in. Eggs can be kept fresh using this method for up to nine months. When boiling eggs preserved this way, it is well advised to pin-prick the egg to allow steam to escape because the shell is no longer porous.

An article in The Mother Earth News offers actual test results for this and other methods of preservation. LINK

Timber treatment

The use of sodium silicate as a timber treatment for pressure-treated wood actually began some time in the 19th century. It is suggested that that more costly "silicate of potash" (potassium silicate) may also be used, in "Handy Farm Devices and How to Make Them" by Rolfe Cobleigh, published in 1910. Since 1998 scientists have researched methods for rendering sodium silicate insoluble once the lumber has been treated with it. With or without the additional process, treating wood with sodium silicate preserves wood from insects and possesses some flame-retardant properties.

Concrete and general masonry treatment

Concrete treated with a sodium silicate solution helps to significantly reduce porosity in most masonry products such as concrete, stucco, plasters. A chemical reaction occurs with the excess Ca(OH)₂ in the concrete that permanently binds the silicates with the surface making them far more wearable and water repellent. It is generally advised to apply only after initial cure has taken place (7 days or so depending on conditions). These coatings are known as silicate mineral paint.

Passive fire protection ("PFP")

Sodium silicates are inherently intumescent. They come in prill (solid beads) form, as well as the liquid, water glass. The solid sheet form (Palusol) must be waterproofed to ensure longterm passive fire protection.

Standard, solid, bead form sodium silicates have been used as aggregate within silicone rubber to manufacture plastic pipe firestop devices. The silicone rubber was insufficient waterproofing to preserve the intumescing function and the products had to be recalled, which is problematic for firestops that are concealed behind drywall in buildings.

Pastes for caulking purposes are similarly unstable. This too has resulted in recalls and even litigation. Only 3M's "Expantrol" version, which has an external heat treatment that helps to seal the outer surface, as part of its process standard, has achieved sufficient longevity to qualify for DIBt approvals in the US for use in firestopping.

Not unlike other intumescents, sodium silicate, both in bead form and in liquid form are inherently endothermic, due to liquid water in the water glass and hydrates in the prill form. The absence in the US of mandatory aging tests, whereby PFP systems are made to undergo system performance tests after the aging and humidity exposures, are at the root of the continued availability, in North America, of PFP products that can become inoperable within weeks of installation. Indiscriminate use of sodium silicates without proper waterproofing measures are contributors to the problems and risk. When sodium silicates are adequately protected, they function extremely well and reliably for long. Evidence of this can be seen in the many DIBt approvals for plastic pipe firestop devices using Palusol, which use waterproofed sodium silicate sheets.

Refractory use

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.

Water Treatment

Water glass is used as a water treatment in waste water treatment plants. Waterglass will bind to heavier molecules and drag them out of the water.

Magic Crystals

Water glass was used in the magic crystal garden toys from the 1980s. When waterglass was combined with a selection of different metals in solution, the waterglass would cause the metals to precipitate. Each metal would precipitate separately causing a different color stalagmite.

An early mention of crystals of metallic salts forming a "chemical garden" in sodium silicate is found in the 1946 Modern Mechanix magazine This results in very colorful gardens -- much more than shown in the illustrations. Use caution with the metal salts if you do this, some are dangerous to people and animals if swallowed.